Current Pharmaceutical Design

ISSN: 1381-6128

Current Pharmaceutical Design
Volume 13, Number 31, 2007

Contents

Ion Channels as a Target for Drug Design
Executive Editor: Kwok-Keung Tai

Editorial Pp. 3168


The Impact of Sub-Cellular Location and Intracellular Neuronal Proteins on Properties of GABAA Receptors Pp. 3169-3177
B. Birnir and E.R. Korpi
[Abstract]


Multiple Modes of A-Type Potassium Current Regulation Pp. 3178--3184
S.-Q. Cai, W. Li and F. Sesti
[Abstract]


The NMDA Receptor/Ion Channel Complex: A Drug Target for Modulating Synaptic Plasticity and Excitotoxicity Pp. 3185-3194
B.C. Albensi
[Abstract]


Developing New Anti-Arrhythmics: Clues from the Molecular Basis of Cardiac Ryanodine Receptor (RyR2) Ca²⁺-Release Channel Dysfunction Pp. 3195-3211
C.H. George and F.A. Lai
[Abstract]


Functional Domains of Aquaporin-1: Keys to Physiology, and Targets for Drug Discovery Pp. 3212-3221
A.J. Yool
[Abstract]


Involvement of Potassium and Chloride Channels and Other Transporters in Volume Regulation by Spermatozoa Pp. 3222-3230
T.G. Cooper and C.H. Yeung
[Abstract]


M2 Protein–A Proton Channel of Influenza A Virus Pp. 3231-3235
T. Betakova
[Abstract]


General Articles


Purinergic (P2) Receptor Control of Lower Genitourinary Tract Function and New Avenues for Drug Action: An Overview Pp. 3236-3244
S.Gur, P.J. Kadowitz and W.J.G. Hellstrom
[Abstract]


Therapeutic Approaches in Vascular Repair Induced by Adult Bone Marrow Cells and Circulating Progenitor Endothelial Cells Pp. 3245-3251
C. Napoli, A. Balestrieri and L.J. Ignarro
[Abstract]


Novel Pharmaceutical Approaches for Treating Patients with Cystic Fibrosis Pp. 3252-3263
Z. Saeed, G. Wojewodka, D. Marion, C. Guilbault and D. Radzioch
[Abstract]




Abstracts


[Back to top]
Editorial: Ion Channels as a Target for Drug Design

Ion channels are a class of integrated membrane proteins that allow selective ion permeation across biological membranes and are vital for signal transductions within the cell and between the cells. Because channels are involved in a variety of physiological functions, they are potential targets for therapeutic intervention. This issue of Current Pharmaceutical Design consists of seven review articles describing the roles of several ion channels in a variety of biological functions.

GABA is a major inhibitory neurotransmitter in the central nervous system. Neuronal activities are inhibited when GABA receptors are activated by GABA. GABA receptors are chloride ion channels and are major therapeutic target for general anesthetics. In the first article, Drs. Birin and Korpi [1] review the structure-function relationship of GABA_A receptors; the differences in the properties between recombinant GABA_A receptors and those in native tissues. The effects of accessory proteins in modulation of GABA_A receptors activities are also discussed. Voltage-gated potassium channels also regulate cell membrane excitability in neurons by controlling the flow of potassium ions through the membrane in response to the changes in membrane potential. In the second article, Drs. Cai and Sesti [2] review the role of β-subunit, phosphorylation in modulation of the A-type potassium channels including the newly discovered mode of regulation of channel activities by enzymatic action of beta-subunits using an invertebrate animal model system.

Sustained stimulation of the NMDA receptor channels in the brain during cerebral hypoxia causes excitotoxic injury. This NMDA receptor-mediated excitotoxic injury could also play a role in the pathogenesis of a number of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. In the third article, Dr. Albensi [3] describes the structure and the role of the NMDA receptor channels in modulation of synaptic plasticity and excitotoxicity; developmental changes of the NMDA receptor channels and the classification of NMDA receptor channel blockers.

Anti-arrhythmic drugs are one of the mechanistically best understood ion channel-targeted therapeutics. Most of the current anti-arrhythmic agents act on cell surface ion channels. Recent evidence have showed that perturbations in intracellular calcium release as a result of acquired or genetic defects in the ryanodine receptor channels located in the sarco/endoplasmic reticulum could trigger cardiac arrhythmias. Drs. George and Lai [4] review the structure of ryanodine receptor channels; the cellular and molecular mechanism of how the dysfunctions of intracellular ryanodine receptor channels causing lethal arrhythmias, and how the advancement of this knowledge enhances the development of novel anti-arrhythmic strategies is discussed.

Water is a main constituent of all living organisms. Water molecules permeate biological membranes through the water channels or aquaporins. Dr. Yool [5] reviews the role of aquaporins in water homeostatsis in the peripheral vascular endothelia and in the brain. She discusses the role of aquaporins in cell migration which is essential for an array of biological processes such as angiogenesis and tumorigenesis. The structure-function relationships and the gating mechanisms of aquaporins are also reviewed. The potential therapeutic implications of molecules that can modulate water permeation through aquaporins are discussed.

In addition to aquaporins, other channels are also involved in regulation of cell volume. Ion channels accomplish this task by regulating the cell osmolality as water is removed osmotically across the cell membrane following the efflux of intracellular ions. One of the best examples is the involvement of voltage-gated potassium channels and chloride channels in the regulation of volume decrease in spermatozoa in response to the osmotic challenge in the female reproductive tract which has a lower osmolality. In the sixth article, Drs. Copper and Yeung [6] review evidence for the involvement of voltage-gated potassium channels and chloride channels to maintain spermatozoa volume which is considered as a potential target for the development of contraception in the future. In addition, a better understanding of this process will improve sperm cells handling and storage techniques for a better therapy of infertile patients.

The M2 protein from the influenza A virus which forms a proton channel in the virion and is essential for viral infection. The M2 protein is therefore a potential target for the development of a new generation of vaccine or antiviral agents. Dr. Betakova [7] reviews the role of the M2 ion channel in virus replication and the structure-function relationship of this channel.

Finally we are very grateful to the authors of these articles for their time and effort. We hope that the readers will find these articles stimulating and will help provoke their thought in ion channel research.

References
[1] Birnir B, Korpi ER. The impact of sub-cellular location and intracellular neuronal proteins on properties of GABA_A receptors. Curr Pharm Des 2007; 13(31): 3169-3177.

[2] Cai S-Q, Li W, Sesti F. Multiple modes of A-type potassium current regulation. Curr Pharm Des 2007; 13(31): 3178-3184.

[3] Albensi BC. The NMDA receptor/ion channel complex: A drug target for modulating synaptic plasticity and excitotoxicity. Curr Pharm Des 2007; 13(31): 3185-3194.

[4] George CH, Lai FA. Developing new anti-arrhythmics: Clues from the molecular basis of cardiac ryanodine receptor (RyR2) Ca²⁺-release channel dysfunction. Curr Pharm Des 2007; 13(31): 3195-3211.

[5] Yool AJ. Functional domains of aquaporin-1: Keys to physiology, and targets for drug discovery. Curr Pharm Des 2007; 13(31): 3212-3221.

[6] Cooper TG, Yeung CH. Involvement of potassium and chloride channels and other transporters in volume regulation by spermatozoa. Curr Pharm Des 2007; 13(31): 3222-3230.

[7] Betakova T. M2 Protein–A proton channel of influenza A virus. Curr Pharm Des 2007; 13(31): 3231-3235.


Kwok-Keung Tai, PhD
Associate Director,
The Parkinson’s and Movement Disorder Research Laboratory,
Long Beach Memorial Medical Center,
Long Beach, California,
USA


[Back to top]
The Impact of Sub-Cellular Location and Intracellular Neuronal Proteins on Properties of GABAA Receptors
B. Birnir and E.R. Korpi

Most studies of GABA_A receptor accessory proteins have focused on trafficking, clustering and phosphorylation state of the channel-forming subunits and as a result a number of proteins and mechanisms have been identified that can influence the GABA_A channel expression and function in the cell plasma membrane. In the light of a growing list of intracellular and transmembrane neuronal proteins shown to affect the fate, function and pharmacology of the GABA_A receptors in neurons, the concept of what constitutes the native GABA_A receptor complex may need to be re-examined. It is perhaps more appropriate to consider the associated proteins or some of them to be parts of the receptor channel complex in the capacity of ancillary proteins. Here we highlight some of the effects the intracel-lular environment has on the GABA-activated channel function and pharmacology. The studies demonstrate the need for co-expression of accessory proteins with the GABA_A channel-forming subunits in heterologous expression systems in order to obtain the full repertoire of GABA_A receptors characteristics recorded in the native neuronal environment. Further studies e.g. on gene-modified animal models are needed for most of the accessory proteins to establish their significance in normal physiology and in pathophysiology of neurological and psychiatric diseases. The challenge remains to elucidate the effects that the accessory proteins and processes (e.g. phosphorylation) plus the sub-cellular location have on the “fine-tuning” of the functional and pharmacological properties of the GABA_A receptor channels.


[Back to top]
Multiple Modes of A-Type Potassium Current Regulation
S.-Q. Cai, W. Li and F. Sesti

Voltage-dependent potassium (K⁺) channels (Kv) regulate cell excitability by controlling the movement of K⁺ ions across the membrane in response to changes in the cell voltage. The Kv family, which includes A-type channels, constitute the largest group of K⁺ channel genes within the superfamily of Na⁺, Ca²⁺ and K⁺ voltage-gated channels. The name “A-type” stems from the typical profile of these currents that results form the opposing effects of fast activation and inactivation. In neuronal cells, A-type currents (I_A), determine the interval between two consecutive action potentials during repetitive firing. In cardiac muscle, A-type currents (I_to), control the initial repolarization of the myocardium. Structurally, A-type channels are tetramers of α-subunits each containing six putative transmembrane domains including a voltage-sensor. A-type channels can be modulated by means of protein-protein interactions with so-called β-subunits that control inactivation voltage sensitivity and other properties, and by post-transcriptional modifications such as phosphorylation or oxidation. Recently a new mode of A-type regulation has been discovered in the form of a class of hybrid β-subunits that posses their own enzymatic activity. Here, we review the biophysical and physiological properties of these multiple modes of A-type channel regulation.


[Back to top]
The NMDA Receptor/Ion Channel Complex: A Drug Target for Modulating Synaptic Plasticity and Excitotoxicity
B.C. Albensi

A recent search on PubMed for the phrase NMDA receptor results in 2,190 hits on this topic for review articles and 20,100 hits for experimental papers. This is a direct reflection of the intensiveness, significance, and complexity associated with the research on this key receptor protein over the last several decades. In this review, we briefly describe the NMDA receptor structure, discuss the role of NMDA receptors in modulating synaptic plasticity and excitotoxicity, explore age-dependent changes in NMDA receptor functioning, and survey interesting NMDA receptor blockers. Given the huge existing literature on the subject, an exhaustive review has not been endeavored. Instead, an attempt was made to point out those studies that have been instrumental in the field or that are of special interest.


[Back to top]
Developing New Anti-Arrhythmics: Clues from the Molecular Basis of Cardiac Ryanodine Receptor (RyR2) Ca²⁺-Release Channel Dysfunction
C.H. George and F.A. Lai

Sudden cardiac death (SCD) remains a major cause of mortality, and despite our knowledge of the causative genetic, molecular and biochemical cellular mechanisms involved, effective therapeutic strategies are lacking. Perturbations in cardiac Ca²⁺ handling promote arrhythmias and there is enormous interest in developing new anti-arrhythmics aimed at correcting Ca²⁺ release dysfunction. In particular, abnormal Ca²⁺ release arising as a result of acquired or genetic defects in cardiac ryanodine receptors (RyR2) has emerged as an important arrhythmogenic trigger in heart failure, and in a devastating genetic arrhythmia syndrome termed catecholaminergic poly-morphic ventricular tachycardia (CPVT). Here, we evaluate how experimental insights into RyR2 structure-function are unravelling the precise molecular basis of channel dysfunction and are advancing the development of new therapeutic strategies. We also discuss the functional role of RyR2 in the context of the exquisite synergism existing between numerous cellular components involved in cardiac Ca²⁺ signalling, and how these complex interactions may be used to design new anti-arrhythmic approaches that target multiple facets of RyR2 regulation.


[Back to top]
Functional Domains of Aquaporin-1: Keys to Physiology, and Targets for Drug Discovery
A.J. Yool

Aquaporins (AQPs) are expressed in physiologically essential tissues and organs in which edema and fluid imbalances are of major concern. Potential roles in brain water homeostasis and edema, angiogenesis, cell migration, development, neuropathological diseases, and cancer suggest that this family of membrane proteins is an attractive set of novel drug targets. A problem in pursuing therapeutic and basic research strategies for dissecting contributions of AQPs to cell and tissue functions is that little is known regarding the pharmacology of AQP channels; currently defined agents such as tetraethylammonium and phloretin as blockers for aquaporins suffer from a lack of specificity and potency. Subtypes of AQPs modulated by signaling pathways could enable discrete localized control of fluid homeostasis, volume and morphology in cells and intracellular organelles, and might be found to participate in many different aspects of physiology, such as the control of paracellular permeability, process extension, growth, migration, and other responses involving changes in cell shape or surface to volume ratios. Recognizing that AQP1 is a water channel and, under permissive conditions, also a cGMP-gated cation channel, evidence in various tissues for a coupling of the cGMP signaling cascade to a physiological outcome that might involve AQP1 dual ion-and-water channel functions is of interest. Groundbreaking advances in defining aquaporin gating mechanisms suggest conformational changes are important elements in regulation and gating across classes of aquaporins. With a rapidly expanding knowledge of aquaporin structure and functional regulation, new avenues for manipulation of aquaporin channels are likely to be discovered. In parallel, a discovery for novel compounds with specificity and potency for aquaporins is a compelling goal. The need for pharmacological agents to dissect the roles of aquaporins in physiological and pathological processes is a clear call for further research in the field.


[Back to top]
Involvement of Potassium and Chloride Channels and Other Transporters in Volume Regulation by Spermatozoa
T.G. Cooper and C.H. Yeung

Spermatozoa produced in the testis undergo maturation in the epididymis which secretes an osmolyte-rich fluid that bathes the sperm cells. These cells need to maintain their volume after ejaculation when they first encounter hypo-osmolal environments of accessory gland fluids and later within the female tract. If they do not, they experience swelling that is manifested in flagellar angulation that prevents their passage through cervical mucus or the uterotubal junction and they never reach the oocytes. This is a cause of male infertility in domestic species and certain infertile transgenic mice in which flagellar angulation has been shown to indicate cell swelling as a consequence of reduced epididymal provision of osmolytes. The reduced volume regulating ability of spermatozoa from subfertile boars and bulls has prompted study of volume regulation of spermatozoa as a possible cause of human male infertility. Understanding this process may make its manipulation possible and could suggest better sperm handling and storage techniques and thus provide therapy for infertile patients. On the other hand, volume regulation is a potential target for contraception if mimicking the conditions expressed by the “sterile studs” were possible.

The evidence for the presence of ion channels probably responsible for regulatory volume decreases in spermatozoa is reviewed here that implicate voltage-gated potassium channels (especially Kv1.5 (KCNA5), minK (KCNE1) and TASK2 (KCNK5)) and the chloride channels CLCN3 and CLNS1A. The involvement of ion co-transporters in volume regulation of spermatozoa is also discussed.


[Back to top]
M2 Protein–A Proton Channel of Influenza A Virus
T. Betakova

Recent outbreaks of highly pathogenic avian influenza A virus infections (H5 and H7 subtypes) in poultry and humans have raised concerns that a new influenza pandemic will occur in near future. Currently, four antivirals have proven efficacy in the treatment and prophylaxis of influenza A infections: two M2 inhibitors (amantadine and rimantadine) and two neuraminidase inhibitors (zanamivir and oseltamivir). Early treatment with antivirals reduces the duration of symptoms and the time to recovery by one to two days. However, when antivirals are used for the treatment the antiviral resistance develops rapidly, limiting their use. There is an urgent need for research on newer antiviral agents and “universal” vaccine against influenza virus. The M2 protein from the influenza A virus forms a proton channel in the virion and is essential for infection. As a relatively conserved protein, the M2 protein seems to be a suitable candidate for development of a new generation of vaccine or antiviral agents. This review describes the role of the M2 ion channel in virus replication and the structure-function relationship of the channel.


[Back to top]
Purinergic (P2) Receptor Control of Lower Genitourinary Tract Function and New Avenues for Drug Action: An Overview
S.Gur, P.J. Kadowitz and W.J.G. Hellstrom

Micturition, penile erection, contraction of prostatic smooth muscle, peristalsis of the male excurrent duct system and lumbosacral spinal cord neurotransmission all require adenosine 5’-triphosphate (ATP) activity and this likely involves purinergic (P2) receptors. P2 receptors are categorized as either ligand-gated ionotropic P2X or metabotropic G-protein-coupled P2Y subtypes. In the urinary bladder, purinergic receptor mechanisms are involved in both motor and sensory function. In the prostate, P2X1-receptors, which mediate contraction, are present in the fibromuscular stroma while G protein-coupled P2Y purinoceptors have a wide range of actions in prostate cancer. In the excretory ducts of the testis (ductus epididymidis, vas deferens and its associated seminal vesicles), heavy immunostaining for P2X1 and P2X2 subtypes is detected in the membranes of smooth muscle, suggesting their role in sperm transport and ejaculation. In the penis, intense P2X1 and weak P2X2 immunoreactivity are observed in smooth muscle of blood vessels and the corpus cavernosum, implying their participation in detumescence. Human corporal cavernosum stimulation induces relaxation of P2Y purinoceptors. Targeting of extracellular or intracellular P2X and/or P2Y receptor signaling pathways holds promise in affecting the lower genitourinary tract system. Our advancing knowledge about purine agonists and their pharmacologic benefits in erectile, ejaculatory, urinary bladder and prostatic hyperplasia may service clinical problems in the near future.


[Back to top]
Therapeutic Approaches in Vascular Repair Induced by Adult Bone Marrow Cells and Circulating Progenitor Endothelial Cells
C. Napoli, A. Balestrieri and L.J. Ignarro

Strong evidence indicates that bone marrow cells (BMCs) can contribute to the healing process of injured vascular system via CXCR4/Thymosin β4/Integrin α4β1/SDF-1 molecular pathways. We discuss the therapeutic approaches of BMCs and circulating endothelial progenitor cells (EPCs) to restore vascularization. Today some clinical trials employing BMCs in the treatment of peripheral vascular diseases have been completed with encouraging results. When large clinical controlled studies will be completed, the scientific community will evaluate this novel and promising therapeutic approach. Although some basic studies suggest the potential use of adult/somatic stem cell for vascular repair, other stringent data suggest that this potential is dependent also on growth factor synthesis rather than the formation of new arterial vessels. Considering the limitations of adult stem cells especially in elderly subjects, our point of view is that BMCs or exogenous BMC/EPC are candidate for adjunct cell-therapy applications in vascular repair.


[Back to top]
Novel Pharmaceutical Approaches for Treating Patients with Cystic Fibrosis
Z. Saeed, G. Wojewodka, D. Marion, C. Guilbault and D. Radzioch

Before the cloning of the CFTR gene in 1989, there were relatively few treatment options for the many phenotypes associated with cystic fibrosis (CF). The advancement of research in areas such as immunology, molecular biology and pharmacology have provided new insights into the mechanism and evolution of CF. More than 40 systematic clinical trials evaluating new therapies for CF are presently registered with the NIH. A great deal of effort is focused on the main cause of mortality: chronic and persistent lung infections. Intestinal malabsorption, pancreatic insufficiency, reduced bone mineral density and reproductive abnormalities are other manifestations of this disease that have been targeted by innovated treatments which are giving renewed hope to CF patients and their families. The following review is a summary of the novel pharmaceutical approaches for the treatment of cystic fibrosis aimed at improving both the quality and the longevity of the lives of patients afflicted with this devastating disease.