Using automated patch clamp electrophysiology platforms in ion channel drug discovery: an industry perspective

INTRODUCTION

Automated patch clamp (APC) is now well established as a mature technology for ion channel drug discovery in academia, biotech and pharma companies, and in contract research organizations (CRO), for a variety of applications including channelopathy research, compound screening, target validation and cardiac safety testing.

AREAS COVERED

Ion channels are an important class of drugged and approved drug targets. The authors present a review of the current state of ion channel drug discovery along with new and exciting developments in ion channel research involving APC. This includes topics such as native and iPSC-derived cells in ion channel drug discovery, channelopathy research, organellar and biologics in ion channel drug discovery.

EXPERT OPINION

It is our belief that APC will continue to play a critical role in ion channel drug discovery, not only in 'classical' hit screening, target validation and cardiac safety testing, but extending these applications to include high throughput organellar recordings and optogenetics. In this way, with advancements in APC capabilities and applications, together with high resolution cryo-EM structures, ion channel drug discovery will be re-invigorated, leading to a growing list of ion channel ligands in clinical development.

TRPV6 Calcium Channel Targeting by Antibodies Raised against Extracellular Epitopes Induces Prostate Cancer Cell Apoptosis

The TRPV6 calcium channel is known to be up-regulated in various tumors. The efforts to target the TRPV6 channel in vivo are still ongoing to propose an effective therapy against cancer. Here, we report the generation of two antibodies raised against extracellular epitopes corresponding to the extracellular loop between S1 and S2 (rb79) and the pore region (rb82). These antibodies generated a complex biphasic response with the transient activation of the TRPV6 channel. Store-operated calcium entry was consequently potentiated in the prostate cancer cell line LNCaP upon the treatment. Both rb79 and rb82 antibodies significantly decreased cell survival rate in a dose-dependent manner as compared to the control antibodies of the same isotype. This decrease was due to the enhanced cell death via apoptosis revealed using a sub-G1 peak in a cell cycle assay, TUNEL assay, and a Hoechst staining, having no effects in the PC3M^trpv6-/- cell line. Moreover, all TUNEL-positive cells had TRPV6 membrane staining as compared to the control antibody treatment where TRPV6-positive cells were all TUNEL negative. These data clearly demonstrate that TRPV6 channel targeting using rb79 and rb82 antibodies is fatal and may be successfully used in the anticancer therapies.

ORAI3 silencing alters cell proliferation and promotes mitotic catastrophe and apoptosis in pancreatic adenocarcinoma

Changes in cytosolic free Ca2+ concentration play a central role in many fundamental cellular processes including muscle contraction, neurotransmission, cell proliferation, differentiation, gene transcription and cell death. Many of these processes are known to be regulated by store-operated calcium channels (SOCs), among which ORAI1 is the most studied in cancer cells, leaving the role of other ORAI channels yet inadequately addressed. Here we demonstrate that ORAI3 channels are expressed in both normal (HPDE) and pancreatic ductal adenocarcinoma (PDAC) cell lines, where they form functional channels, their knockdown affecting store operated calcium entry (SOCE). More specifically, ORAI3 silencing increased SOCE in PDAC cell lines, while decreasing SOCE in normal pancreatic cell line. We also show the role of ORAI3 in proliferation, cell cycle, viability, mitotic catastrophe and cell death. Finally, we demonstrate that ORAI3 silencing impairs pancreatic tumor growth and induces cell death in vivo, suggesting that ORAI3 could represent a potential therapeutic target in PDAC treatment.

Testosterone-androgen receptor: The steroid link inhibiting TRPM8-mediated cold sensitivity

Recent studies have revealed gender differences in cold perception, and pointed to a possible direct action of testosterone (TST) on the cold-activated TRPM8 (Transient Receptor Potential Melastatin Member 8) channel. However, the mechanisms by which TST influences TRPM8-mediated sensory functions remain elusive. Here, we show that TST inhibits TRPM8-mediated mild-cold perception through the noncanonical engagement of the Androgen Receptor (AR). Castration of both male rats and mice increases sensitivity to mild cold, and this effect depends on the presence of intact TRPM8 and AR. TST in nanomolar concentrations suppresses whole-cell TRPM8-mediated currents and single-channel activity in native dorsal root ganglion (DRG) neurons and HEK293 cells co-expressing recombinant TRPM8 and AR, but not TRPM8 alone. AR cloned from rat DRGs shows no difference from standard AR. However, biochemical assays and confocal imaging reveal the presence of AR on the cell surface and its interaction with TRPM8 in response to TST, leading to an inhibition of channel activity.

Ion channels in the regulation of autophagy

Autophagy is a cellular process in which the cell degrades and recycles its own constituents. Given the crucial role of autophagy in physiology, deregulation of autophagic machinery is associated with various diseases. Hence, a thorough understanding of autophagy regulatory mechanisms is crucially important for the elaboration of efficient treatments for different diseases. Recently, ion channels, mediating ion fluxes across cellular membranes, have emerged as important regulators of both basal and induced autophagy. However, the mechanisms by which specific ion channels regulate autophagy are still poorly understood, thus underscoring the need for further research in this field. Here we discuss the involvement of major types of ion channels in autophagy regulation.

Ion channels in the regulation of apoptosis

Apoptosis, a type of genetically controlled cell death, is a fundamental cellular mechanism utilized by multicellular organisms for disposal of cells that are no longer needed or potentially detrimental. Given the crucial role of apoptosis in physiology, deregulation of apoptotic machinery is associated with various diseases as well as abnormalities in development. Acquired resistance to apoptosis represents the common feature of most and perhaps all types of cancer. Therefore, repairing and reactivating apoptosis represents a promising strategy to fight cancer. Accumulated evidence identifies ion channels as essential regulators of apoptosis. However, the contribution of specific ion channels to apoptosis varies greatly depending on cell type, ion channel type and intracellular localization, pathology as well as intracellular signaling pathways involved. Here we discuss the involvement of major types of ion channels in apoptosis regulation. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Orai1 and STIM1 mediate SOCE and contribute to apoptotic resistance of pancreatic adenocarcinoma

The store-operated calcium channels (SOCs) represent one of the major calcium-entry pathways in non-excitable cells. SOCs and in particular their major components ORAI1 and STIM1 have been shown to be implicated in a number of physiological and pathological processes such as apoptosis, proliferation and invasion. Here we demonstrate that ORAI1 and STIM1 mediate store-operated calcium entry (SOCE) in pancreatic adenocarcinoma cell lines. We show that both ORAI1 and STIM1 play pro-survival anti-apoptotic role in pancreatic adenocarcinoma cell lines, as siRNA-mediated knockdown of ORAI1 and/or STIM1 increases apoptosis induced by chemotherapy drugs 5-fluorouracil (5-FU) or gemcitabine. We also demonstrate that both 5-FU and gemcitabine treatments increase SOCE in Panc1 pancreatic adenocarcinoma cell line via upregulation of ORAI1 and STIM1. Altogether our results reveal the novel calcium-dependent mechanism of action of the chemotherapy drugs 5-FU and gemcitabine and emphasize the anti-apoptotic role of ORAI1 and STIM1 in pancreatic adenocarcinoma cells. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Calcium-permeable ion channels in control of autophagy and cancer

Autophagy, or cellular self-eating, is a tightly regulated cellular pathway the main purpose of which is lysosomal degradation and subsequent recycling of cytoplasmic material to maintain normal cellular homeostasis. Defects in autophagy are linked to a variety of pathological states, including cancer. Cancer is the disease associated with abnormal tissue growth following an alteration in such fundamental cellular processes as apoptosis, proliferation, differentiation, migration and autophagy. The role of autophagy in cancer is complex, as it can promote both tumor prevention and survival/treatment resistance. It's now clear that modulation of autophagy has a great potential in cancer diagnosis and treatment. Recent findings identified intracellular calcium as an important regulator of both basal and induced autophagy. Calcium is a ubiquitous secondary messenger which regulates plethora of physiological and pathological processes such as aging, neurodegeneration and cancer. The role of calcium and calcium-permeable channels in cancer is well-established, whereas the information about molecular nature of channels regulating autophagy and the mechanisms of this regulation is still limited. Here we review existing mechanisms of autophagy regulation by calcium and calcium-permeable ion channels. Furthermore, we will also discuss some calcium-permeable channels as the potential new candidates for autophagy regulation. Finally we will propose the possible link between calcium permeable channels, autophagy and cancer progression and therapeutic response.

Complex modulation of TRPM8 cold receptor by volatile anaesthetics and role in complications of general anaesthesia

The mechanisms by which volatile general anaesthetics (VAs) produce a depression of central nervous system are beginning to be better understood, but little is known about a number of side effects. Here, we show that the cold receptor, TRPM8, is complexly modulated by clinical concentration of VAs in dorsal root ganglion neurons and HEK-293 cells heterologously expressing TRPM8. VAs produced transient enhancement of TRPM8 via a depolarizing shift of its activation towards physiological membrane potentials, followed by a sustained TRPM8 inhibition. Stimulatory action of VAs engaged molecular determinants distinct from those used by the TRPM8 agonist. Transient TRPM8 activation by VAs could explain such side effects as inhibition of respiratory drive, shivering and cooling sensation during the beginning of anaesthesia, whereas the second phase of VA action associated with sustained TRPM8 inhibition may be responsible for hypothermia. Consistent with this, both hypothermia and inhibition of respiratory drive induced by VAs are partially abolished in TRPM8-null animals. Thus, we propose TRPM8 as a new clinical target for diminishing common and serious complications of general anaesthesia.

TRPV6 determines the effect of vitamin D3 on prostate cancer cell growth

Despite remarkable advances in the therapy and prevention of prostate cancer it is still the second cause of death from cancer in industrialized countries. Many therapies initially shown to be beneficial for the patients were abandoned due to the high drug resistance and the evolution rate of the tumors. One of the prospective therapeutical agents even used in the first stage clinical trials, 1,25-dihydroxyvitamin D3, was shown to be either unpredictable or inefficient in many cases. We have already shown that TRPV6 calcium channel, which is the direct target of 1,25-dihydroxyvitamin D3 receptor, positively controls prostate cancer proliferation and apoptosis resistance (Lehen'kyi et al., Oncogene, 2007). However, how the known 1,25-dihydroxyvitamin D3 antiproliferative effects may be compatible with the upregulation of pro-oncogenic TRPV6 channel remains a mystery. Here we demonstrate that in low steroid conditions 1,25-dihydroxyvitamin D3 upregulates the expression of TRPV6, enchances the proliferation by increasing the number of cells entering into S-phase. We show that these pro-proliferative effects of 1,25-dihydroxyvitamin D3 are directly mediated via the overexpression of TRPV6 channel which increases calcium uptake into LNCaP cells. The apoptosis resistance of androgen-dependent LNCaP cells conferred by TRPV6 channel is drastically inversed when 1,25-dihydroxyvitamin D3 effects were combined with the successful TRPV6 knockdown. In addition, the use of androgen-deficient DU-145 and androgen-insensitive LNCaP C4-2 cell lines allowed to suggest that the ability of 1,25-dihydroxyvitamin D3 to induce the expression of TRPV6 channel is a crucial determinant of the success or failure of 1,25-dihydroxyvitamin D3-based therapies.

Complex regulation of the TRPM8 cold receptor channel: role of arachidonic acid release following M3 muscarinic receptor stimulation

Cold/menthol-activated TRPM8 (transient receptor potential channel melastatin member 8) is primarily expressed in sensory neurons, where it constitutes the principal receptor of environmental innocuous cold. TRPM8 has been shown to be regulated by multiple influences such as phosphorylation, pH, Ca(2+), and lipid messengers. One such messenger is arachidonic acid (AA), which has been shown to inhibit TRPM8 channel activity. However, the physiological pathways mediating the inhibitory effect of AA on TRPM8 still remain unknown. Here, we demonstrate that TRPM8 is regulated via M3 muscarinic acetylcholine receptor-coupled signaling cascade based on the activation of cytosolic phospholipase A2 (cPLA2) and cPLA2-catalyzed derivation of AA. Stimulation of M3 receptors heterologously co-expressed with TRPM8 in HEK-293 cells by nonselective muscarinic agonist, oxotremorine methiodide (Oxo-M), caused inhibition of TRPM8-mediated membrane current, which could be mimicked by AA and antagonized by pharmacological or siRNA-mediated cPLA2 silencing. Our results demonstrate the intracellular functional link between M3 receptor and TRPM8 channel via cPLA2/AA and suggest a novel physiological mechanism of arachidonate-mediated regulation of TRPM8 channel activity through muscarinic receptors. We also summarize the existing TRPM8 regulations and discuss their physiological and pathological significance.

The transient receptor potential channel TRPM8 is inhibited via the alpha 2A adrenoreceptor signaling pathway

The transient receptor potential channel melastatin member 8 (TRPM8) is expressed in sensory neurons, where it constitutes the main receptor of environmental innocuous cold (10-25 degrees C). Among several types of G protein-coupled receptors expressed in sensory neurons, G(i)-coupled alpha 2A-adrenoreceptor (alpha 2A-AR), is known to be involved in thermoregulation; however, the underlying molecular mechanisms remain poorly understood. Here we demonstrated that stimulation of alpha 2A-AR inhibited TRPM8 in sensory neurons from rat dorsal root ganglia (DRG). In addition, using specific pharmacological and molecular tools combined with patch-clamp current recordings, we found that in heterologously expressed HEK-293 (human embryonic kidney) cells, TRPM8 channel is inhibited by the G(i) protein/adenylate cyclase (AC)/cAMP/protein kinase A (PKA) signaling cascade. We further identified the TRPM8 S9 and T17 as two key PKA phosphorylation sites regulating TRPM8 channel activity. We therefore propose that inhibition of TRPM8 through the alpha 2A-AR signaling cascade could constitute a new mechanism of modulation of thermosensation in both physiological and pathological conditions.

I-II loop structural determinants in the gating and surface expression of low voltage-activated calcium channels

The intracellular loops that interlink the four transmembrane domains of Ca²⁺- and Na⁺-channels (Ca_v, Na_v) have critical roles in numerous forms of channel regulation. In particular, the intracellular loop that joins repeats I and II (I–II loop) in high voltage-activated (HVA) Ca²⁺ channels possesses the binding site for Ca_vβ subunits and plays significant roles in channel function, including trafficking the α₁ subunits of HVA channels to the plasma membrane and channel gating. Although there is considerable divergence in the primary sequence of the I–II loop of Ca_v1/Ca_v2 HVA channels and Ca_v3 LVA/T-type channels, evidence for a regulatory role of the I–II loop in T-channel function has recently emerged for Ca_v3.2 channels. In order to provide a comprehensive view of the role this intracellular region may play in the gating and surface expression in Ca_v3 channels, we have performed a structure-function analysis of the I–II loop in Ca_v3.1 and Ca_v3.3 channels using selective deletion mutants. Here we show the first 60 amino acids of the loop (post IS6) are involved in Ca_v3.1 and Ca_v3.3 channel gating and kinetics, which establishes a conserved property of this locus for all Ca_v3 channels. In contrast to findings in Ca_v3.2, deletion of the central region of the I–II loop in Ca_v3.1 and Ca_v3.3 yielded a modest increase (+30%) and a reduction (−30%) in current density and surface expression, respectively. These experiments enrich our understanding of the structural determinants involved in Ca_v3 function by highlighting the unique role played by the intracellular I–II loop in Ca_v3.2 channel trafficking, and illustrating the prominent role of the gating brake in setting the slow and distinctive slow activation kinetics of Ca_v3.3.