Spontaneous arthritis and ankylosis in male DBA/1 mice: further evidence for a role of behavioral factors in "stress-induced arthritis".

BACKGROUND: Ageing male DBA/1 mice spontaneously develop arthritis in the hind paws. We and others have demonstrated that this model shares striking features with human spondyloarthritis, in particular entheseal involvement, progressive ankylosis but also dactylitis. Here, we report on our recent experience with this model highlighting how changes in the animal facility affect the development of the disease. FINDINGS: Ageing male DBA/1 mice from different litters were caged together (6 mice per cage) at the age of 10 weeks. The mice were checked twice a week for clinical signs of arthritis. Disease severity was assessed in further detail post-mortem by scoring for histomorphological characteristics. DBA/1 mice spontaneously develop macroscopically detectable arthritis, presenting as joint swelling or toe stiffness. Standard settings with open cages lead to an almost 100% incidence by the age of 26 weeks. The introduction of larger cages and filter tops reducing exposure to other cages dramatically affected incidence. Other negative factors include excess bedding material reducing the impact of walking and running. Switching back to the original conditions resulted again in a high incidence, further optimized by sensory exposure to female mice. We also showed that the related DBA/2 strain is sensitive to the disease. CONCLUSIONS: Changing environmental factors in the housing conditions of DBA/1 mice severely affects the spontaneous development of arthritis. This points out that the model is very sensitive to external stress and sensory factors that are likely affecting the behavior of the male mice and that the model needs to be optimized in different situations.

The role of bone morphogenetic proteins in ankylosing spondylitis.

Ankylosing spondylitis (AS), the best-known form of spondyloarthritis (SpA), is a remodelling arthritis characterized by chronic inflammation and bone formation. Ankylosis of the axial skeleton and sacroiliac joints leads to an impairment of spinal mobility, progressive spinal fusion and an increased risk of spinal fractures. The nature of the relationship between inflammation and new bone formation in AS has been controversial and questions remain as to whether there is a direct relationship between inflammation and new bone formation. Like others, we have hypothesized that the molecular pathways underlying ankylosis recapitulate the process of endochondral bone formation and that bone morphogenetic proteins (BMPs) play a key role in this process in AS. Furthermore, we discuss the entheseal stress hypothesis, which proposes that inflammation and ankylosis are linked but largely independent processes, and consider observations from mouse models and other human diseases which also imply that biomechanical factors contribute to the pathogenesis of AS. As current therapeutics, such as tumour necrosis factor inhibitors do not impede disease progression and ankylosis in AS, it is the pathways discussed in this review that are the now the focus for the identification of future drug targets.

Inhibition of inflammation but not ankylosis by glucocorticoids in mice: further evidence for the entheseal stress hypothesis.

INTRODUCTION: Studies in the spontaneous ankylosis model in aging male DBA/1 mice and in patients with ankylosing spondylitis provide evidence that inflammation and new tissue formation leading to joint or spine ankylosis are likely linked but largely uncoupled processes. We previously proposed the 'entheseal stress' hypothesis that defines microdamage or cell stress in the enthesis as a trigger for these disease processes. Here, we further investigated the relationship between inflammation and ankylosis by focusing on the early phase of the spontaneous arthritis model. METHODS: Aging male DBA/1 mice from different litters were caged together at the age of ten weeks and studied for signs of arthritis. A group of DBA/1 mice were treated daily with dexamethasone (0.5 µg/g body weight). Severity of disease was assessed by histomorphology and by positron emission tomography (PET) using 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG) as a tracer. Bone loss in dexamethasone-treated or control mice was determined by in vivo dual-energy X-ray absorptiometry. Chemokine gene expression was studied ex vivo in dissected paws and in vitro in mesenchymal cells (periosteal and bone marrow stromal cells) by quantitative real-time PCR in the presence or absence of bone morphogenetic protein 2 (BMP2) and dexamethasone. RESULTS: Dexamethasone treatment did not affect incidence or severity of ankylosis, but led to an expected reduction in inflammation in the paws at week 15 as measured by PET tracer uptake. Treatment with dexamethasone negatively affected bone mineral density. Chemokines attracting neutrophils and lymphocytes were expressed in affected paws. In vitro, BMP2 stimulation upregulated chemokines in different mesenchymal joint-associated cell types, an effect that was inhibited by dexamethasone. CONCLUSIONS: BMP signaling may be a trigger for both inflammation and ankylosis in the spontaneous model of ankylosing enthesitis. The lack of inhibition by glucocorticoids on new bone formation while causing systemic bone loss highlights the paradoxical simultaneous loss and gain of bone in patients with ankylosing spondylitis.

Insights into the pathophysiology of ankylosing spondylitis: contributions from animal models.

The introduction of anti-tumor necrosis factor strategies has significantly changed the perspective and outcome of patients with ankylosing spondylitis and related spondyloarthritides. This breakthrough has also boosted further research efforts into the mechanisms of disease. As human tissue specimens of the spine and sacroiliac joints are very difficult to obtain and rarely allow mechanistic studies, most of the new concepts have emerged from different animal models of disease. In this review, we summarize insights into the role of HLA-B27 based on transgenic rat and mouse models, efforts into the identification of cell populations stimulating inflammation and molecular studies of pathological bone formation leading to ankylosis. Important progress has been made and novel hypotheses were put forward. These include the impact of HLA-B27 on endoplasmic reticulum stress and the unfolded protein response, the role of stromal cells in inflammation, the entheseal stress hypothesis and the identification of the bone morphogenetic protein and WNT signaling pathways as therapeutic targets for ankylosis.

Blocking p38 signalling inhibits chondrogenesis in vitro but not ankylosis in a model of ankylosing spondylitis in vivo.

OBJECTIVES: To investigate p38 mitogen activated protein kinase (MAPK) signalling in an in vitro model of bone morphogenetic protein (BMP) and transforming growth factor ß (TGFß)-induced chondrogenesis and in vivo, with specific attention to its potential role in ankylosing enthesitis. METHODS: Human periosteum-derived cells (hPDCs) were cultured in pellets and stimulated with BMP2 or TGFß1 in the presence or absence of a p38 inhibitor SB203580 or proinflammatory cytokines. Chondrogenic differentiation was evaluated using quantitative PCR. Male DBA/1 mice from different litters were caged together at the age of 8 weeks and treated with SB203580 in both a preventive and therapeutic strategy. The mice were evaluated for prospective signs of arthritis and the toe joints were analysed histologically to assess disease severity. RESULTS: p38 inhibition by SB203580 and proinflammatory cytokines downregulated chondrogenic markers in pellet cultures stimulated by BMP2 or TGFß1. In contrast, the in vivo experiments resulted in an increased clinical incidence of arthritis and pathology severity score, reflecting progression towards ankylosis in mice given SB203580. CONCLUSION: Inhibition of p38 inhibited chondrogenic differentiation of progenitor cells, showing that not only the SMAD signalling pathways and also alternative activation of MAPKs including p38 contribute to chondrogenesis. Such an inhibitory effect is not found in an in vivo model of joint ankylosis and spondyloarthritis. Increased incidence and severity of disease in preventive experiments and shifts in disease stages in a therapeutic experimental set-up suggest that specific inhibition of p38 may have deleterious rather than beneficial effects.