Osteoclasts are specialized cells derived from the monocyte/macrophage haematopoietic lineage that develop and adhere to bone matrix, then secrete acid and lytic enzymes that degrade it in a specialized, extracellular compartment. Discovery of the RANK signalling pathway in the osteoclast has provided insight into the mechanisms of osteoclastogenesis and activation of bone resorption, and how hormonal signals impact bone structure and mass. Further study of this pathway is providing the molecular basis for developing therapeutics to treat osteoporosis and other diseases of bone loss.

Osteoclast differentiation and activation

The TNF and TNF receptor superfamilies: integrating mammalian biology.

Osteoporosis, a disease endemic in Western society, typically reflects an imbalance in skeletal turnover so that bone resorption exceeds bone formation. Bone resorption is the unique function of the osteoclast, and anti-osteoporosis therapy to date has targeted this cell. The osteoclast is a specialized macrophage polykaryon whose differentiation is principally regulated by macrophage colony-stimulating factor, RANK ligand, and osteoprotegerin. Reflecting integrin-mediated signals, the osteoclast develops a specialized cytoskeleton that permits it to establish an isolated microenvironment between itself and bone, wherein matrix degradation occurs by a process involving proton transport. Osteopetrotic mutants have provided a wealth of information about the genes that regulate the differentiation of osteoclasts and their capacity to resorb bone.

https://science.sciencemag.org/content/sci/289/5484/1504.full.pdf

Bone Resorption by Osteoclasts

Missing Pieces in the NF-κB Puzzle

Rheumatoid arthritis is the most common inflammatory arthritis and is a major cause of disability. It existed in early Native American populations several thousand years ago but might not have appeared in Europe until the 17th century. Early theories on the pathogenesis of rheumatoid arthritis focused on autoantibodies and immune complexes. T-cell-mediated antigen-specific responses, T-cell-independent cytokine networks, and aggressive tumour-like behaviour of rheumatoid synovium have also been implicated. More recently, the contribution of autoantibodies has returned to the forefront. Based on the pathogenic mechanisms, specific therapeutic interventions can be designed to suppress synovial inflammation and joint destruction in rheumatoid arthritis.

/pdf/evolving-concepts-of-rheumatoid-arthritis-1bu40832qh.pdf

Evolving concepts of rheumatoid arthritis

Bone remodelling and bone loss are controlled by a balance between the tumour necrosis factor family molecule osteoprotegerin ligand (OPGL) and its decoy receptor osteoprotegerin (OPG)1,2,3. In addition, OPGL regulates lymph node organogenesis, lymphocyte development and interactions between T cells and dendritic cells in the immune system3,4,5. The OPGL receptor, RANK, is expressed on chondrocytes, osteoclast precursors and mature osteoclasts4,6. OPGL expression in T cells is induced by antigen receptor engagement7, which suggests that activated T cells may influence bone metabolism through OPGL and RANK. Here we report that activated T cells can directly trigger osteoclastogenesis through OPGL. Systemic activation of T cells in vivo leads to an OPGL-mediated increase in osteoclastogenesis and bone loss. In a T-cell-dependent model of rat adjuvant arthritis characterized by severe joint inflammation, bone and cartilage destruction and crippling, blocking of OPGL through osteoprotegerin treatment at the onset of disease prevents bone and cartilage destruction but not inflammation. These results show that both systemic and local T-cell activation can lead to OPGL production and subsequent bone loss, and they provide a novel paradigm for T cells as regulators of bone physiology.

Activated T cells regulate bone loss and joint destruction in adjuvant arthritis through osteoprotegerin ligand

Objective To determine whether interleukin-1 (IL-1) or tumor necrosis factor alpha (TNFalpha), or both, plays a role in the excessive degradation that is observed in cultured osteoarthritic (OA) articular cartilage. Methods Antagonists of IL-1 and TNFalpha, namely, IL-1 receptor antagonist and the PEGylated soluble TNFalpha receptor I, respectively, were added at different concentrations to explant cultures of nonarthritic (5 obtained at autopsy) and OA (15 obtained at arthroplasty) articular cartilage. The cleavage of type II collagen (CII) by collagenase was measured by an immunoassay in cartilage and culture media. Proteoglycan (mainly aggrecan) content and degradation were measured by a colorimetric assay for glycosaminoglycan (GAG) content in cartilage and culture media. Reverse transcriptase-polymerase chain reaction was used to analyze gene expression of matrix metalloproteases (MMPs) 1, 3, and 13, CII, aggrecan, IL-1, and TNFalpha. Results Antagonists of IL-1 and TNFalpha inhibited the increase in CII cleavage by collagenase as well as the increase in GAG release observed in OA cartilage compared with normal cartilage. Inhibition was significant in tissue from some patients but not from others, although significant inhibition was observed when all the results were analyzed together. An increase in the GAG content in cartilage was seen in 4 of 15 cases. However, this increase was not significant when all the data were combined. Preliminary results indicated no effect of these antagonists on nonarthritic cartilage from 3 different donors. Independent analyses of gene expression in cultured cartilage from 9 other OA patients revealed that IL-1 or TNFalpha blockade, either alone and/or in combination, frequently down-regulated MMP-1, MMP-3, and MMP-13 expression. Expression of IL-1 and TNFalpha was inhibited by either antagonist or by the combination in essentially half the cases. The combined blockade up-regulated aggrecan and CII gene expression in approximately half the cases. Conclusion These results suggest that the autocrine/paracrine activities of TNFalpha and IL-1 in articular cartilage may play important roles in cartilage matrix degradation in OA patients but not in all patients. Inhibition of either or both of these cytokines may offer a useful therapeutic approach to the management of OA by reducing gene expression of MMPs involved in cartilage matrix degradation and favoring its repair.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.20776

Role of interleukin-1 and tumor necrosis factor alpha in matrix degradation of human osteoarthritic cartilage.

The present invention concerns fusion of Fc domains with biologically active peptides and a process for preparing pharmaceutical agents using biologically active peptides. In this invention, pharmacologically active compounds are prepared by a process comprising: a) selecting at least one peptide that modulates the activity of a protein of interest; and b) preparing a pharmacologic agent comprising an Fc domain covalently linked to at least one amino acid of the selected peptide. Linkage to the vehicle increases the half-life of the peptide, which otherwise would be quickly degraded in vivo. The preferred vehicle is an Fc domain. The peptide is preferably selected by phage display, E. coli display, ribosome display, RNA-peptide screening, or chemical-peptide screening.

Modified peptides as therapeutic agents

Objective

To investigate the efficacy of single and combined blockade of tumor necrosis factor (TNF), interleukin-1 (IL-1), and RANKL pathways on synovial inflammation, bone erosion, and cartilage destruction in a TNF-driven arthritis model.



Methods

Human TNF–transgenic (hTNFtg) mice were treated with anti-TNF (infliximab), IL-1 receptor antagonist (IL-1Ra; anakinra), or osteoprotegerin (OPG; an OPG-Fc fusion protein), either alone or in combinations of 2 agents or all 3 agents. Synovial inflammation, bone erosion, and cartilage damage were evaluated histologically.



Results

Synovial inflammation was inhibited by anti-TNF (−51%), but not by IL-1Ra or OPG monotherapy. The combination of anti-TNF with either IL-1Ra (−91%) or OPG (−81%) was additive and almost completely blocked inflammation. Bone erosion was effectively blocked by anti-TNF (−79%) and OPG (−60%), but not by IL-1Ra monotherapy. The combination of anti-TNF with IL-1Ra, however, completely blocked bone erosion (−98%). Inhibition of bone erosion was accompanied by a reduction of osteoclast numbers in synovial tissue. Cartilage destruction was inhibited by anti-TNF (−43%) and was weakly, but not significantly, inhibited by IL-1Ra, but was not inhibited by OPG monotherapy. The combination of anti-TNF with IL-1Ra was the most effective double combination therapy in preventing cartilage destruction (−80%). In all analyses, the triple combination of anti-TNF, IL-1Ra, and OPG was not superior to the double combination of anti-TNF and IL-1Ra.



Conclusion

Articular changes caused by chronic overexpression of TNF are not completely blockable by monotherapies that target TNF, IL-1, or RANKL. However, combined approaches, especially the combined blockade of TNF and IL-1 and, to a lesser extent, TNF and RANKL, lead to almost complete remission of disease. Differences in abilities to block synovial inflammation, bone erosion, and cartilage destruction further strengthen the rationale for using combined blockade of more than one proinflammatory pathway.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.11487

Single and combined inhibition of tumor necrosis factor, interleukin‐1, and RANKL pathways in tumor necrosis factor–induced arthritis: Effects on synovial inflammation, bone erosion, and cartilage destruction

The present invention concerns fusion of Fc domains with biologically active peptides and a process for preparing pharmaceutical agents using biologically active peptides. In this invention, pharmacologically active compounds are prepared by a process comprising: a) selecting at least one peptide that modulates the activity of a protein of interest; and b) preparing a pharmacologic agent comprising an Fc domain covalently linked to at least one amino acid of the selected peptide. Linkage to the vehicle increases the half-life of the peptide, which otherwise would be quickly degraded in vivo. The preferred vehicle is an Fc domain. The peptide can be selected, for example, by phage display, E.coli display, ribosome display, RNA-peptide screening, yeast-based screening, chemical-peptide screening, rational design, or protein structural analysis.

Ulrich Feige

Papers

Activated T cells regulate bone loss and joint destruction in adjuvant arthritis through osteoprotegerin ligand

Role of interleukin-1 and tumor necrosis factor alpha in matrix degradation of human osteoarthritic cartilage.

Modified peptides as therapeutic agents

Single and combined inhibition of tumor necrosis factor, interleukin‐1, and RANKL pathways in tumor necrosis factor–induced arthritis: Effects on synovial inflammation, bone erosion, and cartilage destruction

Modified peptides, comprising an fc domain, as therapeutic agents