Current Pharmaceutical Design - Volume 12, Issue 18, 2006

Ion channels play a pivotal role in transmembrane cell signaling. Their involvement in a variety of physiological functions and in diseases means that they are an important target for therapeutic intervention. This issue of Current Pharmaceutical Design is dedicated to the topic "Ion Channels as a Target for Drug Design". In the first article, Nelson et al. [1] review low-voltage activated, or T-type calcium channels in synaptic integration and in nociception. These T-type calcium channels are potential targets for the therapy of epilepsy and pain. In the second article, Panyi et al. [2] discuss the role of Kv1.3 and calcium-activated potassium channels (IKCa1) in T cell activation and the development of their inhibitors. Such inhibitors have therapeutic potential for a number of diseases that involve T cell activation such as multiple sclerosis and type I diabetes mellitus. In the third article, Sagnella and Swift [3] review a renal epithelial sodium channel and its role in hypertension, one of the main causes of mortality in industrialized nations. In the fourth article, Verkman et al. [4] describe the identification of small molecule inhibitors of cystic fibrosis transmembrane conductance regulator (CFTR) and the activators of a common mutant of CFTR causing cystic fibrosis. These inhibitors can potentially be used for the therapy of secretory diarrheas, while activators can be employed for the therapy of cystic fibrosis. In the fifth article, Shoshan-Barmatz et al. [5] give an account on the mitochondrial voltage-dependent anion channel (VDAC) including features of channel activity and the role of VDAC in apoptosis. In the sixth article, Thomas et al. [6] review the human ether-a-go-go-related gene (hERG) potassium channels, which carry the rapid component of the cardiac repolarization current, and hence cardiac rhythm. This article covers many aspects of hERG channels including some novel antiarrhythmic strategies that involve modulating the cardiac IKr current carried by hERG channels. It is now known that most pore-forming α-subunits of ion channels require some ancillary subunits in the channel complex in order to serve specific physiological roles in vivo. In the final article, Panaghie and Abbott [7] review how some ancillary subunits modulate the functional attributes and pharmacology of some voltage-gated potassium channels, which are important in the repolarization of all excitable cells. In particular, the impact of ancillary subunits on the development of therapeutics targeting ion channels is discussed. We would like to thank all of the contributors - it is their commitment that has made this issue possible. References [1] Nelson MT, Todorovic SM, Perez-Reyes E. The Role of T-type Calcium Channels in Epilepsy and Pain. Curr Pharm Des 2006; 12(18): 2189-2197. [2] Panyi G, Possani LD, Rodríguez de la Vega RC, Gáspár R, Varga Z. K+ Channel Blockers: Novel Tools to Inhibit T Cell Activation Leading to Specific Immunosuppression. Curr Pharm Des 2006; 12(18): 2199-2220. [3] Sagnella GA, Swift PA. The Renal Epithelial Sodium Channel: Genetic Heterogeneity and Implications for the Treatment of High Blood Pressure. Curr Pharm Des 2006; 12(18): 2221-2234. [4] Verkman AS, Lukacs GL, Galietta LJV. CFTR Chloride Channel Drug Discovery - Inhibitors as Antidiarrheals and Activators for Therapy of Cystic Fibrosis. Curr Pharm Des 2006; 12(18): 2235-2247. [5] Shoshan-Barmatz V, Israelson A, Brdiczka D, Sheu SS. The Voltage-Dependent Anion Channel (VDAC): Function in Intracellular Signalling, Cell Life and Cell Death. Curr Pharm Des 2006; 12(18): 2249-2270. [6] Thomas D, Karle CA, Kiehn J. The Cardiac hERG/IKr Potassium Channel as Pharmacological Target: Structure, Function, Regulation, and Clinical Applications. Curr Pharm Des 2006; 12(18): 2271-2283. [7] Panaghie G, Abbott GW. The Impact of Ancillary Subunits on Small-Molecule Interactions with Voltage-Gated Potassium Channels. Curr Pharm Des 2006; 12(18): 2285-2302.

T-type calcium channels open in response to small depolarizations of the plasma membrane. The entry of two positive charges with every calcium ion leads to a further depolarization of the membrane, the low threshold spike, and opening of channels that have a higher threshold. In this manner, T-channels play an important pacemaker role in gating the activity of Na+ and Ca2+ channels. T-channels are preferentially expressed in dendrites, suggesting they play important roles in synaptic integration. Pharmacological evidence indicates that they are expressed in the receptive fields of sensory neurons, suggesting they play a primary role in nociception. Molecular cloning of the three T-channel genes has allowed detailed studies on their channel properties, pharmacology, distribution in the brain, up-regulation in animal models of disease, and provided the tools to screen for novel drugs. Studies on transgenic animals have provided the proof-ofconcept that T-channels are important drug targets for the treatment of absence epilepsy and neuropathic pain. Mutations in ion channel genes, or channelopathies, have been found in many diseases. Similarly, T-channel gene mutations have been found in patients with childhood absence epilepsy. Considering the important role T-channels play in the thalamus, it is likely that T-channel mutations also contribute to a wider range of disorders characterized by thalamocortical dysrhythmia.

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.

The renal epithelial sodium channel (ENaC) is of fundamental importance in the control of sodium reabsorption through the distal nephron. ENaC is an important component in the overall control of sodium balance, blood volume and thereby of blood pressure. This is clearly demonstrated by rare genetic disorders of sodium channel activity (Liddle's Syndrome and Pseudohypoaldosteronism type 1 associated with contrasting effects on blood pressure). Subtle dysregulation of ENaC however may also be important in essential hypertension - a common condition and a major cause of cardiovascular morbidity and mortality. The epithelial sodium channel is formed from three partly homologous subunits. In this review we deals firstly with current views of structural and functional features of the renal epithelial sodium channel with particular emphasis on mechanisms and processes involved in the control of sodium channel activity at the biochemical and cellular levels. We then focus on genetic aspects with reference to the significance of genetic variation in the sodium channel genes in relation to blood pressure. In particular, we review recent investigations on the potential clinical significance of mutations within the genes encoding ENaC subunits in individuals with high blood pressure. Lastly, we also examine the potential value of pharmacological targeting of the renal epithelial sodium channel with the sodium channel inhibitor amiloride for the treatment of hypertension.

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a cAMP-activated chloride channel expressed in epithelia in the lung, intestine, pancreas, testis and other tissues, where it facilitates transepithelial fluid transport. In the intestine CFTR provides the major route for chloride secretion in certain diarrheas. Mutations in CFTR cause the hereditary disease cystic fibrosis, where chronic lung infection and deterioration in lung function cause early death. CFTR is a well-validated targeted for development of inhibitors for therapy of secretory diarrheas and activators for therapy in cystic fibrosis. Our lab has identified and optimized small molecule inhibitors of CFTR, as well as activators of ΔF508-CFTR, the most common mutant CFTR causing cystic fibrosis. High-throughput screening of small molecule collections utilizing a cell-based fluorescence assay of halide transport yielded thiazolidinone and glycine hydrazide CFTR inhibitors that block enterotoxin-mediated secretory diarrhea in rodent models, including a class of non-absorbable inhibitors that target the CFTR pore at its external entrance. Benzothiophene, phenylglycine and sulfonamide potentiators were identified that correct the defective gating of ΔF508-CFTR chloride channels, and other small molecules that correct its defective cellular processing. Small molecule modulators of CFTR function may be useful in the treatment of cystic fibrosis, secretory diarrhea and polycystic kidney disease.

Research over the last decade has extended the prevailing view of mitochondria to include functions well beyond the critical bioenergetics role in supplying ATP. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organelle communication, aging, many diseases, cell proliferation and cell death. Apoptotic signal transmission to the mitochondria results in the efflux of a number of potential apoptotic regulators to the cytosol that trigger caspase activation and lead to cell destruction. Accumulating evidence indicates that the voltagedependent anion channel (VDAC) is involved in this release of proteins via the outer mitochondrial membrane. VDAC in the outer mitochondrial membrane is in a crucial position in the cell, forming the main interface between the mitochondrial and the cellular metabolisms. VDAC has been recognized as a key protein in mitochondria-mediated apoptosis since it is the proposed target for the pro- and anti-apoptotic Bcl2-family of proteins and due to its function in the release of apoptotic proteins located in the inter-membranal space. The diameter of the VDAC pore is only about 2.6-3 nm, which is insufficient for passage of a folded protein like cytochrome c. New work suggests pore formation by homooligomers of VDAC or hetero-oligomers composed of VDAC and pro-apoptotic proteins such as Bax or Bak. This review provides insights into the central role of VDAC in cell life and death and emphasizes its function in the regulation of mitochondria- mediated apoptosis and, thereby, its potential as a rational target for new therapeutics.

Human ether-a-go-go-related gene (hERG) potassium channels conduct the rapid component of the delayed rectifier potassium current, IKr, which is crucial for repolarization of cardiac action potentials. Moderate hERG blockade may produce a beneficial class III antiarrhythmic effect. In contrast, a reduction in hERG currents due to either genetic defects or adverse drug effects can lead to hereditary or acquired long QT syndromes characterized by action potential prolongation, lengthening of the QT interval on the surface ECG, and an increased risk for “"torsade de pointes" arrhythmias and sudden death. This undesirable side effect of non-antiarrhythmic compounds has prompted the withdrawal of several blockbuster drugs from the market. Studies on mechanisms of hERG channel inhibition provide significant insights into the molecular factors that determine state-, voltage-, and use-dependency of hERG current block. In addition, crucial properties of the high-affinity drug binding site in hERG and its interaction with drug molecules have been identified, providing the basis for more refined approaches in drug design, safety pharmacology and in silico modeling. Recently, mutations in hERG have been shown to cause current increase and hereditary short QT syndrome with a high risk for lifethreatening arrhythmias. Finally, the discovery of adrenergic mechanisms of hERG channel regulation as well as the development of strategies to enhance hERG currents and to modify intracellular hERG protein processing may provide novel antiarrhythmic options in repolarization disorders. In conclusion, the increasing understanding of hERG channel function and molecular mechanisms of hERG current regulation could improve prevention and treatment of hERGassociated cardiac repolarization disorders.

Voltage-gated potassium channels (Kv channels) are the major determinants of cellular repolarization in excitable cells - they open in response to depolarization and facilitate selective efflux of potassium ions across the plasma membrane. Because of the importance of exquisitely timed cellular repolarization in controlling action potential morphology and duration, Kv channels are attractive therapeutic targets, particularly for drugs aimed at controlling aberrant electrical excitability such as is observed in cardiac arrhythmia and epilepsy. While the pore-forming a subunits of Kv channels are sufficient to form functional channels, a host of cytoplasmic and transmembrane ancillary subunits modulate their trafficking, function and regulation in vivo. Here, we consider the impact of ancillary subunits on Kv channel pharmacology, and discuss how increased understanding of the roles of ancillary subunits in native Kv channel complexes will lead to development of safer, more specific and more efficacious therapeutic small molecules.

Neuroblastoma (NB) is the most frequent extra-cranial solid tumor and the first cause of lethality in pre-school age children. NB accounts for 9-10% of pediatric tumors and affects more than ten thousand children a year. It originates from the sympathetic nervous system and is characterized by heterogeneous pathological and clinical presentation. Stage 4 NB represents approximately 50% of the cases and shows metastatic dissemination at onset; its prognosis is grim, with 20% of the patients surviving at 5 years from diagnosis in spite of aggressive chemotherapy with autologous hematopoietic stem cell support. Novel therapeutic strategies are urgently needed to improve the prognosis of stage 4 NB patients. Here we review the most promising approaches to NB treatment that have already reached clinical testing or have proved to be successful in preclinical models of the disease. All of these approaches are molecularly guided, since their rational development has benefited from the enormous amount of information on the biology of neuroblastoma gathered through molecular biology and genetics studies. The following topics are reviewed: MYCN oncogene amplification as parameter for therapeutic decision, minimal residual disease, immunotherapy, gene therapy, differentiation and apoptotic therapy, anti-angiogenic therapy, gene expression profiling as tool for generating novel therapeutic approaches. Although several efforts are still needed to reach a significant cure of patients with neuroblastoma, molecularly guided approaches have opened new ways to neuroblastoma treatment and can represent useful models for other cancers of either childhood or adulthood.

The endocannabinoids (eCBs) anandamide and 2-arachidonoylglycerol are important retrograde messengers that inhibit neurotransmitter release via presynaptic CB1 receptors. In addition, cannabinoids are known to modulate the cell death/survival decision of different neural cell types, leading to different outcomes that depend on the nature of the target cell and its proliferative/differentiation status. Thus, cannabinoids protect primary neurons, astrocytes and oligodendrocytes from apoptosis, whereas transformed glial cells are prone to apoptosis by cannabinoid challenge. Moreover, a potential role of the eCB system in neurogenesis and neural differentiation has been proposed. Recent research shows that eCBs stimulate neural progenitor proliferation and inhibit hippocampal neurogenesis in normal adult brain. Cannabinoids inhibit cortical neuron differentiation and promote glial differentiation. On the other hand, experiments with differentiated neurons have shown that cannabinoids also regulate neuritogenesis, axonal growth and synaptogenesis. These new observations support that eCBs constitute a new family of lipid signaling cues responsible for the regulation of neural progenitor proliferation and differentiation, acting as instructive proliferative signals through the CB1 receptor.