Inhibition of acid-sensing ion channels in articular chondrocytes by amiloride attenuates articular cartilage destruction in rats with adjuvant arthritis

Acid-sensing ion channel 1a modulation of apoptosis in acidosis-related diseases: implications for therapeutic intervention

Acid-sensing ion channel 1a (ASIC1a), a prominent member of the acid-sensing ion channel (ASIC) superfamily activated by extracellular protons, is ubiquitously expressed throughout the human body, including the nervous system and peripheral tissues. Excessive accumulation of Ca2+ ions via ASIC1a activation may occur in the acidified microenvironment of blood or local tissues. ASIC1a-mediated Ca2+‑induced apoptosis has been implicated in numerous pathologies, including neurological disorders, cancer, and rheumatoid arthritis. This review summarizes the role of ASIC1a in the modulation of apoptosis via various signaling pathways across different disease states to provide insights for future studies on the underlying mechanisms and development of therapeutic strategies.

Regulation of pain neurotransmitters and chondrocytes metabolism mediated by voltage-gated ion channels: A narrative review

Osteoarthritis (OA) is one of the leading causes of chronic pain and dysfunction. It is essential to comprehend the nature of pain and cartilage degeneration and its influencing factors on OA treatment. Voltage-gated ion channels (VGICs) are essential in chondrocytes and extracellular matrix (ECM) metabolism and regulate the pain neurotransmitters between the cartilage and the central nervous system. This narrative review focused primarily on the effects of VGICs regulating pain neurotransmitters and chondrocytes metabolism, and most studies have focused on voltage-sensitive calcium channels (VSCCs), voltage-gated sodium channels (VGSCs), acid-sensing ion channels (ASICs), voltage-gated potassium channels (VGKCs), voltage-gated chloride channels (VGCCs). Various ion channels coordinate to maintain the intracellular environment's homeostasis and jointly regulate metabolic and pain under normal circumstances. In the OA model, the ion channel transport of chondrocytes is abnormal, and calcium influx is increased, which leads to increased neuronal excitability. The changes in ion channels are strongly associated with the OA disease process and individual OA risk factors. Future studies should explore how VGICs affect the metabolism of chondrocytes and their surrounding tissues, which will help clinicians and pharmacists to develop more effective targeted drugs to alleviate the progression of OA disease.

Modulators of ASIC1a and its potential as a therapeutic target for age-related diseases

Age-related diseases have become more common with the advancing age of the worldwide population. Such diseases involve multiple organs, with tissue degeneration and cellular apoptosis. To date, there is a general lack of effective drugs for treatment of most age-related diseases and there is therefore an urgent need to identify novel drug targets for improved treatment. Acid-sensing ion channel 1a (ASIC1a) is a degenerin/epithelial sodium channel family member, which is activated in an acidic environment to regulate pathophysiological processes such as acidosis, inflammation, hypoxia, and ischemia. A large body of evidence suggests that ASIC1a plays an important role in the development of age-related diseases (e.g., stroke, rheumatoid arthritis, Huntington's disease, and Parkinson's disease.). Herein we present: 1) a review of ASIC1a channel properties, distribution, and physiological function; 2) a summary of the pharmacological properties of ASIC1a; 3) and a consideration of ASIC1a as a potential therapeutic target for treatment of age-related disease.

Estrogen antagonizes ASIC1a-induced chondrocyte mitochondrial stress in rheumatoid arthritis

BACKGROUND

Destruction of articular cartilage and bone is the main cause of joint dysfunction in rheumatoid arthritis (RA). Acid-sensing ion channel 1a (ASIC1a) is a key molecule that mediates the destruction of RA articular cartilage. Estrogen has been proven to have a protective effect against articular cartilage damage, however, the underlying mechanisms remain unclear.

METHODS

We treated rat articular chondrocytes with an acidic environment, analyzed the expression levels of mitochondrial stress protein HSP10, ClpP, LONP1 by q-PCR and immunofluorescence staining. Transmission electron microscopy was used to analyze the mitochondrial morphological changes. Laser confocal microscopy was used to analyze the Ca²⁺, mitochondrial membrane potential (Δψm) and reactive oxygen species (ROS) level. Moreover, ASIC1a specific inhibitor Psalmotoxin 1 (Pctx-1) and Ethylene Glycol Tetraacetic Acid (EGTA) were used to observe whether acid stimulation damage mitochondrial function through Ca²⁺ influx mediated by ASIC1a and whether pretreatment with estrogen could counteract these phenomena. Furthermore, the ovariectomized (OVX) adjuvant arthritis (AA) rat model was treated with estrogen to explore the effect of estrogen on disease progression.

RESULTS

Our results indicated that HSP10, ClpP, LONP1 protein and mRNA expression and mitochondrial ROS level were elevated in acid-stimulated chondrocytes. Moreover, acid stimulation decreased mitochondrial membrane potential and damaged mitochondrial structure of chondrocytes. Furthermore, ASIC1a specific inhibitor PcTx-1 and EGTA inhibited acid-induced mitochondrial abnormalities. In addition, estrogen could protect acid-stimulated induced mitochondrial stress by regulating the activity of ASIC1a in rat chondrocytes and protects cartilage damage in OVX AA rat.

CONCLUSIONS

Extracellular acidification induces mitochondrial stress by activating ASIC1a, leading to the damage of rat articular chondrocytes. Estrogen antagonizes acidosis-induced joint damage by inhibiting ASIC1a activity. Our study provides new insights into the protective effect and mechanism of action of estrogen in RA.

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.

Nesfatin-1 exerts protective effects on acidosis-stimulated chondrocytes and rats with adjuvant-induced arthritis by inhibiting ASIC1a expression

Nesfatin-1, a newly identified energy-regulating peptide, has been reported to possess antioxidant, anti-inflammatory, and antiapoptotic properties; however, to date, its effect on rheumatoid arthritis (RA) has not been previously explored in detail. We previously showed that activation of acid-sensing ion channel 1a (ASIC1a) by acidosis plays an important role in RA pathogenesis. Therefore, in this study, we evaluated the effects of nesfatin-1 on acidosis-stimulated chondrocyte injury in vitro and in vivo and examined the involvement of ASIC1a and the mechanism underlying the effects of nesfatin-1 on RA. Acid-stimulated articular chondrocytes were used to examine one of the several possible mechanisms underlying RA pathogenesis in vitro. The mRNA expression profile of acid-induced chondrocytes treated or not treated with nesfatin-1 was investigated by RNA sequencing. The effects of nesfatin-1 on oxidative stress, inflammation, and apoptosis in acid-induced chondrocytes were measured. The mechanistic effect of nesfatin-1 on ASIC1a expression and intracellular Ca²⁺ in acid-stimulated chondrocytes was studied. Rats with adjuvant-induced arthritis (AA) were used for in vivo analysis of RA pathophysiology. Cartilage degradation and ASIC1a expression in chondrocytes were detected in rats with AA after intraarticular nesfatin-1 injection. The in vitro experiments showed that nesfatin-1 decreased acidosis-induced cytotoxicity and elevation of intracellular Ca²⁺ levels in chondrocytes. Moreover, it attenuated acid-induced oxidative stress, inflammation, and apoptosis in chondrocytes. Nesfatin-1 decreased ASIC1a protein levels in acid-stimulated chondrocytes via the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and nuclear factor kappa-B (NF-κB) signaling pathways. In vivo analysis showed that nesfatin-1 ameliorated cartilage degradation and decreased ASIC1a expression in the chondrocytes of rats with AA. Collectively, nesfatin-1 suppressed acidosis-induced oxidative stress, inflammation, and apoptosis in acid-stimulated chondrocytes and alleviated arthritis symptoms in rats with AA, and its mechanism may be related to its ability to decrease ASIC1a protein levels via the MAPK/ERK and NF-κB pathways.

Acid-Sensing Ion Channel 2: Function and Modulation

Acid-sensing ion channels (ASICs) have an important influence on human physiology and pathology. They are members of the degenerin/epithelial sodium channel family. Four genes encode at least six subunits, which combine to form a variety of homotrimers and heterotrimers. Of these, ASIC1a homotrimers and ASIC1a/2 heterotrimers are most widely expressed in the central nervous system (CNS). Investigations into the function of ASIC1a in the CNS have revealed a wealth of information, culminating in multiple contemporary reviews. The lesser-studied ASIC2 subunits are in need of examination. This review will focus on ASIC2 in health and disease, with discussions of its role in modulating ASIC function, synaptic targeting, cardiovascular responses, and pharmacology, while exploring evidence of its influence in pathologies such as ischemic brain injury, multiple sclerosis, epilepsy, migraines, drug addiction, etc. This information substantiates the ASIC2 protein as a potential therapeutic target for various neurological, psychological, and cerebrovascular diseases.

Estrogen protects against acidosis-mediated articular chondrocyte injury by promoting ASIC1a protein degradation

Epidemiological data suggest that the incidence of rheumatoid arthritis (RA) increases in postmenopausal women, which may be related to estrogen deficiency. Tissue acidosis is a common symptom of RA. Acid-sensitive ion channel 1a (ASIC1a), a member of the extracellular H+-activated cation channel family, could be activated by changes in extracellular pH and plays a crucial role in the pathogenesis of RA. As the only cellular component in cartilage tissue, chondrocytes play an extremely important role in maintaining cartilage tissue homeostasis. The aim of this study was to investigate whether estrogen could protect acid-stimulated chondrocytes by regulating the expression of ASIC1a and explore the possible mechanism. The results showed that estrogen could protect against acid-induced chondrocyte injury by reducing ASIC1a protein expression. Moreover, lysosome inhibitor chloroquine (CQ) and autophagy inhibitor 3-methyladeniine (3-MA) could reverse the reduction of ASIC1a protein caused by estrogen, indicating that autophagy-lysosome pathway contributes to estrogen-induced degradation of ASIC1a protein. Furthermore, the down-regulation of ASIC1a expression by estrogen was attenuated by MPP, a specific inhibitor of estrogen-related receptor-alpha (Esrra), indicating that Esrra is involved in the process of estrogen regulating the expression of ASIC1a. Additionally, adenosine 5'-monophosphate (AMP)-activated protein kinase/unc-51-like kinase 1 (AMPK-ULK1) signaling pathway was activated by estrogen treatment, which was abrogated by Esrra-silencing, and AMPK-specific inhibitor Compound C pretreatment could reduce estrogen-induced downregulation of ASIC1a protein. Taken together, these results indicate that estrogen could promote autophagy-lysosome pathway-dependent ASIC1a protein degradation and protect against acidosis-induced cytotoxicity, the mechanisms of which might relate to Esrra-AMPK-ULK1 signaling pathway.

Acid-Sensing Ion Channels: Expression and Function in Resident and Infiltrating Immune Cells in the Central Nervous System

Peripheral and central immune cells are critical for fighting disease, but they can also play a pivotal role in the onset and/or progression of a variety of neurological conditions that affect the central nervous system (CNS). Tissue acidosis is often present in CNS pathologies such as multiple sclerosis, epileptic seizures, and depression, and local pH is also reduced during periods of ischemia following stroke, traumatic brain injury, and spinal cord injury. These pathological increases in extracellular acidity can activate a class of proton-gated channels known as acid-sensing ion channels (ASICs). ASICs have been primarily studied due to their ubiquitous expression throughout the nervous system, but it is less well recognized that they are also found in various types of immune cells. In this review, we explore what is currently known about the expression of ASICs in both peripheral and CNS-resident immune cells, and how channel activation during pathological tissue acidosis may lead to altered immune cell function that in turn modulates inflammatory pathology in the CNS. We identify gaps in the literature where ASICs and immune cell function has not been characterized, such as neurotrauma. Knowledge of the contribution of ASICs to immune cell function in neuropathology will be critical for determining whether the therapeutic benefits of ASIC inhibition might be due in part to an effect on immune cells.

Hypertension meets osteoarthritis - revisiting the vascular aetiology hypothesis

Osteoarthritis (OA) is a whole-joint disease characterized by subchondral bone perfusion abnormalities and neovascular invasion into the synovium and articular cartilage. In addition to local vascular disturbance, mounting evidence suggests a pivotal role for systemic vascular pathology in the aetiology of OA. This Review outlines the current understanding of the close relationship between high blood pressure (hypertension) and OA at the crossroads of epidemiology and molecular biology. As one of the most common comorbidities in patients with OA, hypertension can disrupt joint homeostasis both biophysically and biochemically. High blood pressure can increase intraosseous pressure and cause hypoxia, which in turn triggers subchondral bone and osteochondral junction remodelling. Furthermore, systemic activation of the renin-angiotensin and endothelin systems can affect the Wnt-β-catenin signalling pathway locally to govern joint disease. The intimate relationship between hypertension and OA indicates that endothelium-targeted strategies, including re-purposed FDA-approved antihypertensive drugs, could be useful in the treatment of OA.

Estrogen Protects Articular Cartilage by Downregulating ASIC1a in Rheumatoid Arthritis

Purpose

The severity of rheumatoid arthritis (RA) in women is generally lower than that in men. RA is mediated, at least in part, by the protective effects of estradiol. However, the mechanisms underlying the protective effect of estradiol on RA are still unclear. Recent studies have demonstrated that activation of acid-sensing ion channel 1a (ASIC1a) by tissue acidosis plays an important role in the injury of cartilage in RA. Here, we assessed the effects of estradiol on acid-mediated cartilage injury both in vitro and in vivo and explored the involvement of ASIC1a in RA and its underlying mechanism.

Methods

Cultured primary articular chondrocytes were subjected to acidosis-mediated injury in vitro. Beclin1, LC3, p62, GPER1, and ASIC1a expression was detected through Western blotting, quantitative real-time PCR, and immunofluorescence analysis. Adjuvant arthritis (AA) was induced in rats through intradermal immunization by injecting 0.25 mL heat-killed mycobacteria (10 mg/mL) suspended in complete Freund’s adjuvant into the left hind metatarsal footpad. The levels of estrogen and related inflammatory factors in the serum were measured using enzyme-linked immunosorbent assay. The expression of ASIC1a and autophagy-related proteins was detected through immunohistochemical analysis and Western blot.

Results

Treatment of primary articular chondrocytes with estradiol decreased the expression of ASIC1a and autophagy level. The symptoms of cartilage damage and levels of inflammatory cytokines in the serum were reduced after estradiol treatment in the rats with AA. In addition, estradiol treatment reduced ASIC1a expression via the PI3K-AKT-mTOR pathway, among which G-protein coupled estradiol receptor 1 (GPER1) plays a regulatory role. Finally, the level of autophagy in chondrocytes was decreased by the selective ASIC1a blocker psalmotoxin-1 (PCTX-1).

Conclusion

Estradiol can protect the cartilage of rats with AA against acidosis-mediated damage and autophagy by suppressing ASIC1a expression through GPER1.

Acid-Sensing Ion Channel-1a in Articular Chondrocytes and Synovial Fibroblasts: A Novel Therapeutic Target for Rheumatoid Arthritis

Acid-sensing ion channel 1a (ASIC1a) is a member of the extracellular H⁺-activated cation channel family. Emerging evidence has suggested that ASIC1a plays a crucial role in the pathogenesis of rheumatoid arthritis (RA). Specifically, ASIC1a could promote inflammation, synovial hyperplasia, articular cartilage, and bone destruction; these lead to the progression of RA, a chronic autoimmune disease characterized by chronic synovial inflammation and extra-articular lesions. In this review, we provided a brief overview of the molecular properties of ASIC1a, including the basic biological characteristics, tissue and cell distribution, channel blocker, and factors influencing the expression and function, and focused on the potential therapeutic targets of ASIC1a in RA and possible mechanisms of blocking ASIC1a to improve RA symptoms, such as regulation of apoptosis, autophagy, pyroptosis, and necroptosis of articular cartilage, and synovial inflammation and invasion of fibroblast-like cells in synovial tissue.

Acid-sensing ion channel 1a mediates acid-induced pyroptosis through calpain-2/calcineurin pathway in rat articular chondrocytes

The pyroptosis is a causative agent of rheumatoid arthritis, a systemic autoimmune disease merged with degenerative articular cartilage. Nevertheless, the precise mechanism of extracellular acidosis on chondrocyte pyroptosis is largely unclear. Acid-sensing ion channels (ASICs) belong to an extracellular H⁺ -activated cation channel family. Accumulating evidence has highlighted activation of ASICs induced by extracellular acidosis upregulate calpain and calcineurin expression in arthritis. In the present study, to investigate the expression and the role of acid-sensing ion channel 1a (ASIC1a), calpain, calcineurin, and NLRP3 inflammasome proteins in regulating acid-induced articular chondrocyte pyroptosis, primary rat articular chondrocytes were subjected to different pH, different time, and different treatments with or without ASIC1a, calpain-2, and calcineurin, respectively. Initially, the research results showed that extracellular acidosis-induced the protein expression of ASIC1a in a pH- and time-dependent manner, and the messenger RNA and protein expressions of calpain, calcineurin, NLRP3, apoptosis-associated speck-like protein, and caspase-1 were significantly increased in a time-dependent manner. Furthermore, the inhibition of ASIC1a, calpain-2, or calcineurin, respectively, could decrease the cell death accompanied with the decreased interleukin-1β level, and the decreased expression of ASIC1a, calpain-2, calcineurin, and NLRP3 inflammasome proteins. Taken together, these results indicated the activation of ASIC1a induced by extracellular acidosis could trigger pyroptosis of rat articular chondrocytes, the mechanism of which might partly be involved with the activation of calpain-2/calcineurin pathway.

Acid-sensing ion channels mediate the degeneration of intervertebral disc via various pathways-A systematic review

To elucidate the pathological significance of acid-sensing ion channels (ASICs) in intervertebral disc degeneration (IVDD), the database of Medline, Web of Science, and EmBase were carefully screened. Search terms used in each database varied slightly to optimize results. Data relating to the correlation between ASICs and IVDD was systematically collected and integrated into the review. 11 basic science studies, containing the related information, were finally identified for inclusion. Intervertebral disc degeneration (IVDD) is a common disease in middle-aged and elderly people, which has a great impact on patients’ quality of life. Many research teams have attempted to elucidate the pathogenesis of this degenerative disease, and have made considerable progress. Acid-sensing ion channels (ASICs) were once reported to be able to regulate the apoptosis process of chondrocytes in joint cartilage, which has been transplanted into the IVDD-related research. ASIC1a functions as the mediator for cells in nucleus pulposus (NP) and endplate (EP), with whose activation the apoptosis process would be accelerated. Moreover, ASIC1a’s activation could also regulate the anabolism in chondrocytes of EP, facilitating the degeneration. ASIC3 would only promote the degeneration in NP, possibly via its pro-inflammatory effect. The distribution of ASICs in NP, EP, annulus fibrosus, and the particular functions of ASIC1a and ASIC3 remind us about the pathological significance of ASICs in IVDD, which could be a promising therapeutic target in future treatment for IVDD.

17β-estradiol attenuates rat articular chondrocyte injury by targeting ASIC1a-mediated apoptosis

Epidemiological evidence suggests that the etiology and pathogenesis of rheumatoid arthritis (RA) are closely associated with estrogen metabolism and deficiency. Estrogen protects against articular damage. Estradiol replacement therapy ameliorates local inflammation and knee joint swelling in ovariectomized models of RA. The mechanistic basis for the protective role of 17β-estradiol (17β-E2) is poorly understood. Acid-sensing ion channel 1a (ASIC1a), a sodium-permeable channel, plays a pivotal role in acid-induced articular chondrocyte injury. The aims of this study were to evaluate the role of 17β-E2 in acid-induced chondrocyte injury and to determine the effect of 17β-E2 on the level and activity of ASIC1a protein. Results showed that pretreatment with 17β-E2 attenuated acid-induced damage, suppressed apoptosis, and restored mitochondrial function. Further, 17β-E2 was shown to reduce protein levels of ASIC1a through the ERα receptor, to protect chondrocytes from acid-induced apoptosis, and to induce ASIC1a protein degradation through the autophagy-lysosomal pathway. Taken together, these results show that the use of 17β-E2 may be a novel strategy for the treatment of RA by reducing cartilage destruction through down-regulation of ASIC1a protein levels.

Nerve growth factor promotes ASIC1a expression via the NF-κB pathway and enhances acid-induced chondrocyte apoptosis

Nerve growth factor (NGF) is a neurotrophic factor that is thought to have a broad role in the nervous system and tumors, and has recently been described as a mediator of inflammation. It is not clear whether or not NGF participates in apoptosis of articular chondrocytes. In this study, we determined if NGF affects ASIC1a expression and NF-κB P65 activation in rat chondrocytes, and measured the effectiveness of NGF on apoptotic protein expression in acid-induced chondrocytes. NGF was shown to up-regulate the level of ASIC1a in a dose- and time-dependent fashion. Simultaneously, NGF activated NF-κB P65 in chondrocytes. Additionally, the elevated ASIC1a expression induced by NGF was eliminated by the NF-κB inhibitor (PDTC) in chondrocytes. Moreover, NGF reduced cell viability and induced LDH release under the premise of acid-induced articular chondrocytes. Furthermore, NGF could enhance cleaved-caspase 9 and cleaved-PARP expression in acid-pretreated chondrocytes, and which could be inhibited by using psalmotoxin 1(PcTX1) or PDTC. Together, these results indicated that NGF may up-regulate ASIC1a expression through the NF-κB signaling pathway, and further promote acid-induced apoptosis of chondrocytes.

High Expression of Acid-Sensing Ion Channel 2 (ASIC2) in Bone Cells in Osteoporotic Vertebral Fractures

Little is known about the function of acid-sensing ion channels (ASICs) in bone cells or osteoporotic vertebral fractures (OVF). This study delineated ASICs expression in adult human bone marrow-mesenchymal stem cells- (BM-MSC-) derived osteoblasts and in OVF bone cells. Adult BM-MSC-derived osteoblasts were isolated and cultured in different pH values. Osteogenic markers as alkaline phosphatase (ALP), osteopontin (OPN), and osteocalcin (OC) mRNA were assessed. Western blots method was applied to analyze ASICs protein expression in different pH values. Amiloride was added into the osteogenic media to analyze the Na⁺/K⁺ ATPase change. We harvested the vertebral cancellous bone through a bone biopsy needle in 26 OVF patients when performing percutaneous vertebroplasty. Six vertebral bone specimens obtained from 4 patients with high-energy vertebral fractures were used as the control. The reverse transcription polymerase chain reaction was performed to analyze the quantitative mRNA expression of ASICs. Osteogenic markers as ALP, OPN, and OC mRNA were higher expressed in increasing pH values throughout osteoblastogenesis. ASIC proteins were higher expressed in lower pH media, especially ASIC3, and ASIC4. The highest protein expression at days 7, 14, and 21 was ASIC2, ASIC4, and ASIC3, respectively. Expression of Na⁺/K⁺ ATPase was significantly decreased in cultured osteoblasts by addition of amiloride into the pH 6.9 osteogenic media. ASIC2 mRNA was most highly expressed with a 65.93-fold increase in the biopsied vertebral bone cells in OVF compared with the control. In conclusion, we found osteoblastogenesis was reduced in an acidic environment, and ASIC2, ASIC3, and ASIC4 were most highly expressed in turn during osteoblastogenesis within acidic media. ASIC2 was the most abundantly expressed gene in human bone cells in OVF compared with the control. ASIC2 could be crucial in the pathogenesis of osteoporosis and could serve as a therapeutic target for antiosteoporotic therapies.

Simiao Pill Attenuates Collagen-Induced Arthritis in Rats through Suppressing the ATX-LPA and MAPK Signalling Pathways

Objective

Simiao pill (SM), a traditional Chinese formula, has been used as an antirheumatic drug in clinical practice for hundreds of years. Rheumatoid arthritis (RA) is characterized by chronic synovial inflammation and hyperplasia, cartilage destruction, and joint damage. This study was designed to investigate the protective effects of SM on collagen-induced arthritis (CIA) in rats. It also aimed to explore whether this protective effect of SM was related to the inhibition of the ATX-LPA and MAPK signalling pathways.

Materials and Methods

Rats were injected with a collagen II emulsion at the end of the tail and on the back to induce arthritis. Treatment with different doses of SM was conducted by intragastric administration. Then, body weights and arthritis scores were measured. The serum levels of tumour necrosis factor (TNF)-α, interleukin (IL)-1β, C-reactive protein (CRP), osteoprotegerin (OPG), autotaxin (ATX), and lysophosphatidic acid (LPA) were determined by ELISA. Pathological changes in the joints were measured by micro-CT and assessed via haematoxylin-eosin (H&E) staining. The expression of ATX, LPA receptor 1 (LPA1) was detected by immunohistochemical staining, and the expression of mitogen-activated protein kinase (MAPK) was detected by Western blotting.

Results

SM significantly alleviated arthritis symptoms, inhibited bone erosion, and decreased the levels of TNF-α, IL-1β, CRP, ATX, and LPA in the sera of CIA rats. Importantly, SM clearly reduced the protein expression of LPA1 and ATX. The activation of the MAPK signalling pathway was also inhibited by SM in the synovial tissues of CIA rats.

Conclusions

The antirheumatic effects of SM were associated with the regulation of the ATX-LPA and MAPK pathways, the suppression of proinflammatory cytokine production, and the alleviation of cartilage and bone injury. These findings suggest that SM might be a promising alternative candidate for RA therapy.

Inhibition of acid‑sensing ion channel 1a attenuates acid‑induced activation of autophagy via a calcium signaling pathway in articular chondrocytes

Acid-sensing ion channel 1a (ASIC1a), member of the degenerin/epithelial sodium channel protein superfamily, serves a critical role in various physiological and pathological processes. The aim of the present study was to examine the role of ASIC1a in the autophagy of rat articular chondrocytes. Autophagy was induced by acidic stimulation in rat articular chondrocytes and the extent of autophagy was evaluated via the expression levels of microtubule-associated protein 1 light chain 3II, Beclin1 and uncoordinated-51 like kinase1. Suppression of ASIC1a was achieved using small interfering RNA technology and/or inhibitor psalmotoxin-1. The expression levels of autophagy markers were measured by western blot analysis and reverse transcription-quantitative polymerase chain reaction methods. Intracellular calcium ([Ca²⁺]_i) was analyzed using a Ca²⁺-imaging method. Additionally, protein expression levels of the Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ)/5′-monophosphate-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) pathway were measured by western blot analysis. The results showed that autophagy was increased in a pH-and time-dependent manner with exposure to an acidic environment. In addition, silencing ASIC1a significantly decreased the expression levels of autophagy makers, accompanied by abrogation of the acid-induced [Ca²⁺]_i increase. Furthermore, silencing of ASIC1a downregulated the levels of CaMKKβ/β-actin and phosphorylated (p-) AMPK/AMPK, and upregulated the levels of p-mTOR/mTOR. These results indicated that ASIC1a is a potent regulator of autophagy in chondrocytes, which may be associated with decreased Ca²⁺ influx and the CaMKKβ/AMPK/mTOR pathway.

Abnormal response of costal chondrocytes to acidosis in patients with chest wall deformity

Costal cartilage is much understudied compared to the load bearing cartilages. Abnormally grown costal cartilages are associated with the inherited chest wall deformities pectus excavatum and pectus carinatum resulting in sunken or pigeon chest respectively. A lack of understanding of the ultrastructural and molecular biology properties of costal cartilage is a major confounder in predicting causes and outcomes of these disorders. Due to the avascular nature of cartilage, chondrocytes metabolize glycolytically, producing an acidic environment. During physical activity hydrogen ions move within cartilage driven by compressive forces, thus at any one time, chondrocytes experience transient changes in pH. A variety of ion channels on chondrocytes plasma membrane equip them to function in the rapidly changing conditions they experience. In this paper we describe reduced expression of the ASIC2 gene encoding the acid sensing ion channel isoform 2 (previously referred to as ACCN1 or ACCN) in patients with chest wall deformities. We hypothesized that chondrocytes from these patients cannot respond normally to changes in pH that are an integral part of the biology of this tissue. Activation of ASICs indirectly creates a cascade ultimately dependent on intracellular calcium transients. The objective of this paper was to compare internal calcium signaling in response to external pH changes in costal chondrocytes from patients with chest wall deformities and healthy individuals. Although the molecular mechanism through which chondrocytes are regulated by acidosis remains unknown, we observed reduced amplitudes of calcium rise in patient chondrocytes exposed to low pH that become further impaired upon repeat exposure.

Necrostatin-1 ameliorates adjuvant arthritis rat articular chondrocyte injury via inhibiting ASIC1a-mediated necroptosis

Necroptosis, a necrotic cell death pathway regulated by receptor interacting protein (RIP) 1 and 3, plays a key role in pathophysiological processes, including rheumatoid arthritis (RA). However, whether necroptosis is involved in RA articular cartilage damage processes remain unclear. The aim of present study was to investigate the dynamic changes in arthritic chondrocyte necroptosis and the effect of RIP1 inhibitor necrostatin-1 (Nec-1) and acid-sensing ion channels (ASICs) inhibitor amiloride on arthritic cartilage injury and acid-induced chondrocyte necroptosis. Our results demonstrated that the expression of RIP1, RIP3 and mixed lineage kinase domain-like protein phosphorylation (p-MLKL) were increased in adjuvant arthritis (AA) rat articular cartilage in vivo and acid-induced chondrocytes in vitro. High co-expression of ASIC1a and RIP1 showed in AA rat articular cartilage. Moreover, Nec-1 and amiloride could reduce articular cartilage damage and necroinflammation in AA rats. In addition, acid-induced increase in necroptosis markers RIP1/RIP3 were inhibited by Nec-1, ASIC1a-specific blocker psalmotoxin-1 (PcTx-1) or ASIC1a-short hairpin RNA respectively, which revealed that necroptosis is triggered in acid-induced chondrocytes and mediated by ASIC1a. These findings indicated that blocking ASIC1a-mediated chondrocyte necroptosis may provide potential therapeutic strategies for RA treatment.

Acetazolamide protects rat articular chondrocytes from IL-1β-induced apoptosis by inhibiting the activation of NF-κB signal pathway

Because the excessive apoptosis of articular chondrocytes contributes to extracellular matrix (ECM) loss and cartilage damage in rheumatoid arthritis (RA), inhibiting chondrocyte apoptosis might be a promising strategy for RA. Aquaporin1 (AQP1) is overexpressed in RA cartilage and synovial tissues, and play a vital pathogenic role in RA development. Particularly, we previously reported that acetazolamide (AZ) as an AQP1 inhibitor suppressed secondary inflammation and promoted ECM production in cartilage of adjuvant-induced arthritis rats. Here, we investigated the antiapoptotic effect of AZ on interleukin-1β (IL-1β)-induced apoptosis, a classic in vitro model of chondrocyte apoptosis. AZ treatment could inhibit IL-1β-induced apoptosis, evidenced by increasing cell viability, relieving apoptotic nuclear morphology, decreasing apoptosis rates, and restoring mitochondrial membrane potential. Additionally, AZ reversed IL-1β-induced decrease of Bcl-2 protein and reduced IL-1β-induced increases of Bax and caspase 3 protein, accompanied by inhibiting IκBα degradation and phosphorylation in cytoplasm, reducing NF-κB p65 protein level in nucleus and preventing NF-κB p65 translocation from cytoplasm to nucleus. In conclusion, our findings indicated that AZ could effectively attenuate IL-1β-induced chondrocyte apoptosis mediated by regulating the protein levels of apoptosis-related genes and inhibiting the activation of NF-κB signal pathway, suggesting that AZ might be of potential clinical interest in RA treatment.

Dual role of acid-sensing ion channels 3 in rheumatoid arthritis: destruction or protection?

Acid-sensing ion channels (ASIC) are voltage-independent cationic channels that open in response to decrease in extracellular pH. Amongst different subtypes, ASIC3 has received much attention in joint inflammatory conditions including rheumatoid arthritis. There have been a number of studies showing that there is an increase in expression of ASIC3 on nerve afferents supplying joints in response to inflammatory stimulus. Accordingly, a number of selective as well as nonselective ASIC3 inhibitors have shown potential in attenuating pain and inflammation in animal models of rheumatoid arthritis. On the other hand, there have been studies showing that ASIC3 may exert protective effects in joint inflammation. ASIC^-/- animals, without ASIC3 genes, exhibit more joint inflammation and destruction in comparison to ASIC^+/+ animals. The present review discusses the dual nature of ASIC3 in joint inflammation with possible mechanisms.

Novel amidrazone derivatives: Design, synthesis and activity evaluation

A series of new 6-styryl-naphthalene-2-amidrazone derivatives were synthesized and evaluated as potential ASIC1a inhibitors. Among them, compound 5e showed the most activity to inhibit [Ca²⁺]_i. elevation in acid-induced articular chondrocytes. Together with the important role of ASIC1a in the pathogenesis of tissue acidification diseases including rheumatoid arthritis, these results might provide a meaningful hint or inspiration in developing drugs targeting at tissue acidification diseases.

Therapeutic effect of acetazolamide, an aquaporin 1 inhibitor, on adjuvant-induced arthritis in rats by inhibiting NF-κB signal pathway

OBJECTIVES

Previous studies have shown that aquaporin 1 (AQP1) is up-regulated in synovium and cartilage of rheumatoid arthritis (RA) patients and that AQP1 may be involved in joint swelling and synovial inflammation. This study was aimed to investigate the potential therapeutic effect of acetazolamide (AZ, an AQP1 inhibitor) on rat adjuvant-induced arthritis (AIA) and explore its related mechanisms.

MATERIALS AND METHODS

Rat AIA was induced by complete Freund's adjuvant. The effect of AZ on rat AIA was evaluated by secondary hind paw swelling, arthritis index, TNF-α and IL-1β serum levels and histological examination of ankle joint. Proteoglycans expression and mRNA levels of type-II collagen (COII) and aggrecan in cartilage were measured by alcian blue staining and real-time PCR, respectively. The protein levels of AQP1, IκBα, phospho-IκBα (p-IκBα), NF-κB p65 and phospho-NF-κB p65 (p-NF-κB p65) in synovial tissues were detected by western blot.

RESULTS

AZ treatment could inhibit secondary hind paw swelling and arthritis index, reduce serum levels of TNF-α and IL-1β, and ameliorate pathological changes of ankle joint in AIA rats. AZ increased proteoglycans production and mRNA levels of COII and aggrecan in cartilage tissues. Moreover, AZ decreased AQP1 protein level and suppressed the activation of NF-κB pathway in synovium, indicated by inhibiting the degradation and phosphorylation of IκBα and reducing p-NF-κB p65 protein level.

CONCLUSIONS

AZ as an AQP1 inhibitor has a powerful therapeutic effect on rat AIA via inhibiting NF-κB activation, suggesting AQP1 inhibition might be of potential clinical interest in RA treatment.

Tackling Pain Associated with Rheumatoid Arthritis: Proton-Sensing Receptors

Rheumatoid arthritis (RA), characterized by chronic inflammation of synovial joints, is often associated with ongoing pain and increased pain sensitivity. Chronic pain that comes with RA turns independent, essentially becoming its own disease. It could partly explain that a significant number (50%) of RA patients fail to respond to current RA therapies that focus mainly on suppression of joint inflammation. The acute phase of pain seems to associate with joint inflammation in early RA. In established RA, the chronic phase of pain could be linked to inflammatory components of neuron-immune interactions and noninflammatory components. Accumulating evidence suggests that the initial inflammation and autoimmunity in RA (preclinical RA) begin outside of the joint and may originate at mucosal sites and alterations in the composition of microbiota located at mucosal sites could be essential for mucosal inflammation, triggering joint inflammation. Fibroblast-like synoviocytes in the inflamed joint respond to cytokines to release acidic components, lowering pH in synovial fluid. Extracellular proton binds to proton-sensing ion channels, and G-protein-coupled receptors in joint nociceptive fibers may contribute to sensory transduction and release of neurotransmitters, leading to pain and hyperalgesia. Activation of peripheral sensory neurons or nociceptors further modulates inflammation, resulting in neuroinflammation or neurogenic inflammation. Peripheral and central nerves work with non-neuronal cells (such as immune cells, glial cells) in concert to contribute to the chronic phase of RA-associated pain. This review will discuss actions of proton-sensing receptors on neurons or non-neuronal cells that modulate RA pathology and associated chronic pain, and it will be beneficial for the development of future therapeutic treatments.

ASIC2a overexpression enhances the protective effect of PcTx1 and APETx2 against acidosis-induced articular chondrocyte apoptosis and cytotoxicity

Acid hydrarthrosis is another important pathological character in rheumatoid arthritis (RA), and acid-sensing ion channel 1a (ASIC1a) plays a destructive role in acidosis-induced articular chondrocyte cytotoxicity. Recently, ASIC2a has been reported to possess neuroprotective effect on acidosis-induced injury of neuronal cells. However, whether ASIC2a has an enhanced effect on the protective effect of blocking ASIC1a and ASIC3 against acid-induced chondrocyte apoptosis is still unclear. The aim of present study was to investigate the chondroprotective effect of ASIC2a with PcTx1 (ASIC1a specific blocker) and APETx2 (ASIC3 specific blocker) on acidosis-induced chondrocyte apoptosis. Our results revealed that acid (pH 6.0) decreased the cell viability and induced apoptosis of articular chondrocytes. PcTx1 and APETx2 combination significantly attenuated acidosis-induced chondrocyte cytotoxicity due to inhibit apoptosis, and this role could be enhanced by ASIC2a overexpression compared with the PcTx1 and APETx2 combination alone group. Moreover, both the [Ca²⁺]_i levels and the levels of phosphorylated ERK1/2 as well as p38 were further reduced in acidosis-induced chondrocytes after ASIC2a overexpression in the presence of PcTx1 and APETx2. Furthermore, ASIC2a overexpression also reduced acid-induced the expression of ASIC1a. In addition, ASIC2a overexpression further promoted the PcTx1 and APETx2-increased levels of type II collagen in acidosis-induced chondrocytes. Taken together, the current data suggested that ASIC2a overexpression might enhance the anti-apoptotic and protective role of PcTx1 and APETx2 against acid-induced rat articular chondrocyte apoptosis by regulating ASIC1a expression and the [Ca²⁺]_i levels and at least in part, suppressing p38 and ERK1/2 MAPK signaling pathways.

Antiarthritic Activity of Qi-Wu Rheumatism Granule (a Chinese Herbal Compound) on Complete Freund's Adjuvant-Induced Arthritis in Rats

Objective

The aim was to study the therapeutic effects and mechanisms of QWRG on adjuvant-induced RA in rats.

Methods

The RA rat models were manipulated and subsequently divided into five experimental groups: AIA, DEX, and QWRG groups. The paw volume, body weight, arthritic score, and mechanical nociceptive threshold were assessed. The serum levels of the RF, MDA, ALP, AST, ALT, IL-1β, IL-2, IL-16, and TNF-α were measured. The proliferative capacity of lymphocytes was evaluated, and the synovial tissue was histopathologically examined.

Results

The paw swelling and arthritic scores were relieved, and the variation of relative body weight and mechanical nociceptive threshold had improved in the AIA rats. The serum levels of RF, MDA, ALP, AST, and ALT were alleviated, and the inflammation and cartilage damage were effectively attenuated in the AIA rats. Simultaneously, the inflammation of the synovial cavity was alleviated, and the grading of synovitis reduced by inhibiting the expressions of IL-1β, TNF-α, and IL-16 in the serum and synovium tissue.

Conclusion

Our results suggested that the antiarthritic properties of QWRG may be due to immunodepression and downregulation of inflammatory cytokines, which may be a potential candidate for the treatment of RA.

ASIC1a Promotes Acid-Induced Autophagy in Rat Articular Chondrocytes through the AMPK/FoxO3a Pathway

Acid-sensing ion channel 1a (ASIC1a) is a member of the extracellular H⁺-activated cation channels family. Our previous studies suggested that ASIC1a contributed to acid-induced rat articular chondrocytes autophagy. However, its potential mechanisms remain unclear. The present study demonstrated the effect of ASIC1a on rat articular chondrocytes autophagy and explored the underlying molecular mechanisms. The results demonstrated that ASIC1a contributed to acid-induced autophagy in rat articular chondrocytes, and which was associated with an increase in (Ca2+)i, as indicated that acid-induced increases in mRNA and protein expression of LC3B-II and other autophagy-related markers were inhibited by ASIC1a-specific blocker, PcTx1 and calcium chelating agent, BAPTA-AM. Furthermore, the results showed that extracellular acid increased level of Forkhead box O (FoxO) 3a, but was reversed by inhibition of ASIC1a and Ca2+ influx. Moreover, gene ablation of FoxO3a prevented acid-induced increases in mRNA and protein expression of LC3B-II, Beclin1 and the formation of autophagosome. Finally, it also showed that ASIC1a activated adenine nucleotide (AMP)-activated protein kinase (AMPK). In addition, suppression of AMPK by Compound C and its small interfering RNA (siRNA) prevented acid-induced upregulation of total and nuclear FoxO3a and increases in mRNA and protein expression of LC3B-II, Beclin1, and ATG5. Taken together, these findings suggested that AMPK/FoxO3a axis plays an important role in ASIC1a-mediated autophagy in rat articular chondrocytes, which may provide novel mechanistic insight into ASIC1a effects on autophagy.

TDAG8, TRPV1, and ASIC3 involved in establishing hyperalgesic priming in experimental rheumatoid arthritis

Rheumatoid arthritis (RA), characterized by chronic inflammation of synovial joints, is often associated with ongoing pain and increased pain sensitivity. High hydrogen ion concentration (acidosis) found in synovial fluid in RA patients is associated with disease severity. Acidosis signaling acting on proton-sensing receptors may contribute to inflammation and pain. Previous studies focused on the early phase of arthritis (<5 weeks) and used different arthritis models, so elucidating the roles of different proton-sensing receptors in the chronic phase of arthritis is difficult. We intra-articularly injected complete Freund’s adjuvant into mice once a week for 4 weeks to establish chronic RA pain. Mice with knockout of acid-sensing ion channel 3 (ASIC3) or transient receptor potential/vanilloid receptor subtype 1 (TRPV1) showed attenuated chronic phase (>6 weeks) of RA pain. Mice with T-cell death-associated gene 8 (TDAG8) knockout showed attenuated acute and chronic phases of RA pain. TDAG8 likely participates in the initiation of RA pain, but all three genes, TDAG8, TRPV1, and ASIC3, are essential to establish hyperalgesic priming to regulate the chronic phase of RA pain.

Overexpression of aquaporin 4 in articular chondrocytes exacerbates the severity of adjuvant-induced arthritis in rats: an in vivo and in vitro study

Background

The dysfunction of articular chondrocytes is a crucial step in rheumatoid arthritis (RA) pathogenesis while its molecular mechanisms are not fully known. This study was aimed to investigate the expression of aquaporin 4 (AQP4) in articular chondrocytes of adjuvant-induced arthritis (AIA) rats and its involvement in AIA development.

Methods

Thirty rats were divided into normal and AIA group (n = 15). Rat AIA was induced by intradermal injection of complete Freund’s adjuvant and evaluated by secondary paw swelling and histological assessments on knee joint damage. Localization and protein expression of AQP4 in articular cartilage were examined by immunohistochemistry and western blot. In vitro study, AIA articular chondrocytes were cultured and treated with acetazolamide, an AQPs inhibitor. AQP4 protein level, cell proliferation and mRNA levels of type-II collagen (COII) and aggrecan were measured by western blot, MTT assay and real-time PCR, respectively.

Results

The results of immunohistochemistry and western blot indicated that AQP4 showed higher protein levels in cartilage tissues of AIA rats than that of normal rats. Correlation analysis revealed that AQP4 protein level in cartilage tissues of AIA rats remarkably correlated positively with secondary paw swelling on day 26 after AIA induction as well as pathological scores on joint damage. Additionally, acetazolamide treatment effectively decreased AQP4 protein level, increased cell proliferation and mRNA levels of COII and aggrecan, suggesting AQP4 inhibition by acetazolamide could normalize the dysfunction of AIA articular chondrocytes in vitro.

Conclusions

Our data provide certain experimental evidence that AQP4 over-expression in articular chondrocytes aggravated AIA severity and might be a novel target for RA treatment.

The Roles of Acidosis in Osteoclast Biology

The adverse effect of acidosis on the skeletal system has been recognized for almost a century. Although the underlying mechanism has not been fully elucidated, it appears that acidosis acts as a general stimulator of osteoclasts derived from bone marrow precursors cells and enhances osteoclastic resorption. Prior work suggests that acidosis plays a significant role in osteoclasts formation and activation via up-regulating various genes responsible for its adhesion, migration, survival and bone matrix degradation. Understanding the role of acidosis in osteoclast biology may lead to development of novel therapeutic approaches for the treatment of diseases related to low bone mass. In this review, we aim to discuss the recent investigations into the effects of acidosis in osteoclast biology and the acid-sensing molecular mechanism.

Understanding autoimmunity: The ion channel perspective

Ion channels are integral membrane proteins that orchestrate the passage of ions across the cell membrane and thus regulate various key physiological processes of the living system. The stringently regulated expression and function of these channels hold a pivotal role in the development and execution of various cellular functions. Malfunction of these channels results in debilitating diseases collectively termed channelopathies. In this review, we highlight the role of these proteins in the immune system with special emphasis on the development of autoimmunity. The role of ion channels in various autoimmune diseases is also listed out. This comprehensive review summarizes the ion channels that could be used as molecular targets in the development of new therapeutics against autoimmune disorders.

Total flavonoids of Bidens bipinnata L. ameliorate experimental adjuvant-induced arthritis through induction of synovial apoptosis

BACKGROUND

Bidens bipinnata are widely distributed in China, which have been widely used as a traditional Chinese medicine. The aim of this study was to examine the effect of total flavonoids of Bidens pilosa L. (TFB) on adjuvant arthritis (AA) and its possible mechanisms.

METHODS

The macroscopic scoring of paw edema, secondary paw swelling, and polyarthritis index were measured. Histological examination of the joints and the serum concentrations of IL-6, IL-1beta, and TNF-alpha were examined. Apoptosis in synovial tissue was detected. The expression of Caspase 3 cleavage, serves as a marker undergoing apoptosis, was confirmed by Western blot.

RESULTS

TFB attenuated the severity of arthritis on paw edema, hind paw volume, and polyarthritis index of AA rats, improved the histological status in AA rats as well. TFB can inhibit the production of IL-6, IL-1beta, and TNF-alpha from serum. Clear DNA ladder formation was observed in DNA extraction of synovium from TFB treated AA rats. The number of apoptosis was increased with TFB treatment in TUNEL assay. TFB treatment on AA rats significantly increased the expression of Caspase 3 in synovium.

CONCLUSIONS

Our data suggest that TFB has a significant anti-arthritic effect in AA through the induction of apoptosis in synovial.

Novel Insights into Acid-Sensing Ion Channels: Implications for Degenerative Diseases

Degenerative diseases often strike older adults and are characterized by progressive deterioration of cells, eventually leading to tissue and organ degeneration for which limited effective treatment options are currently available. Acid-sensing ion channels (ASICs), a family of extracellular H(+)-activated ligand-gated ion channels, play critical roles in physiological and pathological conditions. Aberrant activation of ASICs is reported to regulate cell apoptosis, differentiation and autophagy. Accumulating evidence has highlighted a dramatic increase and activation of ASICs in degenerative disorders, including multiple sclerosis, Parkinson's disease, Huntington's disease, intervertebral disc degeneration and arthritis. In this review, we have comprehensively discussed the critical roles of ASICs and their potential utility as therapeutic targets in degenerative diseases.

Effect of acid-sensing ion channel 1a on the process of liver fibrosis under hyperglycemia

Metabolic syndrome characterized by hyperglycemia contributes to nonalcoholic steatohepatitis-associated liver fibrosis. This study was to investigate the effects of Acid-sensing ion Channel 1a (ASIC1a) on the process of liver fibrosis under hyperglycemia. Results showed that high glucose significantly worsen the pathology of liver fibrosis in vivo. In vitro, high glucose stimulated proliferation, activation and extracellular matrix (ECM) production in HSCs, and enhanced the effect of PDGF-BB on the activation and proliferation of HSCs. These effects could be attenuated by ASIC1a specific inhibitor Psalmotoxin-1(PcTx1) or specific ShRNA for ASIC1a through Notch1/Hes-1 pathways. These data indicate that ASIC1a plays an important role in diabetes complication liver fibrosis.

Interleukin-6 enhances acid-induced apoptosis via upregulating acid-sensing ion channel 1a expression and function in rat articular chondrocytes

The inflammatory cytokine interleukin-6 (IL-6) is a causative agent of rheumatoid arthritis (RA), a chronic inflammatory disease complicated with degenerative arthritic cartilage. However, the precise mechanism of IL-6 on chondrocyte apoptosis is largely unclear. Acid-sensing ion channels (ASICs), a family of extracellular H(+)-activated cation channels, can be transiently activated by extracellular acid and play a pivotal role in acid-induced cell injury. In the present study, to investigate the role of IL-6 in regulating acid-induced articular chondrocyte apoptosis, primary rat articular chondrocytes were subjected to different treatments with or without IL-6 in the presence of acid. The results showed that the mRNA and protein expressions of ASIC1a were significantly increased in articular cartilage and chondrocytes of adjuvant arthritis (AA) rats. IL-6 could dramatically upregulate the level of ASIC1a in a time- and dose-dependent manner, and induce the activation of JAK2, STAT3, ERK, JNK and NF-κB in articular chondrocytes. Moreover, both the respective inhibitors of these signaling pathways and the specific antibody against IL-6 receptor (tocilizumab) could partially abrogate the ASIC1a upregulation induced by IL-6. Furthermore, IL-6 inhibited the cell viability and enhanced LDH release, [Ca(2+)]i elevation, and apoptosis in acid-induced articular chondrocytes, and these changes could be reversed by using psalmotoxin 1(PcTX1), which is the specific antagonist of ASIC1a. In addition, pretreatment with PcTX1 could inhibit the downregulated expression of Bcl-2 and the upregulated expression of Bax induced by IL-6 in acid-induced articular chondrocytes. Taken together, these results indicated that IL-6 could enhance acid-induced articular chondrocyte apoptosis, the mechanism of which might partially be involved with its ability of regulating the activation of ASIC1a-dependent JAK2/STAT3 and MAPK/NF-κB signaling pathways.

Alteration of ASIC1 expression in clear cell renal cell carcinoma

Background

Acidic extracellular pH is a major feature of tumor tissue. Acid-sensing ion channels (ASICs) represent an H⁺-gated subgroup of the degenerin/epithelial Na⁺ channel family and are activated by acidic microenvironment. Little is known about the expression and clinical significance of ASICs in solid tumors. The purpose of this study was to examine the expression of ASIC1 in human clear cell renal cell carcinoma (CCRCC) and to determine if the expression of ASIC1 is associated with clinicopathological features.

Methods

The expression of ASIC1 in CCRCC tissues at the mRNA and protein levels was determined by real-time quantitative polymerase chain reaction and Western blot analysis, respectively. A tissue microarray was used to assess the expression of ASIC1 protein in tumor tissue and matched adjacent normal tissues from 75 patients with CCRCC.

Results

ASIC1 expression was detected in normal renal and CCRCC samples. The expressions of ASIC1 protein and mRNA were significantly decreased in the CCRCC tissues compared with matched normal renal tissues (P<0.05). The staining density measurement showed that the expression of ASIC1 was significantly decreased in stage I (P=0.037), stage II (P=0.026), and stage III (P=0.026), grades I–II CCRCC (P=0.004), and CCRCC from male patients (P=0.00002). However, no significant difference was observed for ASIC1 expression between CCRCC and normal tissue in patients with stage IV CCRCC (P=0.236), patients with grades III–IV CCRCC (P=0.314), and female patients (P=0.095). Spearman correlations demonstrated that ASIC1 expression did not correlate to tumor stage (correlation coefficient [CC =0.168], P=0.149) and the age of patients (CC −0.147, P=0.688) but showed a positive correlation to higher tumor grades (CC =0.270, P=0.018).

Conclusion

ASIC1 is downregulated in CCRCC. ASIC1 expression may be potentially used as a novel biomarker and even a CCRCC therapeutic target. Further efforts will be made to clarify the mechanism of ASIC1 in occurrence, progression, and metastasis of CCRCC.

Expression of hedgehog signal pathway in articular cartilage is associated with the severity of cartilage damage in rats with adjuvant-induced arthritis

BACKGROUND

Cartilage damage is a crucial step in rheumatoid arthritis (RA) disease progress while its molecular mechanisms are not fully understood. Here we investigated the expression of hedgehog (Hh) signal pathway in articular cartilage of adjuvant-induced arthritis (AIA) rats and its possible pathological role in cartilage damage.

METHODS

30 rats were divided into sham and AIA group (n = 15). Complete Freund's adjuvant was used to induce AIA. Secondary paw swelling was measured on day 10, 14, 18, 22 and 26 after induction. Rats were sacrificed on day 26 and knee joints and cartilage tissues were collected. Paw swelling, cartilage histopathologic changes and OARSI scores were used to evaluate AIA in rats. The protein expression of Hh signal related genes (Shh, Ptch1, Smo and Gli1) in cartilage were assayed by immunohistochemistry. The mRNA levels of Shh, Ptch1, Smo, Gli1, type-II collagen (COII) and aggrecan in cartilage were assayed by real-time PCR. In vitro study, cultured AIA chondrocytes were treated with cyclopamine (a specific inhibitor of Hh signal) and the mRNA levels of Hh signal and ECM components (COII and aggrecan) were measured by real-time PCR.

RESULTS

Immunohistochemical results revealed that Shh, Ptch1, Smo and Gli1 proteins showed higher expression in the articular cartilage of AIA rats than those of sham rats. Real-time PCR results confirmed that Shh, Ptch1, Smo and Gli1 mRNA levels in cartilage tissues of AIA rats were significantly increased compared with those of sham rats (1.6, 1.4, 1.6, 2.0 fold, respectively). The mRNA levels of Shh, Ptch1, Smo, and Gli1 were associated with the severity of cartilage damage (indicated by OARSI scores, COII and aggrecan mRNA levels in cartilage). In vitro, cyclopamine effectively decreased the mRNA levels of Shh, Ptch1, Smo and Gli1, and increased the mRNA levels of COII and aggrecan in AIA chondrocytes, suggesting Hh signal inhibition might directly promote ECM production.

CONCLUSIONS

Our findings present certain experimental evidence that Hh signal pathway is involved in the pathogenesis of cartilage damage in RA.

Inhibition of hedgehog signal pathway by cyclopamine attenuates inflammation and articular cartilage damage in rats with adjuvant-induced arthritis

OBJECTIVES

We investigated whether inhibition of hedgehog (Hh) signal by cyclopamine attenuated inflammation and cartilage damage in adjuvant-induced arthritis (AIA) rats.

METHODS

Cyclopamine (2.5, 5, 10 mg/kg) was given by intraperitoneal injection once daily from day 12 to 21 after AIA induction. Paw swelling (volume changes), serum pro-inflammatory cytokines levels (ELISA), histological analysis of joint damage (H&E staining), proteoglycans expression (Alcian blue staining), mRNA levels of sonic Hh (Shh), glioma-associated oncogene homologue 1 (Gli1), type II collagen (COII) and aggrecan in cartilage (real-time PCR) and articular chondrocyte apoptosis (terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling) were measured respectively.

KEY FINDINGS

Cyclopamine effectively attenuated inflammation and cartilage damage of AIA rats, as evidenced by reduced paw swelling, serum levels of tumor necrosis factors (TNF)-α, IL-1β, IL-6 and histological scores of joint damage, increased proteoglycans expression and mRNA levels of COII and aggrecan in articular cartilage. Shh or Gli1 mRNA level was correlated negatively with COII and aggrecan mRNA levels, suggesting Hh signal inhibition was associated with promotion of cartilage extracellular matrix production. Furthermore, cyclopamine decreased the number of apoptotic articular chondrocytes of AIA rats, which might be partly related to its mechanisms on relieving cartilage damage.

CONCLUSIONS

Our findings present some experimental evidence that Hh signal inhibition might be of potential clinical interest in rheumatoid arthritis treatment.

Disruption of phosphoinositide-specific phospholipases Cγ1 contributes to extracellular matrix synthesis of human osteoarthritis chondrocytes

Osteoarthritis (OA) is a degenerative joint disease characterized by articular cartilage degradation including extracellular matrix (ECM) degradation and cell loss. It is known that phosphoinositide-specific phospholipase γ1 (PLCγ1) can trigger several signaling pathways to regulate cell metabolism. However, whether this kinase is expressive and active in human OA chondrocytes and its role in the pathological progression of OA have not been investigated. The current study was designed to investigate the PLCγ1 expression in human OA cartilage, and whether PLCγ1 was involved in the ECM synthesis had been further explored using cultured human OA chondrocytes. Our results indicated that PLCγ1 was highly expressed in human OA chondrocytes. In our further study using the cultured human OA chondrocytes, the results demonstrated that the disruption of PLCγ1 by its inhibitor, U73122, and siRNA contributed to the ECM synthesis of human OA chondrocytes through regulating the expression of ECM-related signaling molecules, including MMP-13, Col II, TIMP1, Sox-9, and AGG. Furthermore, PLCγ1/IP3/Ca2+/CaMK II signaling axis regulated the ECM synthesis of human chondrocytes through triggering mTOR/P70S6K/S6 pathway. In summary, our results suggested that PLC-γ1 activities played an important role in the ECM synthesis of human OA chondrocytes, and may serve as a therapeutic target for treating OA.

Inhibition of acid-sensing ion channel 1a in hepatic stellate cells attenuates PDGF-induced activation of HSCs through MAPK pathway

Acid-sensing ion channels (ASICs), a group of Na(+)-selective and Ca(2+)-permeant ligand-gated cation channels, can be transiently activated by extracellular acid. Among seven subunits of ASICs, acid-sensing ion channel 1a (ASIC1a), which is responsible for Ca(2+) transportation, is elevated in response to inflammation, tumor, and ischemic injury in central nervous system and non-neuronal tissues. In this study, we demonstrated for the first time the presence of ASIC1a in rat liver and hepatic stellate cells (HSCs). Furthermore, the expression of ASIC1a was increased in primary HSCs and liver tissues of CCl4-treated rats, suggesting that ASIC1a may play certain role in liver fibrosis. Interestingly, we identified that the level of ASIC1a was significantly elevated in response to platelet-derived growth factor (PDGF) induction in a time- and dose-dependent manner. It was also established that Ca(2+)-transporting ASIC1a was involved in acid-induced injury of different cell types. Moreover, inhibition or silencing of ASIC1a was able to inhibit PDGF-induced pro-fibrogenic effects of activated rat HSCs, including cell activation, de novo synthesis of extracellular matrix components through mitogen-activated protein kinase signaling pathway. Collectively, our studies identified that ASIC1a was expressed in rat liver and HSCs and provided a strong evidence for the involvement of the ASIC1a in the progression of hepatic fibrosis.

Acid-sensing ion channels in healthy and degenerated human intervertebral disc

Acid-sensing ion channels (ASICs) are a family of H(+)-gated voltage-insensitive ion channels that respond to extracellular acidification by regulating transmembrane Ca(2+) flux. Moreover, ASICs can also be gated by mechanical forces and may function as mechanosensors. The cells of the intervertebral disc (IVD) have an unusual acidic and hyperosmotic microenvironment. Changes in the pH and osmolarity determine the viability of IVD cells and the composition of the extracellular matrix, and both are the basis of IVD degeneration. In this study, the expression of ASICs (ASIC1, ASIC2, ASIC3 and ASIC4) mRNAs and proteins in human healthy and degenerated IVD was evaluated by quantitative reverse transcription-quantitative polymerase chain reaction and Western blot. The distribution of ASIC proteins was determined by immunohistochemistry. The mRNAs for all ASICs were detected in normal human IVD, and significantly increased levels were found in degenerated IVD. Western blots demonstrated the presence of proteins with estimated molecular weights of approximately 68-72 kDa. In both the annulus fibrosus (AF) and nucleus pulposus (NP) of normal IVD, ASIC2 is the most frequently expressed ASIC followed by ASIC3, ASIC1 and ASIC4. In the AF of degenerated IVD, there was a significant increase in the number of ASIC1 and ASIC4 positive cells, whereas in the NP, we found significant increase of expression of ASIC1, ASIC2 and ASIC3. These results describe the occurrence and localization of different ASICs in human healthy IVD, and their increased expression in degenerated IVD, thus suggesting that ASICs may be involved in IVD degeneration.