Current Topics in Medicinal Chemistry - Volume 15, Issue 7, 2015

Ion channels constitute a varied class of membrane proteins with pivotal roles in cellular physiology and that are fundamental for neuronal signaling, hormone secretion and muscle contractility. Hence, it is not unanticipated that toxins from diverse organisms have evolved to modulate the activity of ion channels. For instance, animals such as cone snails, scorpions, spiders and snakes use toxins to immobilize and capture their prey by affecting ion channel function. This is a beautiful example of an evolutionary process that has led to the development of an injection apparatus from predators and to the existence of toxins with high affinity and specificity for a given target. Toxins have been used in the field of ion channel biophysics for several decades to gain insight into the gating mechanisms and the structure of ion channels. Through the use of these peptides, much has been learned about the ion conduction pathways, voltagesensing mechanisms, pore sizes, kinetics, inactivation processes, etc. This review examines an assortment of toxins that have been used to study different ion channels and describes some key findings about the structure-function relationships in these proteins through the details of the toxin-ion channel interactions.

Voltage-gated Ca2+ (CaV) channels are transmembrane proteins primarily formed by an ion-conducting α 1 subunit that can associate with auxiliary β and α2δ subunits. Ca2+ entering the cell through these channels serves as a versatile second messenger of electrical signaling, initiating numerous different cellular processes ranging from gene expression to cell fertilization, neuronal transmission and cell death. CaV channels, as other ion channels, are targets for numerous ligands including naturally occurring peptide toxins. Some of these peptide toxins are invaluable tools for studying their structure and function and have potential therapeutic applications. Here, we present an overview of the current knowledge regarding the structure and function of CaV channels as well as their role in human disease, and highlight some of the growing applications of peptide toxins targeting CaV channels. Analysis and understanding of the molecular strategy used by these peptide toxins might allow the design of novel classes of therapeutic agents acting on specific targets with high selectivity and efficacy.

In lumbar radiculopathy, the dorsal root or dorsal root ganglia (DRG) are compressed or affected by herniated discs or degenerative spinal canal stenosis. The disease is multi-factorial and involves almost all types of pain, such as ischemic, inflammatory, mechanical, and neuropathic pain. Acid-sensing ion channels (ASICs) activated by extracellular acidosis play an important role in pain generation, and the effects of ASICs are widespread in lumbar radiculopathy. ASICs may be involved in the disc degeneration process, which results in disc herniation and, therefore, the compression of the dorsal roots or DRG. ASIC3 is involved in inflammatory pain and ischemic pain, and, likely, mechanical pain. ASIC1a and ASIC3 may have an important effect on control of the vascular tone of the radicular artery. In the central nervous system, ASIC1a modulates the central sensitization of the spinal dorsal horn. Thus, toxins targeting ASICs, because of their specificity, may help elucidate the roles of ASICs in lumbar radiculopathy and could be developed as novel analgesic agents.

Patch-Clamp electrophysiology, the “gold standard” for the functional study of ion channels has become automated. This innovative technology, already over a decade old, has revolutionized the strategies for the search of medicinal compounds which now can be screened at unprecedented speed, approaching the high throughput standards required by primary screening campaigns emblematic of the pharmaceutical and biotechnology industries. Consequently, an acceleration of the discovery and development of new drugs targeting ion channels is expected. These pore forming membrane proteins had been relegated as crucial therapeutic drug targets due to the difficulty of their experimental analysis. This new technological approach has begun to impact the finding of new toxins which are conspicuously relevant as medicinal agents given their extraordinary potency and specificity when acting upon ion channels. The introduction of automated patch-clamp instrumentation to academic labs and institutions pursuing the finding of new pharmacological agents, peptide toxins in particular, will certainly enrich these scientific and technological fields by contributing with their always prolific generosity of originality and innovation.

Cytotoxins (or cardiotoxins, CTs) are small rigid membrane-active proteins of the threefinger toxin (TFT) family. They comprise about 60 amino acid residues, stabilized by four disulphide bridges. CTs, the most abundant proteins in cobra venom are able to kill cancer cells in a dose and time-dependent manner. The present review summarizes the current data on the molecular pathways of cancer cell death, induced by CTs. A relationship between structural characteristics of CTs and mechanism of their antiproliferative activity is reviewed as well. The CT molecules are rigid and their structure does not change significantly, when they interact with their molecular targets. This rigidity facilitates identification of molecular entities, responsible for antiproliferative activity of the toxins. We demonstrate that consideration of a net electrostatic charge and recently introduced HTL (Hydrophobicity of the Tips of the Loops) score allows distinguishing between the two mechanisms of cell death. The first is related to membrane destabilization by the toxins. The second involves their capture inside the cells, followed by interrogation into signal cascades mediated by the proteins, essential for cell life. Via addressing to antibacterial activity of CTs, which is supposed to arise from the plasma membrane damage, we demonstrate that, if membrane deterioration is involved in malignant cell death, the toxic activity of CTs correlates with their HTL scores and net electrostatic charge. We assume the relationship found may be used for rational design of antiproliferative compounds.

Snake venom is a complex mixture of biologically and pharmacologically active components, comprising hydrolytic enzymes, non-enzymatic proteins/peptides, and small amounts of organic and inorganic molecules. The venom components are known to vary with geographic location, season, species and age of the snakes. The role of the venom in the snake is not primarily for self-defense, but in prey immobilization and its subsequent digestion. Hence, several digestive enzymes in venoms, in addition to their hydrolytic activity have evolved to interfere in diverse physiological processes that help in the immobilization of prey/victim. As snake components are capable of modulating the physiological response of envenomated prey/victim, they show promise as potential pharmacological tools, as drug leads and in diagnostic applications. This, in a practical sense to be a reality has to be linked to the advances in toxinology that provide investigators with an understanding of the pharmacodynamics of toxins together with improved understanding of the etiology of many human diseases and identification of potential sites for therapeutic intervention. This review aims at providing an overview on snake venom toxins and their derivatives that have potential anti-angiogenic effects for cancer treatment. Some of the anti-angiogenic components of snake venom like Snake venom metalloproteinases (SVMPs), Disintegrins, Phospholipases A2 (PLA2), CType Lectins (CLP), Vascular Apoptosis inducing Proteins (VAP) and L-Amino Acid Oxidases (LAAO) are discussed. This review aims at giving an overall view of these molecules and their mechanism of action as an effective antiangiogenic agent towards the treatment of cancer.

Snake venoms have evolved over millions of years, and some toxins have evolved to specifically target various sites in the cardiovascular system of prey animals, producing prey hypotension. So far, a number of specific hypotensive peptides have been identified from different snake venoms. These snake hypotensive peptides are divided into five classes: bradykinin potentiating peptides, natriuretic peptides, sarafotoxins, Phospholipases A2 and L-type Ca2+ channel blockers. They differ widely in their structure, mechanism and points of action. Each class has many different isoforms with similar structures but different hypotensive activities. In the last decade, research efforts on snake hypotensive peptides have produced great advance in their understanding and applications in designing antihypertensive agents. In addition, several new classes of hypotensive peptides have been found from snake venoms. This review attempts to provide an overview of the current understanding of the structure, function and mechanism of snake hypotensive peptides.

Snake venoms constitute a mixture of bioactive components that are involved not only in envenomation pathophysiology but also in the development of new drugs to treat many diseases. Different enzymatic and non-enzymatic proteins, such as phospholipases A2, hyaluronidases, L-amino acid oxidases, metalloproteinases, serine proteinases, lectins and disintegrins have been isolated and their functional and structural properties described in the literature. Many of these studies have also explored their medicinal potential focusing mainly on anticancer, antithrombotic and microbicide therapies. Bothrops pauloensis is a species found in Brazil, whose venom has been the focus of our studies in order to explore the biochemical and functional characteristics of their components. In this review, we have presented the main results of years of research on different toxins from B. pauloensis emphasizing their therapeutic potential. Studies concerning snake venom toxins to search for new therapeutic models open perspectives for new drug discovery.

Botulinum Neurotoxins are the most poisonous of all toxins with lethal dose in nanogram quantities. They are potential biological warfare and bioterrorism agents due to their high toxicity and ease of preparation. On the other hand BoNTs are also being increasingly used for therapeutic and cosmetic purposes, and with that the chances of accidental overdose are increasing. And despite the potential damage they could cause to human health, there are no post-intoxication drugs available so far. But progress is being made in this direction. The crystal structures in native form and bound with substrate peptides have been determined, and these are enabling structure-based drug discovery possible. High throughput assays have also been designed to speed up the screening progress. Substrate-based and small molecule inhibitors have been identified. But turning high affinity inhibitors into clinically viable drug candidates has remained a challenge. We discuss here the latest developments and the future challenges in drug discovery for Botulinum neurotoxins.