Aberrant bone formation and dysregulated bone homeostasis in ankylosing spondylitis

Abstract

Ankylosing spondylitis (AS) is a chronic, progressive, systemic autoimmune disease characterised by spinal stiffness and ocular, cardiac, intestinal, and peripheral joint involvements. Genetics, infectious agents, and immune-mediated inflammatory processes have all been hypothesized to contribute to AS pathogenesis, but the precise aetiology remains elusive. Recent studies have identified biological and cellular factors that correlate with the onset and progression of AS. This has provided avenues of research that may help elucidate disease mechanisms and lead to advances in therapeutic interventions. This study aimed to examine some of the findings from recent molecular studies, focusing on the molecular mechanism and associated factors such as interleukin-17, tumor necrosis factor-alpha, receptor activator of nuclear factor-kappa B/receptor activator of nuclear factor-kappa B ligand/osteoprotegerin pathway, and related microRNAs to gain insight into aberrant bone formation in AS and potential approaches to its prevention. This editorial also addresses the contribution of osteoclasts to bone pathology in AS. The author examined the molecular pathways governing osteoclast differentiation and activity, with particular emphasis on relevant cytokines and immune cell interactions. A comprehensive understanding of these mechanisms is essential for the development of targeted therapies to mitigate excessive bone resorption and pathological skeletal remodeling in AS.

Key Words: Ankylosing spondylitis; Cytokines; Immune; Osteoclast; Pathway; Targeted therapy

Core Tip: In ankylosing spondylitis (AS), osteoclasts contribute significantly to pathological bone resorption and remodeling. The receptor activator of nuclear factor-kappa B/receptor activator of nuclear factor-kappa B ligand/osteoprotegerin signaling axis, inflammatory cytokines such as interleukin-17 and tumor necrosis factor-alpha, and immune cells such as T helper type 17 and macrophages are central to osteoclastogenesis in AS. Elucidation of the mechanisms governing osteoclast differentiation, activation, and dysregulation in AS is essential for the development of targeted therapeutic strategies that modulate aberrant bone remodeling.

INTRODUCTION

Ankylosing spondylitis (AS) is a chronic inflammatory disease that primarily affects the spine. It begins with the sacroiliac joints and can also involve some joints in the extremities. Symptoms include pain, stiffness, disability, and limited range of motion. As the condition progresses, there is eventual fusion of the spinal joints[1]. Several factors contribute to the development and progression of AS. These include dysfunction of the immune system, dysregulation of the inflammatory process through the expression of inflammatory factors such as tumor necrosis factor-alpha (TNF-α) and interleukin-17 (IL-17), and genetic predisposition, particularly in those with the HLA-B27 gene. Other key drivers of AS progression include abnormal overactivation of bone cells, including osteoclasts and osteoblasts, and dysfunctional inflammation at the points where ligaments and bone meet.

The aberrant bone formation in AS can lead to a variety of comorbidities, including bamboo spine, severe secondary osteoarthritis, and pathologic fractures[2]. The condition is orchestrated by a complex interplay of inflammatory cytokines, signaling pathways, and osteogenic differentiation processes. The inflammatory cytokines TNF-α, IL-17, and IL-23 play pivotal roles in initiating and sustaining inflammation. This, in turn, activates the downstream osteogenic pathways. In addition to the cytokines, molecular abnormalities in the receptor activator of nuclear factor-kappa B (RANK)/RANK ligand (RANKL)/osteoprotegerin (OPG) signaling axis, the wingless-related integration site/beta-catenin signaling pathway, and immune cells such as T helper type 17 (Th17) and macrophages have been implicated in the facilitation of aberrant osteogenesis through the promotion of osteoblast differentiation and mineralization.

Current therapeutic approaches focus on inhibition of the aforementioned cytokines, but further research is necessary for future interventions. Unraveling the molecular mechanisms and pathways behind the aberrant bone formation in AS may help to identify new therapeutic targets and the development of treatments able to prevent excessive ossification without compromising bone integrity. This editorial aims to explore the molecular mechanisms underlying aberrant bone formation in AS, highlighting key signaling pathways and potential therapeutic implications.

KEY PATHWAYS AND MOLECULES IN AS OSTEOCLAST DYSFUNCTION

Osteoclasts are primarily responsible for breaking down bone tissue. These become overactive during the early and active stages of AS, leading to bone erosion, which is followed by abnormal bone formation and remodeling. These multinucleated cells play a crucial role in bone resorption, and their dysregulation is a major factor in the skeletal problems seen in AS. However, the exact molecular pathways involved in osteoclast functioning are not yet fully understood (Table 1).

Table 1 Mechanisms of osteoclast dysregulation in ankylosing spondylitis.

Category	Molecular mechanism	Effect on osteoclasts	Ref.
Inflammatory cytokines	TNF-α, IL-17, IL-23, IL-6	Promotes osteoclast differentiation and activation	[2-4]
RANK/RANKL/OPG pathway	Increases RANKL expression and decreases OPG	Enhances osteoclast formation and bone resorption	[1,3]
TNF-α signaling	Activates NF-κB and MAPK pathways	Enhances osteoclast survival and activity	[2-4]
T-helper cells	Secrete IL-17 and IL-23	Stimulates RANKL production and osteoclastogenesis	[2,8,14]
Wnt/β-catenin signaling	Inhibited by DKK-1 and sclerostin	Reduces bone formation, favors resorption	[3,8,10]
Bone resorption factors	MMPs, CatK	Degrades the bone matrix, increasing erosion	[8,10]

TNF-α: Tumor necrosis factor-alpha; IL: Interleukin; RANK: Receptor activator of nuclear factor-kappa B; RANKL: Receptor activator of nuclear factor-kappa B ligand; OPG: Osteoprotegerin; NF-κB: Nuclear factor-kappa B; MAPK: Mitogen-activated protein kinases; Wnt/β: Wingless-related integration site/beta; DKK-1: Dickkopf-related protein 1; MMPs: Matrix metalloproteinases; CatK: Cathepsin K.

The RANK/RANKL/OPG pathway

Studies have identified the key signaling molecules and pathways in AS osteoclast formation. The RANK/RANKL/OPG pathway, comprising RANK, RANKL, and OPG, is particularly important in regulating how osteoclasts differentiate in AS. Wang et al[3] investigated six single-nucleotide polymorphisms within these genes and found that variations in the RANKL gene are linked to the formation of syndesmophytes, which are bony spurs in spinal ligaments. They found that a specific combination of these variations, rs7984870C/rs9533155G/rs9525641C, reduces the risk of AS and protects against syndesmophyte formation. This suggests that the RANK/RANKL/OPG pathway influences osteoclast differentiation, which, in turn, may affect AS susceptibility and severity.

IL-17 and TNF-α inflammatory cytokines

Other studies have focused on inflammatory cytokines, primarily IL-17 and TNF-α, and the immune cells that produce them, such as Th17. IL-17 is a powerful cytokine that promotes osteoclast formation. Its receptors are found on many different types of cells[4], including neutrophils and mast cells. The presence of IL-17 activates RANK, and Th17 cells produce RANKL. This leads to the differentiation and proliferation of osteoclast precursors and the activation of osteoclasts. In a mouse model of arthritis, Pickens et al[5] found that those lacking the IL-17RA subunit showed less bone erosion and reduced levels of the chemokines C-X-C motif ligand 1 (CXCL1), CXCL2, CXCL5, C-C motif ligand 9 (CCL9), CCL7, and CCL20, the cytokines IL-1β, IL-6, and RANKL, and certain metal proteinases. TNF-α is known to increase the number of RANK receptors on osteoclast precursor cells, which encourages these cells to differentiate and become active. Recent study has shown that TNF-α and IL-17 work together to increase the production of RANKL[6] and further promote osteoclast formation.

Many different immune cells are also involved in osteoclast formation. As mentioned earlier, Th17 cells are the main source of IL-17, which can stimulate osteoclast differentiation through the RANKL and TNF-α pathways. Macrophages, B cells, and dendritic cells also play important roles in the activation and differentiation of osteoclasts[7]. However, a lack of evidence for direct relationships between abnormal bone formation and the RANKL and TNF-α pathways has led recent studies to direct their focus on other cells, pathways, and molecules to discover the mechanisms involved in aberrant osteogenesis and mineralization. These alternatives have included the wingless-related integration site/beta-catenin signaling pathway, bone morphogenetic proteins, the hedgehog signaling pathway, and osteoblasts[8-10].

Specific microRNAs

MicroRNAs (miRNAs) have been found to contribute to several AS processes, including ossification, the inflammatory response, differentiation of osteoblasts, and the control of immune cells such as lymphocytes and macrophages. Several studies have conducted microarray analyses to compare the profiles of long non-coding RNA, messenger RNA, and miRNA in biopsy samples obtained from patients with AS and healthy controls. Using pathway analysis, gene prediction, and network design, these studies have identified two miRNAs, miR-27b-3p and miR-17-5p, that appear to play significant roles in boosting the ability of ligament fibroblasts to differentiate into bone cells able to promote bone formation in AS[11-13]. MiRNA in extracellular vesicles known as exosomes have also emerged as promising candidates owing to their role in intercellular communication and their ability to transport a variety of bioactive molecules, including proteins, lipids, and nucleic acids. These exosomal miRNAs have also been implicated in the regulation of osteoclast differentiation and function. For example, miR-212-3p has been associated with dysregulated osteoclast activity[3]. Other studies have found specific exosomal miRNAs to play significant roles in osteoclastogenesis. These represent promising potential therapeutic targets in AS. Xie et al[6] discovered that miR-212-3p inhibits the phosphorylation of extracellular signal-regulated kinases 1 and 2, which decreases macrophage clumping and the release of inflammatory substances in AS patients. Manipulation of the miR-212-3p extracellular signal-regulated kinase pathway was shown to promote the differentiation and maturation of osteoclasts, potentially reducing inflammation and abnormal bone growth in AS[6]. A recent study by Liu et al[14] found that M2 macrophage-derived exosomes transfer miR-22-3p to recipient cells, where it modulates osteoclastogenesis. While the specific effects on AS are still under investigation, the effects of miR-22-3p on osteoclast differentiation suggest likely associated effects on the AS bone remodeling processes. Another recent study identified 22 miRNAs with increased expression and two with decreased expression in AS patient-derived exosomes compared with those from healthy controls[11]. These upregulated miRNAs may contribute to the dysregulation of osteoclast activity observed in AS and provide further potential therapeutic targets. Some studies have attempted to understand how osteoclasts become overactive or dysfunctional in AS, particularly in relation to the stiffening of the spine caused by inflammation where ligaments and bone meet. Nevertheless, the process of osteoclast formation in AS is still not well understood[15]. Because of this, only a few treatments aimed at reducing this increased activity have so far been developed. A study by Liu et al[13] made two important discoveries about osteoclast activity in AS. First, mesenchymal stem cells (MSCs) in patients with AS have higher levels of the fat mass obesity-associated protein (FTO). Second, FTO controls the ability of these MSCs to prevent osteoclast formation through a specific pathway involving miR-4284 and the long non-coding RNA non-coding RNA activated by DNA damage. The study concluded that FTO regulates how AS-MSCs function through the non-coding RNA activated by DNA damage/miR-4284 pathway, which could help explain how abnormal bone growth occurs and provide potential targets for AS treatment.

CURRENT LIMITATIONS AND FUTURE PERSPECTIVES

Despite significant advances in our understanding of the key pathways and molecular mechanisms of aberrant bone formation in AS, existing research has several limitations. Importantly, the specifics of the interaction between inflammation and osteogenesis have not been fully elucidated. This impedes the development of precise and efficacious therapeutic interventions. Current treatments are primarily TNF-α and IL-17 inhibitors. These focus on reducing inflammation, but they have only limited effects on the prevention of new bone formation. There is also insufficient information on reliable disease- or pathology-specific biomarkers for the prediction of disease progression and therapeutic response. Since all subtypes of AS otherwise follow the same course and exhibit the same activity, it is important to find a means of identifying which AS patients will experience aberrant bone formation so that preventive treatment can be provided. To overcome these current limitations and obtain a comprehensive understanding of AS pathophysiology, research is needed that utilizes methodology not usually utilized in AS research, such as genomics, transcriptomics, and metabolomics.

CONCLUSION

AS causes serious health problems and significantly impacts quality of life. While researchers have identified several potential mechanisms and molecular factors involved in the development and progression of AS, there is still a great deal we do not know. In particular, the overactivity and dysregulation of osteoclasts play a key role in AS bone problems like bamboo spine. To better understand the underlying molecular interactions and pathways, further research using the latest knowledge and technologies is essential. Addressing these limitations and knowledge gaps through advanced research technologies and innovative strategies is essential to improving our understanding of the disease and developing targeted therapeutics.