Effects of potassium and anion channel blockers on the cellular response of human osteoarthritic chondrocytes

Dielectrophoresis as a Tool to Reveal the Potential Role of Ion Channels and Early Electrophysiological Changes in Osteoarthritis

Diseases such as osteoarthritis (OA) are commonly characterized at the molecular scale by gene expression and subsequent protein production; likewise, the effects of pharmaceutical interventions are typically characterized by the effects of molecular interactions. However, these phenomena are usually preceded by numerous precursor steps, many of which involve significant ion influx or efflux. As a consequence, rapid assessment of cell electrophysiology could play a significant role in unravelling the mechanisms underlying drug interactions and progression of diseases, such as OA. In this study, we used dielectrophoresis (DEP), a technique that allows rapid, label-free determination of the dielectric parameters to assess the role of potassium ions on the dielectric characteristics of chondrocytes, and to investigate the electrophysiological differences between healthy chondrocytes and those from an in vitro arthritic disease model. Our results showed that DEP was able to detect a significant decrease in membrane conductance (6191 ± 738 vs. 8571 ± 1010 S/m²), membrane capacitance (10.3 ± 1.47 vs. 14.5 ± 0.01 mF/m²), and whole cell capacitance (5.4 ± 0.7 vs. 7.5 ± 0.3 pF) following inhibition of potassium channels using 10 mM tetraethyl ammonium, compared to untreated healthy chondrocytes. Moreover, cells from the OA model had a different response to DEP force in comparison to healthy cells; this was seen in terms of both a decreased membrane conductivity (782 S/m² vs. 1139 S/m²) and a higher whole cell capacitance (9.58 ± 3.4 vs. 3.7 ± 1.3 pF). The results show that DEP offers a high throughput method, capable of detecting changes in membrane electrophysiological properties and differences between disease states.

Physiological Effects of the Electrogenic Current Generated by the Na+/K+ Pump in Mammalian Articular Chondrocytes

Background: Although the chondrocyte is a nonexcitable cell, there is strong interest in gaining detailed knowledge of its ion pumps, channels, exchangers, and transporters. In combination, these transport mechanisms set the resting potential, regulate cell volume, and strongly modulate responses of the chondrocyte to endocrine agents and physicochemical alterations in the surrounding extracellular microenvironment. Materials and Methods: Mathematical modeling was used to assess the functional roles of energy-requiring active transport, the Na⁺/K⁺ pump, in chondrocytes. Results: Our findings illustrate plausible physiological roles for the Na⁺/K⁺ pump in regulating the resting membrane potential and suggest ways in which specific molecular components of pump can respond to the unique electrochemical environment of the chondrocyte. Conclusion: This analysis provides a basis for linking chondrocyte electrophysiology to metabolism and yields insights into novel ways of manipulating or regulating responsiveness to external stimuli both under baseline conditions and in chronic diseases such as osteoarthritis.

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.

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.

Effects of diadenosine tetraphosphate on FGF9-induced chloride flux changes in achondroplastic chondrocytes

Achondroplasia, the most common type of dwarfism, is characterized by a mutation in the fibroblast growth factor receptor 3 (FGFR3). Achondroplasia is an orphan pathology with no pharmacological treatment so far. However, the possibility of using the dinucleotide diadenosine tetraphosphate (Ap(4)A) with therapeutic purposes in achondroplasia has been previously suggested. The pathogenesis involves the constitutive activation of FGFR3, resulting in altered biochemical and physiological processes in chondrocytes. Some of these altered processes can be influenced by changes in cell volume and ionic currents. In this study, the action of mutant FGFR3 on chondrocyte size and chloride flux in achondroplastic chondrocytes was investigated as well as the effect of the Ap(4)A on these processes triggered by mutant FGFR3. Stimulation with the fibroblast growth factor 9 (FGF9), the preferred ligand for FGFR3, induced an enlarged achondroplastic chondrocyte size and an increase in the intracellular chloride concentration, suggesting the blockade of chloride efflux. Treatment with the Ap(4)A reversed the morphological changes triggered by FGF9 and restored the chloride efflux. These data provide further evidence for the therapeutic potential of this dinucleotide in achondroplasia treatment.

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.

Apoptosis and mitochondrial dysfunction in human chondrocytes following exposure to lidocaine, bupivacaine, and ropivacaine

BACKGROUND

Several mechanisms have been proposed to explain toxicity of local anesthetics to chondrocytes, including the blockade of potassium channels and mitochondrial injury. The purposes of this investigation were to study the effects of lidocaine, bupivacaine, and ropivacaine on human chondrocyte viability and mitochondrial function in vitro and to characterize the type of cell death elicited following exposure.

METHODS

Primary chondrocyte cultures from patients with osteoarthritis undergoing knee replacement were treated with saline solution and the following concentrations of local anesthetics: 2%, 1%, and 0.5% lidocaine, 0.5% and 0.25% bupivacaine, and 0.5% and 0.2% ropivacaine for one hour. Cell viability and apoptosis were measured by flow cytometry at twenty-four hours and 120 hours after treatment. Nuclear staining and caspase 3 and 9 cleavage assays (Western blot) were used to further establish the induction of apoptosis. Mitochondrial dysfunction was evaluated by the accumulation of mitochondrial DNA damage (quantitative Southern blot), changes in adenosine triphosphate production (bioluminescence kit), and mitochondrial protein levels (Western blot analysis).

RESULTS

Exposure of primary human chondrocytes to a 2% concentration of lidocaine caused massive necrosis of chondrocytes after twenty-four hours, 1% lidocaine and 0.5% bupivacaine caused a detectable, but not significant, decrease in viability after twenty-four hours, while 0.5% lidocaine, 0.25% bupivacaine, and both concentrations of ropivacaine (0.5% and 0.2%) did not affect chondrocyte viability. Flow cytometry analysis of chondrocytes 120 hours after drug treatment revealed a significant decrease in viability (p < 0.05) with a concomitant increase in the number of apoptotic cells at all concentrations of lidocaine, bupivacaine, and ropivacaine analyzed, except 0.2% ropivacaine. Apoptosis was verified by observation of condensed and fragmented nuclei and a decrease in procaspase 3 and 9 levels. Local anesthetics induced mitochondrial DNA damage and a decrease in adenosine triphosphate and mitochondrial protein levels.

CONCLUSIONS

Lidocaine, bupivacaine, and ropivacaine cause delayed mitochondrial dysfunction and apoptosis in cultured human chondrocytes.