Evidence for functional ATP-sensitive (K(ATP)) potassium channels in human and equine articular chondrocytes

Ca2+-Activated K+ Channels in Progenitor Cells of Musculoskeletal Tissues: A Narrative Review

Musculoskeletal disorders represent one of the main causes of disability worldwide, and their prevalence is predicted to increase in the coming decades. Stem cell therapy may be a promising option for the treatment of some of the musculoskeletal diseases. Although significant progress has been made in musculoskeletal stem cell research, osteoarthritis, the most-common musculoskeletal disorder, still lacks curative treatment. To fine-tune stem-cell-based therapy, it is necessary to focus on the underlying biological mechanisms. Ion channels and the bioelectric signals they generate control the proliferation, differentiation, and migration of musculoskeletal progenitor cells. Calcium- and voltage-activated potassium (KCa) channels are key players in cell physiology in cells of the musculoskeletal system. This review article focused on the big conductance (BK) KCa channels. The regulatory function of BK channels requires interactions with diverse sets of proteins that have different functions in tissue-resident stem cells. In this narrative review article, we discuss the main ion channels of musculoskeletal stem cells, with a focus on calcium-dependent potassium channels, especially on the large conductance BK channel. We review their expression and function in progenitor cell proliferation, differentiation, and migration and highlight gaps in current knowledge on their involvement in musculoskeletal diseases.

An FDA-Approved Drug Screen for Compounds Influencing Craniofacial Skeletal Development and Craniosynostosis

Neural crest stem/progenitor cells (NCSCs) populate a variety of tissues, and their dysregulation is implicated in several human diseases including craniosynostosis and neuroblastoma. We hypothesised that small molecules that inhibit NCSC induction or differentiation may represent potential therapeutically relevant drugs in these disorders. We screened 640 FDA-approved compounds currently in clinical use for other conditions to identify those which disrupt development of NCSC-derived skeletal elements that form the zebrafish jaw. In the primary screen, we used heterozygous transgenic sox10:gfp zebrafish to directly visualise NCSC-derived jaw cartilage. We noted partial toxicity of this transgene in relation to jaw patterning, suggesting that our primary screen was sensitised for NCSC defects, and we confirmed 10 novel, 4 previously reported, and 2 functional analogue drug hits in wild-type embryos. Of these drugs, 9/14 and 7/14, respectively, are known to target pathways implicated in osteoarthritis pathogenesis or to cause reduced bone mineral density/increased fracture risk as side effects in patients treated for other conditions, suggesting that our screen enriched for pathways targeting skeletal tissue homeostasis. We selected one drug that inhibited NCSC induction and one drug that inhibits bone mineralisation for further detailed analyses which reflect our initial hypotheses. These drugs were leflunomide and cyclosporin A, respectively, and their functional analogues, teriflunomide and FK506 (tacrolimus). We identified their critical developmental windows of activity, showing that the severity of defects observed related to the timing, duration, and dose of treatment. While leflunomide has previously been shown to inhibit NCSC induction, we demonstrate additional later roles in cartilage remodelling. Both drugs altered expression of extracellular matrix metalloproteinases. As proof-of-concept, we also tested drug treatment of disease-relevant mammalian cells. While leflunomide treatment inhibited the viability of several human NCSC-derived neuroblastoma cell lines coincident with altered expression of genes involved in ribosome biogenesis and transcription, FK506 enhanced murine calvarial osteoblast differentiation and prevented fusion of the coronal suture in calvarial explants taken from Crouzon syndrome mice.

Physiological and Pathological Functions of Cl- Channels in Chondrocytes

Articular chondrocytes are embedded in the cartilage of diarthrodial joints and responsible for the synthesis and secretion of extracellular matrix. The extracellular matrix mainly contains collagens and proteoglycans, and covers the articular cartilage to protect from mechanical and biochemical stresses. In mammalian chondrocytes, various types of ion channels have been identified: e.g., voltage-dependent K⁺ channels, Ca²⁺-activated K⁺ channels, ATP-sensitive K⁺ channels, two-pore domain K⁺ channels, voltage-dependent Ca²⁺ channels, store-operated Ca²⁺ channels, epithelial Na⁺ channels, acid-sensing ion channels, transient receptor potential channels, and mechanosensitive channels. These channels play important roles for the regulation of resting membrane potential, Ca²⁺ signaling, pH sensing, mechanotransduction, and cell proliferation in articular chondrocytes. In addition to these cation channels, Cl^- channels are known to be expressed in mammalian chondrocytes: e.g., voltage-dependent Cl^- channels, cystic fibrosis transmembrane conductance regulator channels, swelling-activated Cl^- channels, and Ca²⁺-activated Cl^- channels. Although these chondrocyte Cl^- channels are thought to contribute to the regulation of resting membrane potential, Ca²⁺ signaling, cell volume, cell survival, and endochondral bone formation, the physiological functions have not been fully clarified. Osteoarthritis (OA) is caused by the degradation of articular cartilage, resulting in inflammation and pain in the joints. Therefore the pathophysiological roles of Cl^- channels in OA chondrocytes are of considerable interest. Elucidating the physiological and pathological functions of chondrocyte Cl^- channels will provide us a more comprehensive understanding of chondrocyte functions and may suggest novel molecular targets of drug development for OA.

The Resting Potential and K+ Currents in Primary Human Articular Chondrocytes

Human transplant programs provide significant opportunities for detailed in vitro assessments of physiological properties of selected tissues and cell types. We present a semi-quantitative study of the fundamental electrophysiological/biophysical characteristics of human chondrocytes, focused on K⁺ transport mechanisms, and their ability to regulate to the resting membrane potential, E_m. Patch clamp studies on these enzymatically isolated human chondrocytes reveal consistent expression of at least three functionally distinct K⁺ currents, as well as transient receptor potential (TRP) currents. The small size of these cells and their exceptionally low current densities present significant technical challenges for electrophysiological recordings. These limitations have been addressed by parallel development of a mathematical model of these K⁺ and TRP channel ion transfer mechanisms in an attempt to reveal their contributions to E_m. In combination, these experimental results and simulations yield new insights into: (i) the ionic basis for E_m and its expected range of values; (ii) modulation of E_m by the unique articular joint extracellular milieu; (iii) some aspects of TRP channel mediated depolarization-secretion coupling; (iv) some of the essential biophysical principles that regulate K⁺ channel function in “chondrons.” The chondron denotes the chondrocyte and its immediate extracellular compartment. The presence of discrete localized surface charges and associated zeta potentials at the chondrocyte surface are regulated by cell metabolism and can modulate interactions of chondrocytes with the extracellular matrix. Semi-quantitative analysis of these factors in chondrocyte/chondron function may yield insights into progressive osteoarthritis.

Diazoxide ameliorates severity of experimental osteoarthritis by activating autophagy via modulation of the osteoarthritis-related biomarkers

Accumulating evidence suggests that autophagy plays a protective role in chondrocytes and prevents cartilage degeneration in osteoarthritis (OA). The objective of this study was to investigate the effect of diazoxide on chondrocyte death and cartilage degeneration and to determine whether these effects are correlated to autophagy in experimental OA. In this study, a cellular OA model was established by stimulating SW1353 cells with interleukin 1β. A rat OA model was generated by transecting the anterior cruciate ligament combined with the resection of the medial menisci, followed by treatment with diazoxide or diazoxide combination with 3-methyladenine. The percentage of viable cells was evaluated using calcein-acetoxymethyl/propidium iodide double staining. The messenger RNA expression levels of collagen type II alpha 1 chain (COL2A1), matrix metalloproteinase 13 (MMP-13), TIMP metallopeptidase inhibitor 1 (TIMP-1), and a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5) were determined using quantitative real-time polymerase chain reaction. The cartilage thickness and joint space were evaluated using ultrasound. SW1353 cell degeneration and autophagosomes were observed using transmission electron microscopy. The expression levels of microtubule-associated protein 1 light chain 3 (LC3), beclin-1, P62, COL2A1, and MMP-13 were evaluated using immunofluorescence staining and Western blot analysis. Diazoxide significantly attenuated articular cartilage degeneration and SW1353 cell death in experimental OA. The restoration of autophagy was observed in the diazoxide-treated group. The beneficial effects of diazoxide were markedly blocked by 3-methyladenine. Diazoxide treatment also modulated the expression levels of OA-related biomarkers. These results demonstrated that diazoxide exerted a chondroprotective effect and attenuated cartilage degeneration by restoring autophagy via modulation of OA-related biomarkers in experimental OA. Diazoxide treatment might be a promising therapeutic approach to prevent the development of OA.

Laterality of radiographic osteoarthritis of the knee

There are few reports of the laterality in radiological knee osteoarthritis (ROA). This study aimed to evaluate laterality in terms of the minimum joint space width (mJSW) and osteophyte areas (OFs) in a cross-sectorial general population screen and elucidate the association between laterality and risk of osteoarthritis. We enrolled 330 participants (mean age 64.6 years) and examined the presence of ROA (Kellgren-Lawrence grade ≧ 2) laterality in terms of the mJSW and OF on the medial tibia using auto-measuring software. Moreover, we examined the association between laterality and leg dominance. The right and left medial mJSWs were 4.02 ± 0.98 mm and 4.05 ± 1.01 mm, respectively, showing no laterality; the laterals were also similar. The participants who had osteophytes ≥1 mm² in the right, left, and bilateral knees were 15, 37, and 57 respectively, with osteophytes being significantly more common in the left knee. The OF was significantly larger in the left knee. Conversely, the medial and lateral mJSWs and OF did not differ according to leg dominance. The prevalence of ROA was higher and the OF was more pronounced in the left knee. However, the mJSW showed no laterality. Additionally, the mJSW and OF showed no differences according to leg dominance.

Changes in Membrane Receptors and Ion Channels as Potential Biomarkers for Osteoarthritis

Osteoarthritis (OA), a degenerative joint condition, is currently difficult to detect early enough for any of the current treatment options to be completely successful. Early diagnosis of this disease could increase the numbers of patients who are able to slow its progression. There are now several diseases where membrane protein biomarkers are used for early diagnosis. The numbers of proteins in the membrane is vast and so it is a rich source of potential biomarkers for OA but we need more knowledge of these before they can be considered practical biomarkers. How are they best measured and are they selective to OA or even certain types of OA? The first step in this process is to identify membrane proteins that change in OA. Here, we summarize several ion channels and receptors that change in OA models and/or OA patients, and may thus be considered candidates as novel membrane biomarkers of OA.

No Association between Daily Walking and Knee Structural Changes in People at Risk of or with Mild Knee Osteoarthritis. Prospective Data from the Multicenter Osteoarthritis Study

OBJECTIVE

We investigated the association between objectively measured daily walking and knee structural change, defined either as radiographic worsening or as cartilage loss, in people at risk of or with knee osteoarthritis (OA).

METHODS

Participants from the Multicenter Osteoarthritis Study (MOST) with Kellgren-Lawrence grades 0-2 and daily walking (measured with the StepWatch) at the 60-month visit were included. Participants had fixed-flexion, weight-bearing radiographs and knee magnetic resonance images (MRI) at 60 and 84 months. Radiographic worsening was read in both knees using the Osteoarthritis Research Society International grading, and MRI were read for 1 knee using the Whole-Organ MRI Score semiquantitative scoring. OR and 95% CI were calculated comparing those in the middle tertile against the lowest and highest tertiles of daily walking using logistic regression models and generalized estimating equations. Data on walking with moderate to vigorous intensity (min with > 100 steps/min/day) were associated to structural change using multivariate and logistic regression models.

RESULTS

The 1179 study participants (59% women) were 67.0 years old (± 7.6), with a mean (± SD) body mass index of 29.8 kg/m(2) (± 5.3) who walked 6981 (± 2630) steps/day. After adjusting for confounders, we found no significant associations between daily walking and radiographic worsening or cartilage loss. More time spent walking at a moderate to vigorous intensity was not associated with either radiographic worsening or cartilage loss.

CONCLUSION

Results from the MOST study indicated no association between daily walking and structural changes over 2 years in the knees of people at risk of or with mild knee OA.

Chondrosenescence: definition, hallmarks and potential role in the pathogenesis of osteoarthritis

Aging and inflammation are major contributing factors to the development and progression of arthritic and musculoskeletal diseases. "Inflammaging" refers to low-grade inflammation that occurs during physiological aging. In this paper we review the published literature on cartilage aging and propose the term "chondrosenescence" to define the age-dependent deterioration of chondrocyte function and how it undermines cartilage function in osteoarthritis. We propose the concept that a small number of senescent chondrocytes may be able to take advantage of the inflammatory tissue microenvironment and the inflammaging and immunosenescence that is concurrently occurring in the arthritic joint, further contributing to the age-related degradation of articular cartilage, subchondral bone, synovium and other tissues. In this new framework "chondrosenescence" is intimately linked with inflammaging and the disturbed interplay between autophagy and inflammasomes, thus contributing to the age-related increase in the prevalence of osteoarthritis and a decrease in the efficacy of articular cartilage repair. A better understanding of the basic mechanisms underlying chondrosenescence and its modification by drugs, weight loss, improved nutrition and physical exercise could lead to the development of new therapeutic and preventive strategies for osteoarthritis and a range of other age-related inflammatory joint diseases. Aging is inevitable but age-related diseases may be modifiable.

Millimeter wave promotes the synthesis of extracellular matrix and the proliferation of chondrocyte by regulating the voltage-gated K+ channel

Previously, we reported that millimeter wave promoted the chondrocyte proliferation by pushing cell cycle progression. Activation of K(+) channels plays an essential role in the stimulating of extracellular matrix (ECM) synthesis and the cell proliferation in chondrocytes. While it is unclear if millimeter wave enhances ECM synthesis and proliferation of chondrocytes by regulating K(+) channel activity, we here investigated the effects of millimeter waves on ECM synthesis, chondrocyte proliferation and ion channels in the primary chondrocyte culture. We found that millimeter waves led to the increase of chondrocyte viability, the morphological changes of chondrocyte, and the F-actin distortion and remodeling. Ultrastructural analysis showed that treated chondrocytes contained an expansion of mitochondria and granular endoplasmic reticulum, and a high number of cytoplasmic vesicles in the cytoplasm compared to untreated cells, suggesting millimeter waves increased the energy metabolism and protein synthesis of chondrocytes. The analysis of differential ion channels' genes expression further showed an obvious increase of Kcne1, Kcnj3 and Kcnq2. To determine the role of voltage-gated K(+) channel in chondrocyte, we blocked the voltage-gated K(+) channel with 10 mM tetraethylammonium (TEA) and treated chondrocytes with millimeter waves. The results indicated that TEA significantly negated the promotion of millimeter waves for the ECM synthesis and chondrocyte proliferation. Our results support the hypothesis that millimeter waves promote the synthesis of ECM and the proliferation of chondrocyte by regulating the voltage-gated K(+) channel.

Expression and function of K(ATP) channels in normal and osteoarthritic human chondrocytes: possible role in glucose sensing

ATP-sensitive potassium [K(ATP)] channels sense intracellular ATP/ADP levels, being essential components of a glucose-sensing apparatus in various cells that couples glucose metabolism, intracellular ATP/ADP levels and membrane potential. These channels are present in human chondrocytes, but their subunit composition and functions are unknown. This study aimed at elucidating the subunit composition of K(ATP) channels expressed in human chondrocytes and determining whether they play a role in regulating the abundance of major glucose transporters, GLUT-1 and GLUT-3, and glucose transport capacity. The results obtained show that human chondrocytes express the pore forming subunits, Kir6.1 and Kir6.2, at the mRNA and protein levels and the regulatory sulfonylurea receptor (SUR) subunits, SUR2A and SUR2B, but not SUR1. The expression of these subunits was no affected by culture under hyperglycemia-like conditions. Functional impairment of the channel activity, using a SUR blocker (glibenclamide 10 or 20 nM), reduced the protein levels of GLUT-1 and GLUT-3 by approximately 30% in normal chondrocytes, while in cells from cartilage with increasing osteoarthritic (OA) grade no changes were observed. Glucose transport capacity, however, was not affected in normal or OA chondrocytes. These results show that K(ATP) channel activity regulates the abundance of GLUT-1 and GLUT-3, although other mechanisms are involved in regulating the overall glucose transport capacity of human chondrocytes. Therefore, K(ATP) channels are potential components of a broad glucose sensing apparatus that modulates glucose transporters and allows human chondrocytes to adjust to varying extracellular glucose concentrations. This function of K(ATP) channels seems to be impaired in OA chondrocytes.

Benzamil sensitive ion channels contribute to volume regulation in canine chondrocytes

BACKGROUND AND PURPOSE

Chondrocytes exist within cartilage and serve to maintain the extracellular matrix. It has been postulated that osteoarthritic (OA) chondrocytes lose the ability to regulate their volume, affecting extracellular matrix production. In previous studies, we identified expression of epithelial sodium channels (ENaC) in human chondrocytes, but their function remained unknown. Although ENaC typically has Na(+) transport roles, it is also involved in the cell volume regulation of rat hepatocytes. ENaC is a member of the degenerin (Deg) family, and ENaC/Deg-like channels have a low conductance and high sensitivity to benzamil. In this study, we investigated whether canine chondrocytes express functional ENaC/Deg-like ion channels and, if so, what their function may be.

EXPERIMENTAL APPROACH

Canine chondrocytes were harvested from dogs killed for unassociated welfare reasons. We used immunohistochemistry and patch-clamp electrophysiology to investigate ENaC expression and video microscopy to analyse the effects of pharmacological inhibition of ENaC/Deg on cell volume regulation.

KEY RESULTS

Immunofluorescence showed that canine chondrocytes expressed ENaC protein. Single-channel recordings demonstrated expression of a benzamil-sensitive Na(+) conductance (9 pS), and whole-cell experiments show this to be approximately 1.5 nS per cell with high selectivity for Na(+) . Benzamil hyperpolarized chondrocytes by approximately 8 mV with a pD2 8.4. Chondrocyte regulatory volume decrease (RVI) was inhibited by benzamil (pD2 7.5) but persisted when extracellular Na(+) ions were replaced by Li(+) .

CONCLUSION AND IMPLICATIONS

Our data suggest that benzamil inhibits RVI by reducing the influx of Na(+) ions through ENaC/Deg-like ion channels and present ENaC/Deg as a possible target for pharmacological modulation of chondrocyte volume.

Potassium channels in articular chondrocytes

Chondrocytes are the resident cells of cartilage, which synthesize and maintain the extracellular matrix. The range of known potassium channels expressed by these unique cells is continually increasing. Since chondrocytes are non-excitable, and do not need to be repolarized following action potentials, the function of potassium channels in these cells has, until recently, remained completely unknown. However, recent advances in both traditional physiology and “omic” technologies have enhanced our knowledge and understanding of the chondrocyte channelome. A large number of potassium channels have been identified and a number of putative, but credible, functions have been proposed. Members of each of the potassium channel sub-families (calcium activated, inward rectifier, voltage-gated and tandem pore) have all been identified. Mechanotransduction, cell volume regulation, apoptosis and chondrogenesis all appear to involve potassium channels. Since evidence suggests that potassium channel gene transcription is altered in osteoarthritis, future studies are needed that investigate potassium channels as potential cellular biomarkers and therapeutic targets for treatment of degenerative joint conditions.

Extracellular ATP signaling via P2X(4) receptor and cAMP/PKA signaling mediate ATP oscillations essential for prechondrogenic condensation

Prechondrogenic condensation is the most critical process in skeletal patterning. A previous study demonstrated that ATP oscillations driven by Ca(2+) oscillations play a critical role in prechondrogenic condensation by inducing oscillatory secretion. However, it remains unknown what mechanisms initiate the Ca(2+)-driven ATP oscillations, mediate the link between Ca(2+) and ATP oscillations, and then result in oscillatory secretion in chondrogenesis. This study has shown that extracellular ATP signaling was required for both ATP oscillations and prechondrogenic condensation. Among P2 receptors, the P2X(4) receptor revealed the strongest expression level and mediated ATP oscillations in chondrogenesis. Moreover, blockage of P2X(4) activity abrogated not only chondrogenic differentiation but also prechondrogenic condensation. In addition, both ATP oscillations and secretion activity depended on cAMP/PKA signaling but not on K(ATP) channel activity and PKC or PKG signaling. This study proposes that Ca(2+)-driven ATP oscillations essential for prechondrogenic condensation is initiated by extracellular ATP signaling via P2X(4) receptor and is mediated by cAMP/PKA signaling and that cAMP/PKA signaling induces oscillatory secretion to underlie prechondrogenic condensation, in cooperation with Ca(2+) and ATP oscillations.

Expression of Transient Receptor Potential Vanilloid (TRPV) channels in different passages of articular chondrocytes

Ion channels play important roles in chondrocyte mechanotransduction. The transient receptor potential vanilloid (TRPV) subfamily of ion channels consists of six members. TRPV1-4 are temperature sensitive calcium-permeable, relatively non-selective cation channels whereas TRPV5 and TRPV6 show high selectivity for calcium over other cations. In this study we investigated the effect of time in culture and passage number on the expression of TRPV4, TRPV5 and TRPV6 in articular chondrocytes isolated from equine metacarpophalangeal joints. Polyclonal antibodies raised against TRPV4, TRPV5 and TRPV6 were used to compare the expression of these channels in lysates from first expansion chondrocytes (P0) and cells from passages 1–3 (P1, P2 and P3) by western blotting. TRPV4, TRPV5 and TRPV6 were expressed in all passages examined. Immunohistochemistry and immunofluorescence confirmed the presence of these channels in sections of formalin fixed articular cartilage and monolayer cultures of methanol fixed P2 chondrocytes. TRPV5 and TRPV6 were upregulated with time and passage in culture suggesting that a shift in the phenotype of the cells in monolayer culture alters the expression of these channels. In conclusion, several TRPV channels are likely to be involved in calcium signaling and homeostasis in chondrocytes.

Cell volume regulation in chondrocytes

Chondrocytes are the cells within cartilage which produce and maintain the extracellular matrix. Volume regulation in these cells is vital to their function and occurs in several different physiological and pathological contexts. Firstly, chondrocytes exist within an environment of changing osmolarity and compressive loads. Secondly, in osteoarthritic joint failure, cartilage water content changes and there is a notable increase in chondrocyte apoptosis. Thirdly, endochondral ossification requires chondrocyte swelling in association with hypertrophy. Regulatory volume decrease (RVD) and regulatory volume increase (RVI) have both been observed in articular chondrocytes and this review focuses on the mechanisms identified to account for these. There has been evidence so far to suggest TRPV4 is central to RVD; however other elements of the pathway have not yet been identified. Unlike RVD, RVI appears less robust in articular chondrocytes and there have been fewer mechanistic studies; the primary focus being on the Na(+)-K(+)-2Cl(-) co-transporter. The clinical significance of chondrocyte volume regulation remains unproven. Importantly however, transcript abundances of several ion channels implicated in volume control are changed in chondrocytes from osteoarthritic cartilage. A critical question is whether disturbances of volume regulation mechanisms lead to, result from or are simply coincidental to cartilage damage.

The role of the membrane potential in chondrocyte volume regulation

Many cell types have significant negative resting membrane potentials (RMPs) resulting from the activity of potassium-selective and chloride-selective ion channels. In excitable cells, such as neurones, rapid changes in membrane permeability underlie the generation of action potentials. Chondrocytes have less negative RMPs and the role of the RMP is not clear. Here we examine the basis of the chondrocyte RMP and possible physiological benefits. We demonstrate that maintenance of the chondrocyte RMP involves gadolinium-sensitive cation channels. Pharmacological inhibition of these channels causes the RMP to become more negative (100 µM gadolinium: ΔV(m)  = -30 ± 4 mV). Analysis of the gadolinium-sensitive conductance reveals a high permeability to calcium ions (PCa/PNa ≈80) with little selectivity between monovalent ions; similar to that reported elsewhere for TRPV5. Detection of TRPV5 by PCR and immunohistochemistry and the sensitivity of the RMP to the TRPV5 inhibitor econazole (ΔV(m)  = -18 ± 3 mV) suggests that the RMP may be, in part, controlled by TRPV5. We investigated the physiological advantage of the relatively positive RMP using a mathematical model in which membrane stretch activates potassium channels allowing potassium efflux to oppose osmotic water uptake. At very negative RMP potassium efflux is negligible, but at more positive RMP it is sufficient to limit volume increase. In support of our model, cells clamped at -80 mV and challenged with a reduced osmotic potential swelled approximately twice as much as cells at +10 mV. The positive RMP may be a protective adaptation that allows chondrocytes to respond to the dramatic osmotic changes, with minimal changes in cell volume.

Quantitative cartilage imaging in knee osteoarthritis

Quantitative measures of cartilage morphology (i.e., thickness) represent potentially powerful surrogate endpoints in osteoarthritis (OA). These can be used to identify risk factors of structural disease progression and can facilitate the clinical efficacy testing of structure modifying drugs in OA. This paper focuses on quantitative imaging of articular cartilage morphology in the knee, and will specifically deal with different cartilage morphology outcome variables and regions of interest, the relative performance and relationship between cartilage morphology measures, reference values for MRI-based knee cartilage morphometry, imaging protocols for measurement of cartilage morphology (including those used in the Osteoarthritis Initiative), sensitivity to change observed in knee OA, spatial patterns of cartilage loss as derived by subregional analysis, comparison of MRI changes with radiographic changes, risk factors of MRI-based cartilage loss in knee OA, the correlation of MRI-based cartilage loss with clinical outcomes, treatment response in knee OA, and future directions of the field.

The emerging chondrocyte channelome

Chondrocytes are the resident cells of articular cartilage and are responsible for synthesizing a range of collagenous and non-collagenous extracellular matrix macromolecules. Whilst chondrocytes exist at low densities in the tissue (1-10% of the total tissue volume in mature cartilage) they are extremely active cells and are capable of responding to a range of mechanical and biochemical stimuli. These responses are necessary for the maintenance of viable cartilage and may be compromised in inflammatory diseases such as arthritis. Although chondrocytes are non-excitable cells their plasma membrane contains a rich complement of ion channels. This diverse channelome appears to be as complex as one might expect to find in excitable cells although, in the case of chondrocytes, their functions are far less well understood. The ion channels so far identified in chondrocytes include potassium channels (K(ATP), BK, K(v), and SK), sodium channels (epithelial sodium channels, voltage activated sodium channels), transient receptor potential calcium or non-selective cation channels and chloride channels. In this review we describe this emerging channelome and discuss the possible functions of a range of chondrocyte ion channels.

Characterization of a stretch-activated potassium channel in chondrocytes

Chondrocytes possess the capacity to transduce load-induced mechanical stimuli into electrochemical signals. The aim of this study was to functionally characterize an ion channel activated in response to membrane stretch in isolated primary equine chondrocytes. We used patch-clamp electrophysiology to functionally characterize this channel and immunohistochemistry to examine its distribution in articular cartilage. In cell-attached patch experiments, the application of negative pressures to the patch pipette (in the range of 20-200 mmHg) activated ion channel currents in six of seven patches. The mean activated current was 45.9 +/- 1.1 pA (n = 4) at a membrane potential of 33 mV (cell surface area approximately 240 microm(2)). The mean slope conductance of the principal single channels resolved within the total stretch-activated current was 118 +/- 19 pS (n = 6), and reversed near the theoretical potassium equilibrium potential, E(K+), suggesting it was a high-conductance potassium channel. Activation of these high-conductance potassium channels was inhibited by extracellular TEA (K(d) approx. 900 microM) and iberiotoxin (K(d) approx. 40 nM). This suggests that the current was largely carried by BK-like potassium (MaxiK) channels. To further characterize these BK-like channels, we used inside-out patches of chondrocyte membrane: we found these channels to be activated by elevation in bath calcium concentration. Immunohistochemical staining of equine cartilage samples with polyclonal antibodies to the alpha1- and beta1-subunits of the BK channel revealed positive immunoreactivity for both subunits in superficial zone chondrocytes. These experiments support the hypothesis that functional BK channels are present in chondrocytes and may be involved in mechanotransduction and chemotransduction.

Accelerated Ca2+ entry by membrane hyperpolarization due to Ca2+-activated K+ channel activation in response to histamine in chondrocytes

In articular cartilage inflammation, histamine release from mast cells is a key event. It can enhance cytokine production and matrix synthesis and also promote cell proliferation by stimulating chondrocytes. In this study, the functional impact of Ca2+-activated K+ (KCa) channels in the regulation of intracellular Ca2+ concentration ([Ca2+]i) in chondrocytes in response to histamine was examined using OUMS-27 cells, as a model of chondrocytes derived from human chondrosarcoma. Application of histamine induced a significant [Ca2+]i rise and also membrane hyperpolarization, and both effects were mediated by the stimulation of H1 receptors. The histamine-induced membrane hyperpolarization was attenuated to ∼50% by large-conductance KCa (BK) channel blockers, and further reduced by intermediate (IK) and small conductance KCa (SK) channel blockers. The tonic component of histamine-induced [Ca2+]i rise strongly depended on the presence of extracellular Ca2+ ([Ca2+]o) and was markedly reduced by La3+ or Gd3+ but not by nifedipine. It was significantly attenuated by BK channel blockers, and further blocked by the cocktail of BK, IK, and SK channel blockers. The KCa blocker cocktail also significantly reduced the store-operated Ca2+ entry (SOCE), which was induced by Ca2+ addition after store-depletion by thapsigargin in [Ca2+]o free solution. Our results demonstrate that the histamine-induced membrane hyperpolarization in chondrocytes due to KCa channel activation contributes to sustained Ca2+ entry mainly through SOCE channels in OUMS-27 cells. Thus, KCa channels appear to play an important role in the positive feedback mechanism of [Ca2+]i regulation in chondrocytes in the presence of articular cartilage inflammation.

Genetic influences in the aetiology of femoroacetabular impingement: a sibling study

Femoroacetabular impingement causes pain in the hip in young adults and may predispose to the development of osteoarthritis. Genetic factors are important in the aetiology of osteoarthritis of the hip and may have a role in that of femoroacetabular impingement. We compared 96 siblings of 64 patients treated for primary impingement with a spouse control group of 77 individuals. All the subjects were screened clinically and radiologically using a standardised protocol for the presence of cam and pincer deformities and osteoarthritis. The siblings of those patients with a cam deformity had a relative risk of 2.8 of having the same deformity (66 of 160 siblings hips versus 23 of 154 control hips, p < 0.00001). The siblings of those patients with a pincer deformity had a relative risk of 2.0 of having the same deformity (43 of 116 sibling hips versus 29 of 154 control hips, p = 0.001). Bilateral deformity occurred more often in the siblings (42 of 96 siblings versus 13 of 77 control subjects, relative risk 2.6, p = 0.0002). The prevalence of clinical features in those hips with abnormal morphology was also greater in the sibling group compared with the control group (41 of 109 sibling hips versus 7 of 46 control hips, relative risk 2.5, p = 0.007). In 11 sibling hips there was grade-2 osteoarthritis according to Kellgren and Lawrence versus none in the control group (p = 0.002). Genetic influences are important in the aetiology of primary femoroacetabular impingement. This risk appears to be manifested through not only abnormal joint morphology, but also through other factors which may modulate progression of the disease.

Active creatine kinase is present in matrix vesicles isolated from femurs of chicken embryo: Implications for bone mineralization

Proteomic analysis of matrix vesicles (MVs) isolated from 17-day-old chicken embryo femurs revealed the presence of creatine kinase. In this report we identified the enzyme functionally and suggest that the enzyme may participate in the synthesis of ATP from ADP and phosphocreatine within the lumen of these organelles. Then, ATP is converted by nucleotide hydrolyzing enzymes such as Na(+), K(+)-ATPase, protein kinase C, or alkaline phosphatase to yield inorganic phosphate (P(i)), a substrate for mineralization. Alternatively, ATP can be hydrolyzed by a nucleoside triphosphate pyrophosphatase phosphodiesterase 1 producing inorganic pyrophosphate (PP(i)), a mineralization inhibitor. In addition, immunochemical evidence indicated that VDAC 2 is present in MVs that may serve as a transporter of nucleotides from the extracellular matrix. We discussed the implications of ATP production and hydrolysis by MVs as regulatory mechanisms for mineralization.

The assessment of early osteoarthritis

Treatment strategies for osteoarthritis most commonly involve the removal or replacement of damaged joint tissue. Relatively few treatments attempt to arrest, slow down or reverse the disease process. Such options include peri-articular osteotomy around the hip or knee, and treatment of femoro-acetabular impingement, where early intervention may potentially alter the natural history of the disease. A relatively small proportion of patients with osteoarthritis have a clear predisposing factor that is both suitable for modification and who present early enough for intervention to be deemed worthwhile. This paper reviews recent advances in our understanding of the pathology, imaging and progression of early osteoarthritis.