Cyclic GMP regulation of a voltage-activated K channel in dissociated enterocytes

Molecular mechanisms defining penetrance of LRRK2-associated Parkinson's disease

Mutations in Leucine-rich repeat kinase 2 (LRRK2) are the most frequent cause of dominantly inherited Parkinson's disease (PD). LRRK2 mutations, among which p.G2019S is the most frequent, are inherited with reduced penetrance. Interestingly, the disease risk associated with LRRK2 G2019S can vary dramatically depending on the ethnic background of the carrier. While this would suggest a genetic component in the definition of LRRK2-PD penetrance, only few variants have been shown to modify the age at onset of patients harbouring LRRK2 mutations, and the exact cellular pathways controlling the transition from a healthy to a diseased state currently remain elusive. In light of this knowledge gap, recent studies also explored environmental and lifestyle factors as potential modifiers of LRRK2-PD. In this article, we (i) describe the clinical characteristics of LRRK2 mutation carriers, (ii) review known genes linked to LRRK2-PD onset and (iii) summarize the cellular functions of LRRK2 with particular emphasis on potential penetrance-related molecular mechanisms. This section covers LRRK2's involvement in Rab GTPase and immune signalling as well as in the regulation of mitochondrial homeostasis and dynamics. Additionally, we explored the literature with regard to (iv) lifestyle and (v) environmental factors that may influence the penetrance of LRRK2 mutations, with a view towards further exposomics studies. Finally, based on this comprehensive overview, we propose potential future in vivo, in vitro and in silico studies that could provide a better understanding of the processes triggering PD in individuals with LRRK2 mutations.

Isolation and Characterization of Human Dental Pulp Stem Cells from Cryopreserved Pulp Tissues Obtained from Teeth with Irreversible Pulpitis

INTRODUCTION

Human dental pulp stem cells (DPSCs) are becoming an attractive target for therapeutic purposes because of their neural crest origin and propensity. Although DPSCs can be successfully cryopreserved, there are hardly any reports on cryopreservation of dental pulp tissues obtained from teeth diagnosed with symptomatic irreversible pulpitis during endodontic treatment and isolation and characterization of DPSCs from such cryopreserved pulp. The aim of this study was to cryopreserve the said pulp tissues to propagate and characterize isolated DPSCs.

METHODS

A medium consisting of 90% fetal bovine serum and 10% dimethyl sulfoxide was used for cryopreservation of pulp tissues. DPSCs were isolated from fresh and cryopreserved pulp tissues using an enzymatic method. Cell viability and proliferation were determined using the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. DPSC migration and interaction were analyzed with the wound healing assay. Mesenchymal characteristics of DPSCs were verified by flow cytometric analysis of cell surface CD markers. The osteogenic and adipogenic potential of DPSCs was shown by von Kossa and oil red O staining methods, respectively, and the polymerase chain reaction method.

RESULT

We found no significant difference in CD marker expression and osteogenic and adipogenic differentiation potential of DPSCs obtained from fresh and cryopreserved dental pulp tissue.

CONCLUSIONS

Our study shows that dental pulp can be successfully cryopreserved without losing normal characteristics and differentiation potential of their DPSCs, thus making them suitable for dental banking and future therapeutic purposes.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.

Inhibition of the antigen-induced activation of RBL-2H3 cells by charybdotoxin and cetiedil

Quinidine and Ba(2+), non-selective K(+)-channel blockers, have previously been shown to inhibit antigen-induced mediator (beta-hexosaminidase) release from RBL-2H3 cells, a mucosal-type mast cell line. We therefore used selective blockers of Ca(2+)-activated and other K(+) channels to determine if there was a role for these channels in antigen-induced mediator release. Charybdotoxin and cetiedil dose-dependently inhibited beta-hexosaminidase release with IC(50) values of 133 nM and 84 microM, respectively. Charybdotoxin also inhibited the repolarization phase of the antigen-induced biphasic change in the membrane potential (IC(50) 84 nM), antigen-stimulated 86Rb(+)-efflux and increase in free intracellular calcium, [Ca(2+)](i). Iberiotoxin, margatoxin, apamin and tetraethylammonium had no effect on beta-hexosaminidase release. These results suggest that K(+) conductances play a significant role in mediator release from RBL-2H3, that these conductances are of the intermediate conductance Ca(2+)-activated K(+) channel (IK(Ca)) type, and that they are somewhat similar to those which have been described in red blood cells, though they are much less sensitive to clotrimazole.

Adult alveolar epithelial cells express multiple subtypes of voltage-gated K+ channels that are located in apical membrane

Whole cell perforated patch-clamp experiments were performed with adult rat alveolar epithelial cells. The holding potential was -60 mV, and depolarizing voltage steps activated voltage-gated K(+) (Kv) channels. The voltage-activated currents exhibited a mean reversal potential of -32 mV. Complete activation was achieved at -10 mV. The currents exhibited slow inactivation, with significant variability in the time course between cells. Tail current analysis revealed cell-to-cell variability in K(+) selectivity, suggesting contributions of multiple Kv alpha-subunits to the whole cell current. The Kv channels also displayed steady-state inactivation when the membrane potential was held at depolarized voltages with a window current between -30 and 5 mV. Analysis of RNA isolated from these cells by RT-PCR revealed the presence of eight Kv alpha-subunits (Kv1.1, Kv1.3, Kv1.4, Kv2.2, Kv4.1, Kv4.2, Kv4.3, and Kv9.3), three beta-subunits (Kvbeta1.1, Kvbeta2.1, and Kvbeta3.1), and two K(+) channel interacting protein (KChIP) isoforms (KChIP2 and KChIP3). Western blot analysis with available Kv alpha-subunit antibodies (Kv1.1, Kv1.3, Kv1.4, Kv4.2, and Kv4.3) showed labeling of 50-kDa proteins from alveolar epithelial cells grown in monolayer culture. Immunocytochemical analysis of cells from monolayers showed that Kv1.1, Kv1.3, Kv1.4, Kv4.2, and Kv4.3 were localized to the apical membrane. We conclude that expression of multiple Kv alpha-, beta-, and KChIP subunits explains the variability in inactivation gating and K(+) selectivity observed between cells and that Kv channels in the apical membrane may contribute to basal K(+) secretion across the alveolar epithelium.

Inhibition of goby posterior intestinal NaCl absorption by natriuretic peptides and by cardiac extracts

Natriuretic peptides abolish active Na+ and Cl- absorption across the posterior intestine of the euryhaline goby Gillichthys mirabilis. Inhibition by eel and human natriuretic peptides is dose-dependent with the following sequence of potencies based on experimentally determined ID50 values for inhibition of short-circuit current: eel ventricular natriuretic peptide (78 nmol.l-1), eel atrial natriuretic peptide (156 nmol.l-1), human brain natriuretic peptide (326 nmol.l-1), human alpha atrial natriuretic peptide (1.05 mumol.l-1), and eel C-type natriuretic peptide (75 mumol.l-1). Natriuretic peptides also significantly increase transcellular conductance. The observed sequence of natriuretic peptide potencies is suggestive of cellular mediation by GC-A-type NP-R1 receptors in this tissue; as expected for guanylyl-cyclase-coupled NP-R1 receptors, cyclic GMP mimics the action of natriuretic peptides on the goby intestine. Crude aqueous extracts of goby atrium and ventricle inhibited short circuit current and increased tissue conductance in a dose-dependent manner. Ventricular extract was more potent than atrial extract on both a per organ and per milligram basis.

Effects of charybdotoxin on K+ channel (KV1.2) deactivation and inactivation kinetics

Of particular interest for voltage-gated K+ channels are the effects of membrane voltage and pharmacologic agents on channel kinetics. We have characterized in detail properties of Kv1.2 channel expressed in oocytes as the basis for investigation of its structure-function relationships. This channel exhibited a voltage-dependent rate of activation with a V1/2 of -21 mV. Voltage-dependent steady-state inactivation overlapped the activation curve with half-maximal inactivation occurring at -22 mV. Dendrotoxin inhibited channel activation with an IC50 of 8.6 nM at + 35 mV. Charybdotoxin also blocked this K+ channel (IC50 = 5.6 nM). While dendrotoxin block was not affected by channel activation, charybdotoxin exhibited additional accumulation of block following activation, which was relieved with a time constant of 0.5 s upon repolarization of the membrane. The deactivation of this channel was accelerated in the presence of charybdotoxin while not significantly affected by dendrotoxin.

Angiotensin receptors in the brain

Angiotensin receptors have recently become a focus of scientific interest due to the recent development of specific receptor ligands which allow to distinguish between various angiotensin II receptor subtypes, notably the angiotensin II type 1 receptor (AT1) and angiotensin II type 2 receptor (AT2). Although both receptors belong to the seven transmembrane domain receptor family they feature less than 35% homology and differ in their signal transduction mechanisms and in the effects mediated. In the brain, both angiotensin receptor types and probably some further subtypes are present and have been localized in distinct regions. In the adult brain, the AT1 receptor dominates by far and is responsible for most of the known central actions of angiotensin peptides, for example blood pressure increase, release of vasopressin from the pituitary gland, natriuresis, drinking and induction of immediate early genes in distinct brain areas. Some of the AT1 receptor-mediated effects have been shown to be enhanced by blockade of AT2 receptors in the brain suggesting that the central AT2 receptor can exert an inhibitory control on AT1 receptor-mediated actions in the brain.

Histamine modulates three types of K+ current in a human intestinal epithelial cell line

K+ conductance species in a human intestinal epithelial cell line (Intestine 407) were studied in connection with their sensitivities to an intestinal secretagogue, histamine, using the tight-seal whole-cell patch-clamp technique. Applications of positive command pulses rapidly induced outward K+ currents. The conductance became progressively larger with increasing command voltages, exhibiting an outwardly rectifying current voltage relation. Inward K+ currents were also rapidly activated upon applications of hyperpolarizing pulses at potentials negative to the equilibrium potential of K+ (EK), and the conductance inwardly rectified. Application of a Ca2+ ionophore, ionomycin, brought about activation of additional K+ currents. An inhibitor of protein kinase C, polymyxin B, did not affect the ionomycin-induced response. Histamine (10-200 microM) also activated a similar K+ current which was abolished by cytosolic Ca2+ chelation. Under conditions where Ca2+ mobilization was minimized, histamine was found to significantly augment inwardly rectifying K+, but suppress outwardly rectifying K+, currents. Polymyxin B blocked these effects of histamine. An activator of protein kinase C, 1-oleoyl-2-acetylglycerol, mimicked the histamine effects. It is concluded that the intestinal epithelial cell has three distinct types of K+ conductance and that histamine modulates not only Ca(2+)-activated K+ conductance via Ca2+ mobilization, but also inward- and outward-rectifier K+ conductances via activation of protein kinase C.

Regulation of a voltage-dependent, calcium-activated K conductance by cyclic GMP in dissociated flounder enterocytes

Enterocytes from the winter flounder (Pseudopleuronectes americanus) were isolated by collagenase digestion and maintained in flounder Ringer's solution. Whole cell currents were studied using the amphotericin-perforated whole-cell patch clamp technique. The mean resting membrane potential and capacitance values or dissociated cells were -45 +/- 7 mV and 5 +/- 0.4 pF, respectively. Enterocytes held at -20 mV and treated with 1 mumol.l-1 ionomycin exhibited outward currents when cells were stepped through a series of voltages from -60 to +110 mV. The reversal potential of this current in flounder Ringer's solution was -55 mV and the voltage at which half-maximal activation occurred was +20 mV. Voltage-dependent inhibition of outward current was observed at +60 mV and above. When cells were bathed in symmetric K Ringer's solution the reversal potential shifted to zero mV and no inhibition of current was observed at voltages between -60 and 140 mV. When the holding potential of the cell was changed from -20 to -80 mV and stepped from -60 to +110 mV, a second [previously characterized, O'Grady et al. (1991)] K current with delayed-rectifier properties was identified. This observation demonstrated that the delayed rectifier K channel and the Ca(2+)-activated K channel described in this study exist in the same cell. Extracellular addition of 2 mmol.l-1 Ba2+ to cells bathed in symmetric K Ringer's solution resulted in nearly complete inhibition of outward current. Charybdotoxin produced only minor effects on this current.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-mediated agonists activate an inwardly rectified K+ channel in colonic secretory cells

Single-channel recording techniques were used to identify and characterize the K+ channel activated by Ca(2+)-mediated secretory agonists in T84 cells. Carbachol (CCh; 100 microM) and taurodeoxycholate (TDC; 0.75 mM) stimulated oscillatory outward K+ currents. With K gluconate in bath and pipette, cell-attached single-channel K+ currents stimulated by CCh and ionomycin (2 microM) were inwardly rectified and reversed at 0 mV. The single-channel chord conductance was 32 pS at -90 mV and 14 pS at +90 mV. Similar properties were observed in excised inside-out patches in symmetric K+, permitting further characterization of channel properties. Partial substitution of bath or pipette K+ with Na+ gave a K(+)-to-Na+ selectivity ratio of 5.5:1. Channel activity increased with increasing bath Ca2+ concentration in the physiological range of 50-800 nM. Maximal channel activity occurred at intracellular pH 7.2 and decreased at more acidic or alkaline pH values. Extracellular charybdotoxin (CTX; 50 nM) blocked inward but not outward currents. Extracellular tetraethylammonium (TEA; 10 mM) reduced single-channel amplitude at all voltages. No apparent block of the channel was observed with extracellular Ba2+ (1 mM), apamin (1 microM), 4-aminopyridine (4-AP; 4 mM), quinine (500 microM), or glyburide (10 microM). Cytosolic quinine and 4-AP blocked both inward and outward currents, whereas Ba2+ blocked only outward currents. Apamin, CTX, TEA, and glyburide did not affect channel activity. The agonist activation and pharmacological profile of this inwardly rectified K+ channel indicate that it is responsible for the increase in basolateral K+ conductance stimulated by Ca(2+)-mediated agonists in T84 cells.

Potassium currents and effects of vitamin D-3 metabolites and cyclic GMP in rat osteoblastic cells

A K+ current (IK1), activated by depolarization above -20 mV, showing voltage-dependent inactivation within a few seconds and reduced by 40% by 1 mM TEA, was observed in all cells. In a few cells, we also observed a progressive K(+)-current increase during cell dialysis. The developing current (IK2) was not sensitive to 1 mM TEA and did not inactivate. It was detectable over the whole voltage range and slowly increased during 10 s depolarizations. 1,25-(OH)2D3 and 24,25-(OH)2D3 did not affect IK1, but induced a small K(+)-current increase in some cells showing no IK2. This effect was not mimicked by cyclic GMP analogs which, on the contrary, induced a K(+)-current decrease. 24,25-(OH)2D3 (even at 10(-11)M, but not 1,25-(OH)2D3, strongly reduced IK2. The results further document the diversity of voltage-gated currents of osteoblastic cells, confirm the existence of immediate effects of vitamin D-3 metabolites, which are independent of classical 1,25-(OH)2D3 receptors.