Evaluation of Autoreactive Responses

Loss of tolerance to self-antigens is considered to be one of the initial reasons for the onset of rheumatoid arthritis (RA). Identification of self-antigens and evaluation of autoreactive antibodies can foster understanding of the pathogenesis of the disease and the development of new therapeutics. By detection of responses to a particular self-antigen, such as α-enolase, keratin, fibrinogen, fibronectin, collagen, or vimentin, in patient- and animal model-derived samples, high-affinity T-cell receptor-dependent activation of autoreactive T cells to self-antigens can be elucidated. This chapter introduces a simple method to estimate T-cell-autoreactive responses to type II collagen (CII) in a murine collagen-induced arthritis model. A limiting dilution system is established in order to assess CII-dependent T-cell responses, which are reflected by the level of cytokine release.

Single-Cell Ca2+ Imaging

In rheumatological studies, visualization of Ca2+ dynamics in intact cells as direct experimental evidence of Ca2+-dependent signal pathways is generally used to monitor the function of immune cells at first glance. Ability to monitor Ca2+ signaling in living cells would greatly facilitate advances in the functional dissection of immune cells. In this chapter, we describe a basic technique and methods of data analysis for single-cell real-time Ca2+ monitoring using Fluo-4 labeling, which is a single-wavelength Ca2+ indicator.

Clinical Scoring of Disease Activity in Animal Models

Disease severity in murine arthritis models, such as collagen-induced arthritis (CIA), is commonly assessed by clinical scoring of paw swelling and histological examination of joints. Clinical scoring using a qualitative scoring system of paw inflammation (paw thickness, width, or volume) over time is the standard method used for subjective quantification of arthritis activity. To evaluate paw swelling status, a quantitative method using three-dimensional T2-weighted flash sequence magnetic resonance imaging (MRI) is introduced. The efficacy of a therapeutic approach can be semiologically quantified using a clinical scoring system and an index of paw inflammation in CIA mice.

Collagen-Induced Arthritis Models

Due to the limitations of using patient-derived samples for systemic kinetic studies in rheumatoid arthritis (RA) research, animal models are helpful for further understanding the pathophysiology of RA and seeking potential therapeutic targets or strategies. The collagen-induced arthritis (CIA) model is one of the standard RA models used in preclinical research. The CIA model shares several pathological features with RA, such as breach of tolerance and generation of autoantibodies targeting collagen, synovial inflammatory cell infiltration, synovial hyperplasia, cartilage destruction, and bone erosion. In this chapter, a protocol for the successful induction of CIA in mice is described. In this protocol, CIA is induced by active immunization by inoculation with type II heterologous collagen in Freund's adjuvant in susceptible DBA/1 mice.

Lentiviral Production Platform

Lentiviral-mediated transfection technique is a powerful tool for gene modification in preclinical studies. By using this technique, the desired gene modification can be achieved easily in immune cells, nondividing, and terminally differentiated cells, including hematopoietic stem cells, neurons, and even tumor cells, which other viral vectors cannot do. The main considerations of therapeutic gene delivery using a lentiviral system are the risk of insertional mutagenesis and the immune reaction elicited by infected cells. Although some biosafety concerns need to be addressed before clinical trials in rheumatoid arthritis, the lentiviral system targeting therapeutic targets has been widely used for in vivo gene transfer in animal models. In this chapter, the protocols for production of viral particles and viral concentration are provided. As an alternative utilization, this lentiviral production platform could also be employed to produce a pseudotype severe acute respiratory syndrome-related coronavirus 2 in which the spike glycoprotein of SARS-CoV-2 was incorporated into pseudovirions for viral study.

Preparation of Joint Extracts

Since mice are widely used to establish rheumatoid arthritis models, assessment of the pathogenesis of local arthritis is fundamental. Proteins are the most diverse group of biologically important molecules and are essential for cellular structure and function. The first step in pathogenesis-related protein analysis is joint tissue extraction. Unlike other large rodents, obtaining synovium from model mice is challenging since it is so small and fragile. In this chapter, methods for harvesting synovium through a quadriceps approach and preparing protein extracts are introduced.

Histological Analyses of Arthritic Joints in Collagen-Induced Arthritis Model Mice

Histological analysis is a morphological technique and an effective method for understanding the pathology of rheumatoid arthritis (RA). RA is an inflammatory disease characterized by increased synovial tissue and osteoclasts, angiogenesis, infiltration of inflammatory cells, and pannus formation. These pathologies can be observed in a collagen-induced arthritis model mouse using formaldehyde-fixated paraffin-embedded (FFPE) samples. For the preparation of FFPE samples, the conditions of the fixation and decalcification process significantly affect tissue staining results. Since the lesion sites include bone tissue, a decalcification process is necessary when preparing an FFPE sample. Therefore, selecting an optimal condition for the fixating and decalcifying solution is important. In this chapter, we describe the procedures of preparing paraffin samples, including fixation, decalcification, embedding, and sectioning from the RA model mouse, as well as different staining methods (hematoxylin and eosin, tartrate-resistant acid phosphatase).

Lentiviral-Mediated Systemic RNA Interference In Vivo

The shRNA-encoding lentivirus has been widely used for gene manipulation in preclinical studies. It is a powerful tool for gene transfer and shows promise in its ability to efficiently transduce immune cells and hematopoietic stems cells, which are the initial therapeutic target of autoimmune diseases, and considering that gene manipulation of these cells is usually difficult to achieve using other techniques. In previous chapters, we have described how to produce concentrated shRNA-encoding lentiviral particles. Here, systemic in vivo application of lentivirus, including viral quantification prior to injection, intraperitoneal injection, and quantification of integrated provirus, is introduced.

Calcium-Permeable Channels Cooperation for Rheumatoid Arthritis: Therapeutic Opportunities

Rheumatoid arthritis is a common autoimmune disease that results from the deposition of antibodies–autoantigens in the joints, leading to long-lasting inflammation. The main features of RA include cartilage damage, synovial invasion and flare-ups of intra-articular inflammation, and these pathological processes significantly reduce patients’ quality of life. To date, there is still no drug target that can act in rheumatoid arthritis. Therefore, the search for novel drug targets has become urgent. Due to their unique physicochemical properties, calcium ions play an important role in all cellular activities and the body has evolved a rigorous calcium signaling system. Calcium-permeable channels, as the main operators of calcium signaling, are widely distributed in cell membranes, endoplasmic reticulum membranes and mitochondrial membranes, and mediate the efflux and entry of Ca2+. Over the last century, more and more calcium-permeable channels have been identified in human cells, and the role of this large family of calcium-permeable channels in rheumatoid arthritis has gradually become clear. In this review, we briefly introduce the major calcium-permeable channels involved in the pathogenesis of RA (e.g., acid-sensitive ion channel (ASIC), transient receptor potential (TRP) channel and P2X receptor) and explain the specific roles and mechanisms of these calcium-permeable channels in the pathogenesis of RA, providing more comprehensive ideas and targets for the treatment of RA.

ORAI3 is dispensable for store-operated Ca2+ entry and immune responses by lymphocytes and macrophages

Ca2+ signals regulate the function of many immune cells and promote immune responses to infection, cancer, and autoantigens. Ca2+ influx in immune cells is mediated by store-operated Ca2+ entry (SOCE) that results from the opening of Ca2+ release-activated Ca2+ (CRAC) channels. The CRAC channel is formed by three plasma membrane proteins, ORAI1, ORAI2, and ORAI3. Of these, ORAI1 is the best studied and plays important roles in immune function. By contrast, the physiological role of ORAI3 in immune cells remains elusive. We show here that ORAI3 is expressed in many immune cells including macrophages, B cells, and T cells. To investigate ORAI3 function in immune cells, we generated Orai3-/- mice. The development of lymphoid and myeloid cells in the thymus and bone marrow was normal in Orai3-/- mice, as was the composition of immune cells in secondary lymphoid organs. Deletion of Orai3 did not affect SOCE in B cells and T cells but moderately enhanced SOCE in macrophages. Orai3-deficient macrophages, B cells, and T cells had normal effector functions in vitro. Immune responses in vivo, including humoral immunity (T cell dependent or independent) and antitumor immunity, were normal in Orai3-/- mice. Moreover, Orai3-/- mice showed no differences in susceptibility to septic shock, experimental autoimmune encephalomyelitis, or collagen-induced arthritis. We conclude that despite its expression in myeloid and lymphoid cells, ORAI3 appears to be dispensable or redundant for physiological and pathological immune responses mediated by these cells.

Nucleic Acids for Potential Treatment of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a common systemic inflammatory autoimmune disease that severely affects the life quality of patients. Current therapeutics in clinic mainly focus on alleviating the development of RA or relieving the pain of patients. The emerging biological disease-modifying antirheumatic drugs (DMARDs) require long-term treatment to achieve the expected efficacy. With the development of bionanotechnology, nucleic acids fulfill characters as therapeutics or nanocarriers and can therefore be alternatives to combat RA. This review summarizes the therapeutic RNAs developed through RNA interference (RNAi), nucleic acid aptamers, DNA nanostructures-based drug delivery systems, and nucleic acid vaccines for the applications in RA therapy and diagnosis. Furthermore, prospects of nucleic acids for RA therapy are intensively discussed as well.

RETRACTED: MLL1 promotes migration and invasion of fibroblast-like synoviocytes in rheumatoid arthritis by activating the TRIF/NF-κB signaling pathway via H3K4me3 enrichment in the TLR4 promoter region

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor-in-Chief. A corrigendum for this article was previously published which corrected issues within Figure 1, as detailed here: https://www.sciencedirect.com/science/article/pii/S1567576920337887?via%3Dihub. The journal was subsequently alerted to additional issues, including an associated PubPeer comment concerning the provenance of the flow cytometry data in Figure 1B, as detailed here: https://pubpeer.com/publications/AD39B667B4ACD09C930F532D0BD985; and here https://docs.google.com/spreadsheets/d/1r0MyIYpagBc58BRF9c3luWNlCX8VUvUuPyYYXzxWvgY/edit#gid=262337249. As part of a journal investigation, the editorial team noticed that many of the Western blots contained within the article were pixelated. In addition, the published email address of the corresponding author (zhangyd78@126.com), differed from the version submitted to the journal (weiwu_drww@163.com). The journal asked the authors to provide a detailed explanation to these concerns and the associated raw data. The Authors did not respond to this request. The Editor-in-Chief assessed the case and decided to retract the article.

House dust mite allergens induce interleukin 33 (IL-33) synthesis and release from keratinocytes via ATP-mediated extracellular signaling

In atopic diseases, the epithelium releases cytokines and chemokines that initiate skin inflammation. Atopic dermatitis (AD) is characterized by a disrupted epidermal barrier and is triggered or exacerbated by environmental stimuli such as house dust mite (HDM) allergens. The proinflammatory cytokine interleukin 33 (IL-33) plays an important role in the pathogenesis of AD, but how IL-33 production in keratinocytes is elicited by HDM is unknown. To that end, here we stimulated monolayer-cultured human keratinocytes and human living skin equivalents with Dermatophagoides pteronyssinus HDM extract to investigate its effects on IL-33 production from keratinocytes. The HDM extract induced intracellular expression of IL-33 and modulated its processing and maturation, triggering rapid IL-33 release from keratinocytes. Group 1 HDM allergen but not group 2 HDM allergen elicited IL-33 production. An ATP assay of keratinocyte culture supernatants revealed an acute and transient accumulation of extracellular ATP immediately after the HDM extract stimulation. Using the broad-spectrum P2 antagonist suramin, the specific purinergic receptor P2Y2 (P2RY2) antagonist AR-C118925XX, and P2RY2-specific siRNA, we discovered that the HDM extract-induced IL-33 expression was mainly dependent on extracellular ATP/P2Y2 signaling mediated by transactivation of epidermal growth factor receptor, followed by activation of the ERK kinase signaling pathway. Moreover, HDM extract-induced release of 25-kDa IL-33 from the keratinocytes depended on an extracellular ATP/P2 signaling-mediated intracellular Ca²⁺ increase. Our study demonstrates the new mechanism controlling the induction and maturation of keratinocyte-produced IL-33 by HDM allergens, an innate immune process that might play a role in AD development or severity.

Calcium signalling in T cells

Calcium (Ca²⁺) signalling is of paramount importance to immunity. Regulated increases in cytosolic and organellar Ca²⁺ concentrations in lymphocytes control complex and crucial effector functions such as metabolism, proliferation, differentiation, antibody and cytokine secretion and cytotoxicity. Altered Ca²⁺ regulation in lymphocytes leads to various autoimmune, inflammatory and immunodeficiency syndromes. Several types of plasma membrane and organellar Ca²⁺-permeable channels are functional in T cells. They contribute highly localized spatial and temporal Ca²⁺ microdomains that are required for achieving functional specificity. While the mechanistic details of these Ca²⁺ microdomains are only beginning to emerge, it is evident that through crosstalk, synergy and feedback mechanisms, they fine-tune T cell signalling to match complex immune responses. In this article, we review the expression and function of various Ca²⁺-permeable channels in the plasma membrane, endoplasmic reticulum, mitochondria and endolysosomes of T cells and their role in shaping immunity and the pathogenesis of immune-mediated diseases.

The roles of Orai and Stim in bone health and disease

Orai and Stim proteins are the mediators of calcium release-activated calcium signaling and are important in the regulation of bone homeostasis and disease. This includes separate regulatory systems controlling mesenchymal stem cell differentiation to form osteoblasts, which make bone, and differentiation and regulation of osteoclasts, which resorb bone. These systems will be described separately, and their integration and relation to other systems, including Orai and Stim in teeth, will be briefly discussed at the end of this review.

Genetic Manipulation of Calcium Release-Activated Calcium Channel 1 Modulates the Multipotency of Human Cartilage-Derived Mesenchymal Stem Cells

Calcium is a ubiquitous intracellular messenger that has a crucial role in determining the proliferation, differentiation, and functions of multipotent mesenchymal stem cells (MSCs). Our study is aimed at elucidating the influence of genetically manipulating Ca²⁺ release-activated Ca²⁺ (CRAC) channel-mediated intercellular Ca²⁺ signaling on the multipotency of MSCs. The abilities of genetically engineered MSCs, including CRAC-overexpressing and CRAC-knockout MSCs, to differentiate into multiple mesenchymal lineages, including adipogenic, osteogenic, and chondrogenic lineages, were evaluated. CRAC channel-mediated Ca²⁺ influx into these cells was regulated, and the differentiation fate of MSCs was modified. Upregulation of intracellular Ca²⁺ signals attenuated the adipogenic differentiation ability and slightly increased the osteogenic differentiation potency of MSCs, whereas downregulation of CRACM1 expression promoted chondrogenic differentiation potency. The findings demonstrated the effects of genetically manipulating MSCs by targeting CRACM1. CRAC-modified MSCs had distinct differentiation fates to adipocytes, osteoblasts, and chondrocytes. To aid in the clinical implementation of tissue engineering strategies for joint regeneration, these data may allow us to identify prospective factors for effective treatments and could maximize the therapeutic potential of MSC-based transplantation.

Evaluation of Autoreactive Responses

Loss of tolerance to self-antigens is considered to be one of the initial reasons for the onset of rheumatoid arthritis (RA). Identification of self-antigens and evaluation of autoreactive antibodies can foster understanding of the pathogenesis of the disease and the development of new therapeutics. By detection of responses to a particular self-antigen, such as α-enolase, keratin, fibrinogen, fibronectin, collagen, or vimentin, in patient- and animal model-derived samples, high-affinity T-cell receptor-dependent activation of autoreactive T cells to self-antigens can be elucidated. This chapter introduces a simple method to estimate T-cell autoreactive responses to CII in a murine CIA model. A limiting dilution system is established in order to assess CII-dependent T-cell responses, which are reflected by the level of cytokine release.

Clinical Scoring of Disease Activity in Animal Models

Disease severity in murine arthritis models, such as collagen-induced arthritis (CIA), is commonly assessed by clinical scoring of paw swelling and histological examination of joints. Clinical scoring using a qualitative scoring system of paw inflammation (paw thickness, width, or volume) over time is the standard method used for subjective quantification of arthritis activity. To evaluate paw swelling status, a quantitative method using three-dimensional T2-weighted flash sequence magnetic resonance imaging (MRI) is introduced. The efficacy of a therapeutic approach can be semiologically quantified using a clinical scoring system and an index of paw inflammation in CIA mice.

Single-Cell Ca2+ Imaging

In rheumatological studies, visualization of Ca²⁺ dynamics in intact cells as direct experimental evidence of Ca²⁺-dependent signal pathways is generally used to monitor the function of immune cells at first glance. Ability to monitor Ca²⁺ signaling in living cells would greatly facilitate advances in the functional dissection of immune cells. In this chapter, we describe a basic technique and methods of data analysis for single-cell real-time Ca²⁺ monitoring using Fluo-4 labeling, which is a single-wavelength Ca²⁺ indicator.

Efficacy of constant long-term delivery of YM-58483 for the treatment of rheumatoid arthritis

The aim of this study was to investigate the efficacy and safety of YM-58483, a small molecular antagonist of Ca²⁺ release-activated Ca²⁺ (CRAC) channels, for the treatment of rheumatoid arthritis (RA), in vivo and ex vivo. YM-58483 was continuously injected subcutaneously in a collagen-induced arthritis (CIA) mouS.E.M.odel using an implanted osmotic pump. The severity of CIA was evaluated using the following parameters: body weight, hind paw volume, clinical score, histological analysis, cytokine levels, Ca²⁺ influx, and specific IgG production. The efficacy of long-term application of YM-58483 was also verified ex vivo in RA patient-derived peripheral blood monocytes. Assessment of the clinical severity of CIA, cytokine profile in serum and joint protein extracts, and specific IgG production showed that continuous application of YM-58483 suppressed synovial inflammation by inhibiting immune cell activity. Chemical screening and hepatography indicated that long-term subcutaneous delivery of YM-58483 was safer than oral administration for systemic application. Moreover, constant preincubation with YM-58483 at an IC₅₀ of 0.1-1 nM altered proinflammatory cytokine production ex vivo in peripheral T cells derived from RA patients. Our findings suggest that continuous long-term application of appropriate CRAC inhibitors such as YM-58483 is a potential therapeutic strategy for global immunosuppression in RA.

Lentiviral-Mediated Systemic RNA Interference In Vivo

The shRNA-encoding lentivirus has been widely used for gene manipulation in preclinical studies. It is a powerful tool for gene transfer and shows promise in its ability to efficiently transduce immune cells and hematopoietic stem cells, which are the initial therapeutic target of autoimmune diseases, considering that gene manipulation of these cells is usually difficult to achieve using other techniques. In previous chapters, we have described how to produce concentrated shRNA-encoding lentiviral particles. Here, systemic in vivo application of lentivirus, including viral quantification prior to injection, intraperitoneal injection, and quantification of integrated provirus, is introduced.

Production of Lentiviral Particles

Lentiviral-mediated transfection technique is a powerful tool for gene modification in preclinical studies. By using this technique, the desired gene modification can be achieved easily in immune cells, nondividing and terminally differentiated cells, including hematopoietic stem cells, neurons, and even tumor cells, which other viral vectors cannot do. The main considerations of therapeutic gene delivery using a lentiviral system are the risk of insertional mutagenesis and the immune reaction elicited by infected cells. Although some biosafety concerns need to be addressed before clinical trials in rheumatoid arthritis, the lentiviral system targeting therapeutic targets has been widely used for in vivo gene transfer in animal models. In this chapter, the protocols for production of viral particles and viral concentration are provided.

Histological Analysis of Arthritic Joints

Histological analysis is a morphological technique and an effective method for understanding the pathology of rheumatoid arthritis (RA). Here, we describe the processes of paraffin samples, including fixation, decalcifying, embedding, sectioning, and staining (hematoxylin and eosin, tartrate-resistant acid phosphatase, and immunohistochemistry) for an RA model mouse.

Collagen-Induced Arthritis Models

Due to limitations of using patient-derived samples for systemic kinetic studies in rheumatoid arthritis (RA) research, animal models are helpful for further understanding the pathophysiology of RA and seeking potential therapeutic targets or strategies. The collagen-induced arthritis (CIA) model is one of the standard RA models used in preclinical research. The CIA model shares several pathological features with RA, such as breach of tolerance and generation of autoantibodies targeting collagen, synovial inflammatory cell infiltration, synovial hyperplasia, cartilage destruction, and bone erosion. In this chapter, a protocol for successful induction of CIA in mice is described. In this protocol, CIA is induced by active immunization by inoculation with type II heterologous collagen in Freund's adjuvant in susceptible DBA/1 mice.

Efficiency and Safety of CRAC Inhibitors in Human Rheumatoid Arthritis Xenograft Models

Store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channels are involved in the pathogenesis of rheumatoid arthritis (RA) and have been studied as therapeutic targets in the management of RA. We investigated the efficacy and safety of CRAC inhibitors, including a neutralizing Ab (hCRACM1-IgG) and YM-58483, in the treatment of RA. Patient-derived T cell and B cell activity was suppressed by hCRACM1-IgG as well as YM-58483. Systemically constant, s.c. infused CRAC inhibitors showed anti-inflammatory activity in a human-NOD/SCID xenograft RA model as well as protective effects against the destruction of cartilage and bone. hCRACM1-IgG appeared to be safe for systemic application, whereas YM-58483 showed hepatic and renal toxicity in xenograft mice. Treatment with both CRAC inhibitors also caused hyperglycemia in xenograft mice. These results indicate the potential of hCRACM1-IgG and YM-58483 as anti-immunological agents for the treatment of RA. However, some safety issues should be addressed and application methods should be optimized prior to their clinical use.

Enhancement of CCL2 expression and monocyte migration by CCN1 in osteoblasts through inhibiting miR-518a-5p: implication of rheumatoid arthritis therapy

Cysteine-rich 61 (Cyr61 or CCN1), a secreted protein from the CCN family, is an important proinflammatory cytokine. Migration and infiltration of mononuclear cells to inflammatory sites play a critical role in the pathogenesis of rheumatoid arthritis (RA). Monocyte chemoattractant protein-1 (MCP-1/CCL2) is the key chemokine that regulates migration and infiltration of monocytes. Here, we examined the role of CCN1 in monocyte migration, and CCL2 expression in osteoblasts. We found higher levels of CCN1 and CCL2 in synovial fluid from RA patients compared with levels from non-RA controls. We also found that the CCN1-induced increase in CCL2 expression is mediated by the MAPK signaling pathway and that miR-518a-5p expression was negatively regulated by CCN1 via the MAPK cascade. In contrast, inhibition of CCN1 expression with lentiviral vectors expressing short hairpin RNA ameliorated articular swelling, cartilage erosion, and infiltration of monocytes in the ankle joints of mice with collagen-induced arthritis. Our study describes how CCN1 promotes monocyte migration by upregulating CCL2 expression in osteoblasts in RA disease. CCN1 could serve as a potential target for RA treatment.

CRACM3 regulates the stability of non-excitable exocytotic vesicle fusion pores in a Ca(2+)-independent manner via molecular interaction with syntaxin4

Ca(2+) release-activated calcium channel 3 (CRACM3) is a unique member of the CRAC family of Ca(2+)-selective channels. In a non-excitable exocytosis model, we found that the extracellular L3 domain and the cytoplasmic C-terminus of CRACM3 interacted in an activity-dependent manner with the N-peptide of syntaxin4, a soluble N-ethylmaleimide-sensitive factor attachment receptor protein. Our biochemical, electrophysiological and single-vesicle studies showed that knockdown of CRACM3 suppressed functional exocytosis by decreasing the open time of the vesicle fusion pore without affecting Ca(2+) influx, the activity-dependent membrane capacitance (Cm) change, and the total number of fusion events. Conversely, overexpressing CRACM3 significantly impaired cell exocytosis independent of Ca(2+), led to an impaired Cm change, decreased the number of fusion events, and prolonged the dwell time of the fusion pore. CRACM3 changes the stability of the vesicle fusion pore in a manner consistent with the altered molecular expression. Our findings imply that CRACM3 plays a greater role in exocytosis than simply acting as a compensatory subunit of a Ca(2+) channel.

Calcium channels: the potential therapeutic targets for inflammatory bone destruction of rheumatoid arthritis

INTRODUCTION

Inflammatory bone resorption causes progressive joint destruction which ultimately leads to functional disability in rheumatoid arthritis (RA). The primary cell responsible for bone resorption is the osteoclast, which means it is a potential therapeutic target against bone destruction. In fact, experimental and clinical findings suggest that blockade of osteoclast differentiation and function is highly effective in inhibiting bone destruction in RA.

DISCUSSION AND CONCLUSION

In this report, we show several lines of experimental evidence which suggest that a variety of Ca(2+) channels are essential in osteoclast differentiation and function, and present a hypothesis that modulation of Ca(2+) channels is a highly effective therapeutic strategy in preventing osteoclast-induced structural damage in RA.