Effects of one-year tofacitinib therapy on bone metabolism in rheumatoid arthritis.

Janus kinase (JAK) inhibitors are used to treat rheumatoid arthritis (RA). We assessed the effects of tofacitinib on bone density and bone markers in association with clinical and laboratory parameters in RA. Tofacitinib stabilized bone density and resulted in a positive balance of bone turnover. INTRODUCTION: Janus kinase (JAK) inhibitors emerged as new therapeutic options in rheumatoid arthritis (RA). We have little information on how it affects areal and volumetric bone mineral density (BMD) and bone turnover markers. The aim of this study was to assess the effects of 1-year tofacitinib therapy on bone metabolism in RA. METHODS: Thirty RA patients with active disease were treated with either 5 mg bid or 10 mg bid tofacitinib for 12 months. We determined DAS28, CRP, IgM rheumatoid factor (RF), and anti-cyclic citrullinated peptide (CCP) levels, as well as serum levels of sclerostin, osteocalcin (OC), P1NP, DKK-1, OPG, RANKL, and 25-hydroxy-vitamin D3. Areal and volumetric BMD were assessed by DXA and peripheral quantitative CT (QCT), respectively. RESULTS: Twenty-six patients (13 on each arm) completed the study. Tofacitinib was clinically effective by suppressing DAS28, CRP, and HAQ. This was accompanied by the attenuation of further bone loss. Tofacitinib therapy significantly increased OC, OPG, and vitamin D3, while decreased CTX levels (p < 0.05). Age and multiple bone markers (OC, CTX, P1NP, RANKL) inversely correlated with L2-4 and femoral neck BMD by DXA. CRP, DAS28, and RANKL inversely determined volumetric BMD by QCT. Age, CRP, anti-CCP, and DKK-1 influenced the effects of tofacitinib therapy on BMD changes. CONCLUSIONS: One-year tofacitinib treatment stabilized BMD in RA patients and resulted in a positive balance of bone turnover as indicated by bone biomarkers. Further studies are needed to evaluate the potential beneficial effects of JAK inhibitors on inflammatory bone loss.

Pi5 and Pi6, two undescribed peptides from the venom of the scorpion Pandinus imperator and their effects on K+-channels.

This work reports the isolation, chemical and functional characterization of two previously unknown peptides purified from the venom of the scorpion Pandinus imperator, denominated Pi5 and Pi6. Pi5 is a classical K+-channel blocking peptide containing 33 amino acid residues with 4 disulfide bonds. It is the first member of a new subfamily, here defined by the systematic number α-KTx 24.1. Pi6 is a peptide of unknown real function, containing only two disulfide bonds and 28 amino acid residues, but showing sequence similarities to the κ-family of K-channel toxins. The systematic number assigned is κ-KTx2.9. The function of both peptides was assayed on Drosophila Shab and Shaker K+-channels, as well as four different subtypes of voltage-dependent K+-channels: hKv1.1, hKv1.2, hKv1.3 and hKv1.4. The electrophysiological assays showed that Pi5 inhibited Shaker B, hKv1.1, hKv1.2 and hKv1.3 channels with Kd = 540 nM, Kd = 92 nM and Kd = 77 nM, respectively, other studied channels were not affected. Of the channels tested only hKv1.2 and hKv1.3 were inhibited at 100 nM concentration of Pi6, the remaining current fractions were 68% and 77%, respectively. Thus, Pi5 and Pi6 are high nanomolar affinity non-selective blockers of hKv1.2 and hKv1.3 channels.

K+ channel blockers: novel tools to inhibit T cell activation leading to specific immunosuppression.

During the last two decades since the identification and characterization of T cell potassium channels great advances have been made in the understanding of the role of these channels in T cell functions, especially in antigen-induced activation. Their limited tissue distribution and the recent discovery that different T cell subtypes carrying out distinct immune functions show specific expression levels of these channels have made T cell potassium channels attractive targets for immunomodulatory drugs. Many toxins of various animal species and a structurally diverse array of small molecules inhibiting these channels with varying affinity and selectivity were found and their successful use in immunosuppression in vivo was also demonstrated. Better understanding of the topological differences between potassium channel pores, detailed knowledge of toxin and small-molecule structures and the identification of the binding sites of blocking compounds make it possible to improve the selectivity and affinity of the lead compounds by introducing modifications based on structural information. In this review the basic properties and physiological roles of the voltage-gated Kv1.3 and the Ca2+-activated IKCa1 potassium channels are discussed along with an overview of compounds inhibiting these channels and approaches aiming at producing more efficient modulators of immune functions for the treatment of diseases like sclerosis multiplex and type I diabetes.

Kv1.3 potassium channels are localized in the immunological synapse formed between cytotoxic and target cells.

Membrane proteins of cytotoxic T cells specifically reorganize to form an immunological synapse (IS) on interaction with their specific target. In this paper, we investigated the redistribution of Kv1.3 channels, which are the dominant voltage-gated potassium channels, in the plasma membrane of allogen-activated human cytotoxic T lymphocytes (CTLs) on interacting with their specific target cells. Kv1.3 channels bearing a FLAG epitope were expressed in the CTLs and the cell-surface distribution of fluorescently labeled ion channels was determined from confocal laser-scanning microscopy images. FLAG epitope-tagged Kv1.3 channels showed a patchy distribution in CTLs not engaged with target cells, whereas the channels were accumulated in the IS formed between CTLs and specific target lymphocytes. Localization of Kv1.3 channels in the IS might open an unrevealed possibility in the regulation of ion channel activity by signaling molecules accumulated in the IS.

Colocalization and nonrandom distribution of Kv1.3 potassium channels and CD3 molecules in the plasma membrane of human T lymphocytes.

Distribution and lateral organization of Kv1.3 potassium channels and CD3 molecules were studied by using electron microscopy, confocal laser scanning microscopy, and fluorescence resonance energy transfer. Immunogold labeling and electron microscopy showed that the distribution of FLAG epitope-tagged Kv1.3 channels (Kv1.3/FLAG) significantly differs from the stochastic Poisson distribution in the plasma membrane of human T lymphoma cells. Confocal laser scanning microscopy images showed that Kv1.3/FLAG channels and CD3 molecules accumulated in largely overlapping membrane areas. The numerical analysis of crosscorrelation of the spatial intensity distributions yielded a high correlation coefficient (C = 0.64). A different hierarchical level of molecular proximity between Kv1.3/FLAG and CD3 proteins was reported by a high fluorescence resonance energy transfer efficiency (E = 51%). These findings implicate that reciprocal regulation of ion-channel activity, membrane potential, and the function of receptor complexes may contribute to the proper functioning of the immunological synapse.

Cell fusion experiments reveal distinctly different association characteristics of cell-surface receptors.

The existence of small- and large-scale membrane protein clusters, containing dimers, oligomers and hundreds of proteins, respectively, has become widely accepted. However, it is largely unknown whether the internal structure of these formations is dynamic or static. Cell fusion was used to perturb the distribution of existing membrane protein clusters, and to investigate their mobility and associations. Scanning near-field optical microscopy, confocal and electron microscopy were applied to detect the exchange of proteins between large-scale protein clusters, whereas photobleaching fluorescence energy transfer was used to image the redistribution of existing small-scale membrane protein clusters. Large-scale clusters of major histocompatibility complex (MHC)-I exchanged proteins with each other and with MHC-II clusters. Similarly to MHC-I, large-scale MHC-II clusters were also dynamic. Exchange of components between small-scale protein clusters was not universal: intermixing did not take place in the case of MHC-II homoclusters; however, it was observed for homoclusters of MHC-I and for heteroclusters of MHC-I and MHC-II. These processes required a fluid state of the plasma membrane, and did not depend on endocytosis-mediated recycling of proteins. The redistribution of large-scale MHC-I clusters precedes the intermixing of small-scale clusters of MHC-I indicating a hierarchy in protein association. Investigation of a set of other proteins (alpha subunit of the interleukin 2 receptor, CD48 and transferrin receptor) suggested that a large-scale protein cluster usually exchanges components with the same type of clusters. These results offer new insight into processes requiring time-dependent changes in membrane protein interactions.

Multiple binding sites for melatonin on Kv1.3.

Melatonin is a small amino acid derivative hormone of the pineal gland. Melatonin quickly and reversibly blocked Kv1.3 channels, the predominant voltage-gated potassium channel in human T-lymphocytes, acting from the extracellular side. The block did not show state or voltage dependence and was associated with an increased inactivation rate of the current. A half-blocking concentration of 1.5 mM was obtained from the reduction of the peak current. We explored several models to describe the stoichiometry of melatonin-Kv1.3 interaction considering one or four independent binding sites per channel. The model in which the occupancy of one of four binding sites by melatonin is sufficient to block the channels gives the best fit to the dose-response relationship, although all four binding sites can be occupied by the drug. The dissociation constant for the individual binding sites is 8.11 mM. Parallel application of charybdotoxin and melatonin showed that both compounds can simultaneously bind to the channels, thereby localizing the melatonin binding site out of the pore region. However, binding of tetraethylammonium to its receptor decreases the melatonin affinity, and vice versa. Thus, the occupancy of the two separate receptor sites allosterically modulates each other.

Effects of toxins Pi2 and Pi3 on human T lymphocyte Kv1.3 channels: the role of Glu7 and Lys24.

Pandinus imperator scorpion toxins Pi2 and Pi3 differ only by a single amino acid residue (neutral Pro7 in Pi2 vs. acidic Glu7 in Pi3). The binding kinetics of these toxins to human Kv1.3 showed that the decreased ON rate (k(ON) = 2.18 x 10(8) m(-1)sec(-1) for Pi2 and 1.28 x 10(7) m(-1)sec(-1) for Pi3) was almost entirely responsible for the increased dissociation constant (K(d)) of Pi3 (K(d) = 795 pm) as compared to Pi2 (K(d) = 44 pm). The ionic strength dependence of the association rates was exactly the same for the two toxins indicating that through-space electrostatic interactions can not account for the different ON rates. Results were further analyzed on the basis of the three-dimensional structural models of the toxins. A 3D structure of Pi3 was generated from the NMR spectroscopy coordinates of Pi2 by computer modeling. The Pi3 model resulted in a salt bridge between Glu7 and Lys24 in Pi3. Based on this finding our interpretation of the reduced ON rate of Pi3 is that the intramolecular salt bridge reduces the local positive electrostatic potential around Lys24 resulting in decreased short-range electrostatic interactions during the binding step. To support our finding, we constructed a 3D model of the Ser-10-Asp Charybdotoxin mutant displaying distinctly reduced affinity for Shaker channels. The mutant Charybdotoxin structure also displayed a salt bridge between residues Asp10 and Lys27 equivalent to the one between Glu7 and Lys24 in Pi3.

Blockage of human T lymphocyte Kv1.3 channels by Pi1, a novel class of scorpion toxin.

Using the patch-clamp technique we determined that Pandinus imperator toxin Pi1, a recently described peptide toxin having four disulfide bridges instead of the usual three in scorpion toxins, blocked Kv1.3 channels of human T lymphocytes from the extracellular side with a 1:1 stoichiometry. Kv1.3 block was instantaneous and removable with toxin-free extracellular solution. The toxin did not influence activation or inactivation of the channels. We found that Pi1 blocked Kv1.3 with less affinity (K(d) = 11.4 nM) than the structurally related three disulfide bridge containing toxins Pi2 (50 pM) and Pi3 (0.5 nM). The fourth disulfide bridge in Pi1 had no influence on the channel binding ability of the toxin; the less effective block was due to differences in amino acid side chain properties at positions 11 and 35.

Pandinus imperator scorpion venom blocks voltage-gated K+ channels in human lymphocytes.

Using the patch-clamp technique, we determined that Pandinus imperator scorpion venom blocked whole-cell n-type K+ currents in human peripheral blood lymphocytes in a dose-dependent manner with Kd = 0.02 microgram/ml. K+ channel block was instantaneous and removable by washing with venom-free extracellular solution. The venom-induced block was independent of membrane potential. The venom did not influence activation and inactivation kinetics of the K+ channels, however, accelerated recovery from inactivation. Purified peptides Pi1, Pi2, and Pi3 from the P. imperator venom powerfully blocked Kv1.3 channels in human lymphocytes with Kd values of 9.7 nM, 50 pM, and 0.5 nM, respectively. Flow cytometric membrane potential measurements with the oxonol dye showed that Pi2, the most effective peptide toxin of the P. imperator venom, depolarizes human lymphocytes in accordance with its K+ channel blocking effect.

Immunosuppressors inhibit voltage-gated potassium channels in human peripheral blood lymphocytes.

The effects of immunosuppressive agents on the potassium current of human peripheral blood lymphocytes have been studied using the whole-cell patch-clamp technique. Cyclosporin A (10 micrograms/ml), rapamycin (10 micrograms/ml) and FK-506 (2.5 micrograms/ml) reduced the peak K+ current by approximately 40, 30 and 40% of the control, respectively, without any change in the reversal potential of the current. The current inhibition was similar at all membrane potentials studied and was accompanied with an increase in the rate of K+ current inactivation. Membrane potential measurements in current-clamp showed a marked depolarization of the membrane (>10 mV) upon the addition of either immunosuppressor to the cells. Our findings revealed that the voltage-dependent potassium current in human peripheral blood lymphocytes is inhibited by Cyclosporin A and other immunosuppressors, resulting in a depolarized membrane potential.

Assembly and suppression of endogenous Kv1.3 channels in human T cells.

The predominant K+ channel in human T lymphocytes is Kv1.3, which inactivates by a C-type mechanism. To study assembly of these tetrameric channels in Jurkat, a human T-lymphocyte cell line, we have characterized the formation of heterotetrameric channels between endogenous wild-type (WT) Kv1.3 subunits and heterologously expressed mutant (A413V) Kv1.3 subunits. We use a kinetic analysis of C-type inactivation of currents produced by homotetrameric channels and heterotetrameric channels to determine the distribution of channels with different subunit stoichiometries. The distributions are well-described by either a binomial distribution or a binomial distribution plus a fraction of WT homotetramers, indicating that subunit assembly is a random process and that tetramers expressed in the plasma membrane do not dissociate and reassemble. Additionally, endogenous Kv1.3 current is suppressed by a heterologously expressed truncated Kv1.3 that contains the amino terminus and the first two transmembrane segments. The time course for suppression, which is maximal at 48 h after transfection, overlaps with the time interval for heterotetramer formation between heterologously expressed A413V and endogenous WT channels. Our findings suggest that diversity of K+ channel subtypes in a cell is regulated not by spatial segregation of monomeric pools, but rather by the degree of temporal overlap and the kinetics of subunit expression.

C-type inactivation of a voltage-gated K+ channel occurs by a cooperative mechanism.

The lymphocyte voltage-gated K+ channel, Kv1.3, inactivates by a C-type process. We have elucidated the molecular basis for this process using a kinetic analysis of wild-type and mutant (A413V) Kv1.3 homo- and heteromultimeric currents in a mammalian lymphoid expression system. The medians of the measured inactivation time constants for wild-type and A413V homotetrameric currents are 204 and 4 ms, respectively. Co-expression of these subunits produces heteromultimeric channels manifesting inactivation kinetics intermediate between those of wild-type and A413V homomultimers. We have considered several models in which each subunit acts either independently or cooperatively to produce the observed inactivation kinetics. The cooperative model gives excellent fits to the data for any heteromultimeric composition of subunits, clearly distinguishing it from the independent models. In the cooperative model, the difference in free energy between the open and inactivated states of the channel is invariant with subunit composition and equals approximately 1.5 kcal/mol. Each subunit contributes equally to the activation free energy for transitions between open and inactivated states, with an A413V subunit decreasing the free energy barrier for inactivation (and for recovery from inactivation) by approximately 0.6 kcal/mol. Our results are consistent with a physical model in which the outer mouth of the channel constricts during C-type inactivation (G. Yellen, D. Sodickson, T. Chen, and M.E. Jurman, 1994, Biophys. J. 66:1068-1075).

Plasma-membrane-bound macromolecules are dynamically aggregated to form non-random codistribution patterns of selected functional elements. Do pattern recognition processes govern antigen presentation and intercellular interactions?

Molecular recognition processes between cell surface elements are discussed with special reference to cell surface pattern formation of membrane-bound integral proteins. The existence, as detected by flow cytometric resonance energy transfer (Appendix), and significance of cell surface patterns involving the interleukin-2 receptor, the T-cell receptor-CD3 system, the intercellular adhesion molecule ICAM-1, and the major histocompatibility complex class I and class II molecules in the plasma membrane of lymphocytes are described. The modulation of antigen presentation by transmembrane potential changes is discussed, and a general role of transmembrane potential changes, and therefore of ion channel activities, adduced as one of the major regulatory mechanisms of cell-cell communication. A general role in the mediation and regulation of intercellular interactions is suggested for cell-surface macromolecular patterns. The dynamic pattern of protein and lipid molecules in the plasma membrane is generated by the genetic code, but has a remarkable flexibility and may be one of the major instruments of accommodation and recognition processes at the cellular level.

Ion-channel activities regulate transmembrane signaling in thymocyte apoptosis and T-cell activation.

Several examples have shown that plasma membrane ion channels (e.g., Ca2+ and K+ channels) make an important contribution to lymphocyte activation or thymocyte apoptosis. Here we report on the importance of these ion channels in the sensitivity or resistance of lymphoid cells to extracellular ATP-induced apoptosis. Thymocytes of Balb/c mice responded to extracellular ATP (ATPex) sensitively, with an immediate increase in the intracellular calcium level and later with an increased membrane permeability to low MW markers. Mature (medullary) thymocytes showed a higher sensitivity than did cortical thymocytes. Three human lymphoma cell lines, including SUPT13, a cell line reported to be sensitive to TcR/CD3 activation-induced apoptosis, showed a high resistance to ATPex action. These observations suggest that maturation/differentiation state-dependent activity or disappearance of early ATP-receptor operated signaling systems (including ion channels) are critical for the cells in developing towards apoptosis. Using the patch-clamp technique we demonstrated that bretylium tosylate (a particular K(+)-channel blocker) known as inhibitor of T-lymphocyte proliferation also influences the single-channel properties of voltage-gated K+ channels through depressing whole-cell K+ currents. This finding is yet another example underlying the importance of K+ channel activity in T-lymphocyte proliferation.

Effects of bretylium tosylate on voltage-gated potassium channels in human T lymphocytes.

Using the patch-clamp technique, we determined that bretylium tosylate, a quaternary ammonium compound possessing immunomodulating activity, decreased the whole-cell K+ current in human T lymphocytes, in a dose-dependent manner, in the 0.05-5 mM extracellular concentration range. Bretylium tosylate prolonged the recovery from inactivation and accelerated the inactivation and deactivation of the K+ current but did not influence the kinetics of activation or the voltage dependence of activation and steady state inactivation of the K+ conductance. The percentage of drug-induced block was independent of membrane potential. K+ channel block by bretylium tosylate was partially and slowly removable by washing with drug-free extracellular solution. Bovine serum albumin (10 mg/ml) in the bath lifted the drug-induced block almost instantaneously, although not completely. In control experiments bovine serum albumin increased the inactivation time constant of the K+ channels but left the peak K+ current amplitude unaffected. On the basis of the experimental evidence, a gating-dependent allosteric interaction is suggested for the mechanism of drug action. The effective dose range, time of exposure, and reversibility of bretylium tosylate-induced K+ channel block correlated well with the same parameters of the drug-induced inhibition of T lymphocyte activation. The reported effects of bretylium tosylate on T cell mitogenesis can be regarded partly as a consequence of its blocking effects on voltage-gated K+ channels.

Peripheral blood lymphocytes display reduced K+ channel activity in aged humans.

Steady-state parameters of whole-cell K+ current have been determined in human peripheral blood lymphocytes of young (20-50 y.) and elderly (> 90 y.) volunteers by patch-clamp. The magnitude and voltage dependence of the K+ conductance were similar in both lymphocyte populations. The midpoint of steady-state inactivation was -53.3 +/- 2.3 mV for lymphocyte population of young individuals and -65.0 +/- 3.0 mV for that of elderly, showing a significant shift to hyperpolarized potentials. The peak of the steady-state open probability of the K+ channels was decreased and shifted to depolarized potentials by approx. 12.5 mV for lymphocytes of elderly donors. It is suggested that the observed differences in the K+ current parameters may be at least partly responsible for the impaired responsiveness of elderly lymphocytes to proliferative stimuli.

A Ca(2+)-dependent K(+)-channel in freshly isolated and cultured chick osteoclasts.

Calcium-activated potassium channels were found in embryonic chick osteoclasts using the patch-clamp technique. The activity of the channel was increased by both membrane depolarisation and an increase in intracellular Ca2+ concentration in the range 10(-5) to 10(-3) M. In the cell-attached-patch configuration the channel was only active at extreme depolarising potentials. Ca2+ addition to the cytoplasm via ionomycin increased channel activity at the resting membrane potential of the osteoclast. The channel had a single-channel conductance of 150 pS in the inside-out patch under symmetrical K+ conditions (150 mM) and was selective for potassium ions. During sustained application of increased [Ca2+] at the cytoplasmic side of inside-out patches, channel activity sometimes decreased again after the initial increases (desensitization). The results established the properties of the single channels underlying an outward rectifying K+ conductance in chick osteoclasts described previously by us.

Biphasic effect of extracellular ATP on the membrane potential of mouse thymocytes.

Extracellular ATP induced changes in the membrane potential of thymocytes from BALB/c mice were analyzed. At concentrations below 0.1 mM, ATP hyperpolarizes the cell membrane on the time scale of development of the Ca(2+)-signal. After a longer time hyperpolarization turns to depolarization. ATP concentrations higher than 0.5 mM caused rapid depolarization without previous hyperpolarization. Verapamil, quinine or the absence of extracellular Ca2+ blocked the hyperpolarization by ATP. In Na(+)-free medium the magnitude of depolarization decreased. Our data suggest a contribution of Ca(2+)-activated K+ channels to the hyperpolarizing effect of ATP at lower concentrations. The direction of membrane potential changes is determined presumably by a sensitive balance of ATP-receptor mediated Ca(2+)- and Na(+)-influx and the Ca(2+)-activated K(+)-channel activity.