Effect of lithium on the electrical properties of polycystin-2 (TRPP2)

High calcium transport by Polycystin-2 (TRPP2) induces channel clustering and oscillatory currents

The regulation by Ca²⁺ of Ca²⁺-permeable ion channels represents an important mechanism in the control of cell function. Polycystin-2 (PC2, TRPP2), a member of the TRP channel family (Transient Potential Receptor), is a Ca²⁺ permeable non-selective cation channel. Previous studies from our laboratory demonstrated that physiological concentrations of Ca²⁺ do not regulate in vitro translated PC2 (PC2_iv) channel activity. However, the issue as to PC2's Ca²⁺ permeability and regulation remain ill-defined, in particular because Ca²⁺ transport is usually observed in the presence of other ionic gradients. In this study, we assessed Ca²⁺ transport by PC2_iv in a lipid bilayer reconstitution system in a high Ca²⁺ gradient (CaCl₂ 100 mM cis, CaCl₂ 10 mM trans) in the presence of either 3:7 or 7:3 1-palmitoyl-2-oleoyl-choline and ethanolamine lipid mixtures. Reconstituted PC2_iv showed spontaneous Ca²⁺ currents in both lipid mixtures, with a maximum conductance of 63 ± 13 pS (n = 19) and 105 pS ± 9.8 (n = 9), respectively. In both cases, we best fitted the experimental data with the Goldman-Hodgkin-Katz equation, observing a reversal potential (V_rev ∼ -27 mV) consistent with strict Ca²⁺ selectivity. The R742X mutated PC2 (PC2_R742X), lacking the carboxy terminal domain of the channel showed no differences with wild type PC2. Interestingly, we also observed the onset of spontaneous Ca²⁺ current oscillations whenever PC2-containing samples were reconstituted in the 3:7, but not 7:3 POPC:POPE lipid mixture. The amplitude and frequency of the ionic oscillations were highly dependent on the applied voltage, the imposed Ca²⁺ gradient, and the presence of high Ca²⁺, which induced PC2 channel clustering as observed by atomic force microscopy (AFM). We also used the QuB suite to kinetically model the PC2 channel Ca²⁺ oscillations based on the presence of subconductance states in the channel. The encompassed evidence supports a high Ca²⁺ permeability by PC2, and a novel oscillatory mechanism dependent on the presence of Ca²⁺ and phospholipids that provides the first evidence for the relation between stochasticity and deterministic processes mediated by ion channels.

Polycystin-2 (TRPP2) regulates primary cilium length in LLC-PK1 renal epithelial cells

Polycystin-2 (PC2, TRPP2) is a Ca²⁺ permeable nonselective cation channel whose dysfunction generates autosomal dominant polycystic kidney disease (ADPKD). PC2 is present in different cell locations, including the primary cilium of renal epithelial cells. However, little is known as to whether PC2 contributes to the primary cilium structure. Here, we explored the effect(s) of external Ca²⁺, PC2 channel blockers, and PKD2 gene silencing on the length of primary cilia in wild-type LLC-PK1 renal epithelial cells. Confluent cell monolayers were fixed and immuno-labeled with an anti-acetylated α-tubulin antibody to identify primary cilia and measure their length. Although primary cilia length measurements did not follow a Normal distribution, the data were normalized by Box-Cox transformation rendering statistical differences under all experimental conditions. Cells exposed to high external Ca²⁺ (6.2 mM) decreased a 13.5% (p < 0.001) primary cilia length as compared to controls (1.2 mM Ca²⁺). In contrast, the PC2 inhibitors amiloride (200 μM) and LiCl (10 mM), both increased primary ciliary length by 33.2% (p < 0.001), and 17.4% (p < 0.001), respectively. PKD2 gene silencing by siRNA elicited a statistically significant, 10.3% (p < 0.001) increase in primary cilia length compared to their respective scrambled RNA transfected cells. The data indicate that conditions that regulate PC2 function or gene expression modify the length of primary cilia in renal epithelial cells. Blocking of PC2 mitigates the effects of elevated external Ca²⁺ concentration on primary cilia length. Proper regulation of PC2 function in the primary cilium may be essential in the onset of mechanisms that trigger cyst formation in ADPKD.

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Inward Ca2+ current through the polycystin-2-dependent channels of renal primary cilia

In 15% of cases, autosomal dominant polycystic kidney disease arises from defects in polycystin-2 (PC2). PC2 is a member of the polycystin transient receptor potential subfamily of cation-conducting channels and is expressed in the endoplasmic reticulum and primary cilium of renal epithelial cells. PC2 opposes a procystogenic influence of the cilium, and it has been proposed that this beneficial effect is mediated in part by a flow of Ca²⁺ through PC2 channels into the primary cilium. However, previous efforts to determine the permeability of PC2 channels to Ca²⁺ have yielded widely varying results. Here, we report the mean macroscopic Ca²⁺ influx through native PC2 channels in the primary cilia of mIMCD-3 cells, which are derived from the murine inner medullary collecting duct. Under conditions designed to isolate inward Ca²⁺ currents, a small inward Ca²⁺ current was detected in cilia with active PC2 channels but not in cilia lacking those channels. The current was activated by the addition of 10 µM internal Ca²⁺, which is known to activate ciliary PC2 channels. It was blocked by 10 µM isosakuranetin, which blocks the same channels. On average, the current amplitude was -1.8 pA at -190 mV; its conductance from -50 to -200 mV averaged 20 pS. Thus, native PC2 channels of renal primary cilia are able to conduct a small but detectable Ca²⁺ influx under the conditions tested. The possible consequences of this influx are discussed.NEW & NOTEWORTHY In autosomal dominant polycystic kidney disease, it is proposed that Ca²⁺ entering the primary cilium through polycystin-2 (PC2) channels may limit the formation of cysts. Recent studies predict that any macroscopic Ca²⁺ influx through these channels should be small. We report that the native PC2 channels in primary cilia of cultured renal epithelial cells can allow a small macroscopic calcium influx. This may allow a significant accumulation of Ca²⁺ in the cilium in vivo.

[Renal toxicity of lithium]

Besides its efficiency, lithium has a narrow therapeutic index and can result in considerable toxicity. Among the potential side effects, two types of renal toxicity are observed: a decreased renal concentrating ability and a chronic renal failure. Lithium-induced polyuria is frequent, estimated to affect up to 40% of patients, and develops usually early. It may be irreversible, especially if the treatment has been prescribed for more than 15 years. A chronic renal failure is observed in patients treated for more than 10 to 20 years. Its prevalence is estimated at 12% after 19 years of treatment. Some patients (0.5%) may reach end stage renal disease. The major risk factor is the duration of exposure to lithium. Discussion about stopping or not lithium in case of renal failure needs multidisciplinary expertise and depends on psychiatric status and renal function.

Bundles of Brain Microtubules Generate Electrical Oscillations

Microtubules (MTs) are long cylindrical structures of the cytoskeleton that control cell division, intracellular transport, and the shape of cells. MTs also form bundles, which are particularly prominent in neurons, where they help define axons and dendrites. MTs are bio-electrochemical transistors that form nonlinear electrical transmission lines. However, the electrical properties of most MT structures remain largely unknown. Here we show that bundles of brain MTs spontaneously generate electrical oscillations and bursts of electrical activity similar to action potentials. Under intracellular-like conditions, voltage-clamped MT bundles displayed electrical oscillations with a prominent fundamental frequency at 39 Hz that progressed through various periodic regimes. The electrical oscillations represented, in average, a 258% change in the ionic conductance of the MT structures. Interestingly, voltage-clamped membrane-permeabilized neurites of cultured mouse hippocampal neurons were also capable of both, generating electrical oscillations, and conducting the electrical signals along the length of the structure. Our findings indicate that electrical oscillations are an intrinsic property of brain MT bundles, which may have important implications in the control of various neuronal functions, including the gating and regulation of cytoskeleton-regulated excitable ion channels and electrical activity that may aid and extend to higher brain functions such as memory and consciousness.

Structure of the polycystic kidney disease TRP channel Polycystin-2 (PC2)

Mutations in either polycystin-1 (PC1 or PKD1) or polycystin-2 (PC2, PKD2 or TRPP1) cause autosomal-dominant polycystic kidney disease (ADPKD) through unknown mechanisms. Here we present the structure of human PC2 in a closed conformation, solved by electron cryomicroscopy at 4.2-Å resolution. The structure reveals a novel polycystin-specific 'tetragonal opening for polycystins' (TOP) domain tightly bound to the top of a classic transient receptor potential (TRP) channel structure. The TOP domain is formed from two extensions to the voltage-sensor-like domain (VSLD); it covers the channel's endoplasmic reticulum lumen or extracellular surface and encloses an upper vestibule, above the pore filter, without blocking the ion-conduction pathway. The TOP-domain fold is conserved among the polycystins, including the homologous channel-like region of PC1, and is the site of a cluster of ADPKD-associated missense variants. Extensive contacts among the TOP-domain subunits, the pore and the VSLD provide ample scope for regulation through physical and chemical stimuli.

The native TRPP2-dependent channel of murine renal primary cilia

Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening monogenic renal disease. ADPKD results from mutations in either of two proteins: polycystin-1 (also known as PC1 or PKD1) or transient receptor potential cation channel, subfamily P, member 2 (TRPP2, also known as polycystin-2, PC2, or PKD2). Each of these proteins is expressed in the primary cilium that extends from many renal epithelial cells. Existing evidence suggests that the cilium can promote renal cystogenesis, while PC1 and TRPP2 counter this cystogenic effect. To better understand the function of TRPP2, we investigated its electrophysiological properties in the native ciliary membrane. We recorded directly from the cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin. In one-third of cilia examined, a large-conductance channel was observed. The channel was not permeable to Cl¯ but conducted cations with permeability ratios P_K:P_Ca:P_Na of 1:0.55:0.14. The single-channel conductance ranged from 97 pS in typical physiological solutions to 189 pS in symmetrical 145 mM KCl. Open probability of the channel was very sensitive to membrane depolarization or increasing cytoplasmic free Ca²⁺ in the low micromolar range, with the open probability increasing in either case. Knocking out TRPP2 by CRISPR/Cas9 genome editing eliminated the channel current, establishing it as TRPP2 dependent. Possible mechanisms for activating the TRPP2-dependent channel in the renal primary cilium are discussed.

Electrical Oscillations in Two-Dimensional Microtubular Structures

Microtubules (MTs) are unique components of the cytoskeleton formed by hollow cylindrical structures of αβ tubulin dimeric units. The structural wall of the MT is interspersed by nanopores formed by the lateral arrangement of its subunits. MTs are also highly charged polar polyelectrolytes, capable of amplifying electrical signals. The actual nature of these electrodynamic capabilities remains largely unknown. Herein we applied the patch clamp technique to two-dimensional MT sheets, to characterize their electrical properties. Voltage-clamped MT sheets generated cation-selective oscillatory electrical currents whose magnitude depended on both the holding potential, and ionic strength and composition. The oscillations progressed through various modes including single and double periodic regimes and more complex behaviours, being prominent a fundamental frequency at 29 Hz. In physiological K(+) (140 mM), oscillations represented in average a 640% change in conductance that was also affected by the prevalent anion. Current injection induced voltage oscillations, thus showing excitability akin with action potentials. The electrical oscillations were entirely blocked by taxol, with pseudo Michaelis-Menten kinetics and a KD of ~1.29 μM. The findings suggest a functional role of the nanopores in the MT wall on the genesis of electrical oscillations that offer new insights into the nonlinear behaviour of the cytoskeleton.

Calcium transport and local pool regulate polycystin-2 (TRPP2) function in human syncytiotrophoblast

Polycystin-2 (PC2, TRPP2) is a Ca(2+)-permeable, nonselective cation channel implicated in Ca(2+) transport and epithelial cell signaling. Although PC2 may contribute to Ca(2+) transport in human term placenta, the regulatory mechanisms associated with Ca(2+) handling in this tissue are largely unknown. In this work we assessed the regulation by Ca(2+) of PC2 channel function from a preparation of apical membranes of human syncytiotrophoblast (PC2hst) reconstituted in a lipid bilayer system. Addition of either EGTA or BAPTA to the cis hemi-chamber, representing the cytoplasmic domain of the channel, and lowering Ca(2+) to ∼0.6-0.8 nM, inhibited spontaneous PC2hst channel activity, with a time response dependent on the chelator tested. EGTA reduced PC2hst channel currents by 86%, with a t1/2 = 3.6 min, whereas BAPTA rapidly and completely (100%) eliminated channel activity with a t1/2 = 0.8 min. Subsequent titration with Ca(2+) reversed the inhibition, which followed a Hill-type function with apparent dissociation constants of 1-5 nM, and 4 Ca(2+) binding sites. The degree of inhibition by the cis Ca(2+) chelator largely depended on increasing trans Ca(2+). This was consistent with measurable Ca(2+) transport through the channel, feeding the regulatory sites in the cytoplasmic domain. Interestingly, the reconstituted in vitro translated PC2 (PC2iv) was completely insensitive to Ca(2+) regulation, suggesting that the regulatory sites are not intrinsic to the channel protein. Our findings demonstrate the presence of a Ca(2+) microdomain largely accessible through the channel that controls PC2 function in human syncytiotrophoblast of term placenta.

Contribution of calcium-conducting channels to the transport of zinc ions

Zinc (Zn) is a vital nutrient participating in a myriad of biological processes. The mechanisms controlling its transport through the plasma membrane are far from being completely understood. Two families of eukaryotic zinc transporters are known to date: the Zip (SLC39) and ZnT (SLC30) proteins. In addition, some types of plasmalemmal calcium (Ca)-conducting channels are implied in the cellular uptake of zinc. These ion channels are currently described as systems dedicated to the transport of Ca (and, to some extent, sodium (Na) ions). However, a growing body of evidence supports the view that some of them can also function as pathways for Zn transport. For instance, voltage-gated Ca channels and some types of glutamate-gated receptors have long been known to allow the entry of Zn. More recently, members of the TRP superfamily, another type of Ca-conducting channels, have been shown to permit the uptake of Zn into eukaryotic cells. The aim of this review article is to present the current knowledge supporting the notion that Ca-conducting channels take part in the plasmalemmal transport of Zn.