Periarticular Bone Loss in Arthritis Is Induced by Autoantibodies Against Citrullinated Vimentin

Periarticular bone loss is a long known but yet insufficiently understood phenomenon in patients with rheumatoid arthritis. This study investigated whether autoimmunity against citrullinated proteins is causally involved in triggering periarticular bone loss. Periarticular bone loss was studied in the standard antigen-induced arthritis (AIA) mouse model with methylated bovine serum albumin (mBSA) as well as a modified model with mutated citrullinated vimentin (MCV) alone or in combination with mBSA. Periarticular bone loss, subchondral osteoclastogenesis, as well as local expression of cytokines, osteoclast genes, and peptidyl-arginine deiminase (PAD) enzymes were assessed after arthritis induction. Immune cell and osteoclast precursor infiltration were detected in the periarticular bone marrow and local lymph nodes. In addition, periarticular bone loss was assessed upon challenge of mice with purified anti-MCV antibody. Despite inducing a milder form of arthritis than mBSA, MCV triggered significant periarticular bone loss associated with an increased infiltration of osteoclast precursors and mature osteoclasts in the periarticular bone marrow. MCV enhanced the expression of the osteoclast inducers RANKL and M-CSF, the cytokines IL-8, IL-1, IL-6, and TNF-α, as well as PAD2 and PAD4 enzymes in the periarticular bone marrow. Furthermore, also anti-MCV antibody challenge induced significant periarticular bone loss and local osteoclastogenesis in the mice. Autoimmunity against citrullinated vimentin triggers periarticular bone loss by osteoclast activation in the bone marrow. These findings may explain why periarticular bone loss is already found very early in the disease course of patients with rheumatoid arthritis. © 2017 American Society for Bone and Mineral Research.

Resolution of inflammation by interleukin-9-producing type 2 innate lymphoid cells

Inflammatory diseases such as arthritis are chronic conditions that fail to resolve spontaneously. While the cytokine and cellular pathways triggering arthritis are well defined, those responsible for the resolution of inflammation are incompletely characterized. Here we identified interleukin (IL)-9-producing type 2 innate lymphoid cells (ILC2s) as the mediators of a molecular and cellular pathway that orchestrates the resolution of chronic inflammation. In mice, the absence of IL-9 impaired ILC2 proliferation and activation of regulatory T (T_reg) cells, and resulted in chronic arthritis with excessive cartilage destruction and bone loss. In contrast, treatment with IL-9 promoted ILC2-dependent T_reg activation and effectively induced resolution of inflammation and protection of bone. Patients with rheumatoid arthritis in remission exhibited high numbers of IL-9⁺ ILC2s in joints and the circulation. Hence, fostering IL-9-mediated ILC2 activation may offer a novel therapeutic approach inducing resolution of inflammation rather than suppression of inflammatory responses.

A Dual Role of Upper Zone of Growth Plate and Cartilage Matrix-Associated Protein in Human and Mouse Osteoarthritic Cartilage: Inhibition of Aggrecanases and Promotion of Bone Turnover

OBJECTIVE

Cartilage damage and subchondral bone changes are closely connected in osteoarthritis. Nevertheless, how these processes are interlinked is, to date, incompletely understood. This study was undertaken to investigate the mechanistic role of a cartilage-derived protein, upper zone of growth plate and cartilage matrix-associated protein (UCMA), in osteoarthritis-related cartilage and bone changes.

METHODS

UCMA expression was assessed in healthy and osteoarthritic human and mouse cartilage. For analysis of cartilage and bone changes, osteoarthritis was induced by destabilization of the medial meniscus (DMM) in wild-type (WT) and Ucma-deficient mice. UCMA-collagen interactions, the effect of UCMA on aggrecanase activity, and the impact of recombinant UCMA on osteoclast differentiation were studied in vitro.

RESULTS

UCMA was found to be overexpressed in human and mouse osteoarthritic cartilage. DMM-triggered cartilage changes, including increased structural damage, proteoglycan loss, and chondrocyte cell death, were aggravated in Ucma-deficient mice compared to WT littermates, thereby demonstrating the potential chondroprotective effects of UCMA. Moreover, UCMA inhibited ADAMTS-dependent aggrecanase activity and directly interacted with cartilage-specific collagen types. In contrast, osteoarthritis-related bone changes were significantly reduced in Ucma-deficient mice, showing less pronounced osteophyte formation and subchondral bone sclerosis. Mechanistically, UCMA directly promoted osteoclast differentiation in vitro.

CONCLUSION

UCMA appears to link cartilage with bone changes in osteoarthritis by supporting cartilage integrity as an endogenous inhibitor of aggrecanases while also promoting osteoclastogenesis and subchondral bone turnover. Thus, UCMA represents an important link between cartilage and bone in osteoarthritis.

Neurodegeneration Enhances the Development of Arthritis

The prevalence of neurodegenerative disease and arthritis increases with age. Despite both processes being associated with immune activation and inflammation, little is known about the mechanistic interactions between neurodegenerative disease and arthritis. In this article, we show that tau-transgenic (tau-tg) mice that develop neurodegenerative disease characterized by deposition of tau tangles in the brain are highly susceptible to developing arthritis. Already at steady-state conditions, tau-tg mice exhibit peripheral immune activation that is manifested by higher numbers of granulocytes, plasmablasts, and inflammatory Ly6C^hi CCR2⁺ monocytes, as well as increased levels of proinflammatory cytokines, such as TNF-α and IL-17. Upon induction of collagen-induced arthritis (CIA), tau-tg mice displayed an increased incidence and an earlier onset of CIA that was associated with a more pronounced inflammatory cytokine response. Furthermore, induction of CIA led to significantly elevated numbers of Iba-1-expressing cells in the brain, indicative of microglia activation, and the formation of anti-tau Abs in tau-tg mice. These changes were accompanied by the resolution of tau tangles and significantly decreased neurodegenerative pathology. In summary, these data show that neurodegenerative disease enhances the development of arthritis. In addition, arthritis, once induced, triggers innate immune responses in the brain, leading to resolution of neurodegenerative changes.

SUCROSE TRANSPORTER 5 supplies Arabidopsis embryos with biotin and affects triacylglycerol accumulation

The Arabidopsis SUC5 protein represents a classical sucrose/H(+) symporter. Functional analyses previously revealed that SUC5 also transports biotin, an essential co-factor for fatty acid synthesis. However, evidence for a dual role in transport of the structurally unrelated compounds sucrose and biotin in plants was lacking. Here we show that SUC5 localizes to the plasma membrane, and that the SUC5 gene is expressed in developing embryos, confirming the role of the SUC5 protein as substrate carrier across apoplastic barriers in seeds. We show that transport of biotin but not of sucrose across these barriers is impaired in suc5 mutant embryos. In addition, we show that SUC5 is essential for the delivery of biotin into the embryo of biotin biosynthesis-defective mutants (bio1 and bio2). We compared embryo and seedling development as well as triacylglycerol accumulation and fatty acid composition in seeds of single mutants (suc5, bio1 or bio2), double mutants (suc5 bio1 and suc5 bio2) and wild-type plants. Although suc5 mutants were like the wild-type, bio1 and bio2 mutants showed developmental defects and reduced triacylglycerol contents. In suc5 bio1 and suc5 bio2 double mutants, developmental defects were severely increased and the triacylglycerol content was reduced to a greater extent in comparison to the single mutants. Supplementation with externally applied biotin helped to reduce symptoms in both single and double mutants, but the efficacy of supplementation was significantly lower in double than in single mutants, showing that transport of biotin into the embryo is lower in the absence of SUC5.