No abstract

Na + -channel specific scorpion toxins are peptides of 60±76 amino acid residues in length, tightly bound by four disulfide bridges. The complete amino acid sequence of 85 distinct peptides are presently known. For some toxins, the three-dimensional structure has been solved by X-ray diffraction and NMR spectroscopy. A constant structural motif has been found in all of them, consisting of one or two short segments of a-helix plus a triplestranded b-sheet, connected by variable regions forming loops (turns). Physiological experiments have shown that these toxins are modifiers of the gating mechanism of the Na + -channel function, affecting either the inactivation (a-toxins) or the activation (b-toxins) kinetics of the channels. Many functional variations of these peptides have been demonstrated, which include not only the classical a-and b-types, but also the species specificity of their action. There are peptides that bind or affect the function of Na + -channels from different species (mammals, insects or crustaceans) or are toxic to more than one group of animals. Based on functional and structural features of the known toxins, a classification containing 10 different groups of toxins is proposed in this review. Attempts have been made to correlate the presence of certain amino acid residues or`active sites' of these peptides with Na + -channel functions. Segments containing positively charged residues in special locations, such as the five-residue turn, the turn between the second and the third b-strands, the C-terminal residues and a segment of the N-terminal region from residues 2±11, seems to be implicated in the activity of these toxins. However, the uncertainty, and the limited success obtained in the search for the site through which these peptides bind to the channels, are mainly due to the lack of an easy method for expression of cloned genes to produce a well-folded, active peptide. Many scorpion toxin coding genes have been obtained from cDNA libraries and from polymerase chain reactions using fragments of scorpion DNAs, as templates. The presence of an intron at the DNA level, situated in the middle of the signal peptide, has been demonstrated.

No abstract

Voltage-gated sodium channels (VGSCs) are large transmembrane proteins that conduct sodium ions across the membrane and by doing so they generate signals of communication between many kinds of tissues. They are responsible for the generation and propagation of action potentials in excitable cells, in close collaboration with other channels like potassium channels. Therefore, genetic defects in sodium channel genes can cause a wide variety of diseases, generally called “channelopathies.” The first insights into the mechanism of action potentials and the involvement of sodium channels originated from Hodgkin and Huxley for which they were awarded the Nobel Prize in 1963. These concepts still form the basis for understanding the function of VGSCs. When VGSCs sense a sufficient change in membrane potential, they are activated and consequently generate a massive influx of sodium ions. Immediately after, channels will start to inactivate and currents decrease. In the inactivated state, channels stay refractory for new stimuli and they must return to the closed state before being susceptible to a new depolarization. On the other hand, studies with neurotoxins like tetrodotoxin (TTX) and saxitoxin (STX) also contributed largely to our today’s understanding of the structure and function of ion channels and of VGSCs specifically. Moreover, neurotoxins acting on ion channels turned out to be valuable lead compounds in the development of new drugs for the enormous range of diseases in which ion channels are involved. A recent example of a synthetic neurotoxin that made it to the market is ziconotide (Prialt®, Elan). The original peptide, ω-MVIIA, is derived from the cone snail Conus magus and now FDA/EMA-approved for the management of severe chronic pain by blocking the N-type voltage-gated calcium channels in pain fibers. This review focuses on the current status of research on neurotoxins acting on VGSC, their contribution to further unravel the structure and function of VGSC and their potential as novel lead compounds in drug development.

An important step toward understanding the molecular basis of the functional diversity of pacemaker currents in spontaneously active cells has been the identification of a gene family encoding hyperpolarizationactivated cyclic nucleotide-sensitive cation nonselective (HCN) channels. Three of the four gene products that have been expressed so far give rise to pacemaker channels with distinct activation kinetics and are differentially distributed among the brain, with considerable overlap between some isoforms. This raises the possibility that HCN channels may coassemble to form heteromeric channels in some areas, similar to other K ؉ channels. In this study, we have provided evidence for functional heteromerization of HCN1 and HCN2 channels using a concatenated cDNA construct encoding two connected subunits. We have observed that heteromeric channels activate several-fold faster than HCN2 and only a little slower than HCN1. Furthermore, the voltage dependence of activation is more similar to HCN2, whereas the cAMP sensitivity is intermediate between HCN1 and HCN2. This phenotype shows marked similarity to the current arising from coexpressed HCN1 and HCN2 subunits in oocytes and the native pacemaker current in CA1 pyramidal neurons. We suggest that heteromerization may increase the functional diversity beyond the levels expected from the number of HCN channel genes and their differential distribution.The newly cloned hyperpolarization-activated cyclic nucleotide-sensitive cation nonselective (HCN) 1 ion channels give rise to currents that are almost indistinguishable or very similar to native pacemaker currents termed I f in heart (1) and I h or I q in brain tissue (2). To date, three of the four HCN genes have been functionally expressed, giving rise to pacemaker currents with distinct gating properties (for review see Refs. 3-5). HCN1 channels display fast gating properties and are almost unaffected by cAMP (6), similar to I h recorded from some hippocampal pyramidal cells. HCN2 channels activate slowly and are strongly modulated by cAMP (7,8), similar to native pacemaker currents in some cardiac cells. HCN4 channels activate very slowly and also respond strongly to cAMP (9, 10), similar to I h in thalamic relay neurons.Expression patterns have confirmed that HCN1 is expressed restrictively in neurons of the neocortex, hippocampus, cerebellar cortex, and brainstem nuclei (6,11,12). In contrast, HCN2 is widely expressed throughout the brain with prominent labeling of thalamic and brainstem nuclei as well as in heart tissue (6, 7). Finally, HCN4 is predominantly expressed in thalamus, heart, and testis (9). This distinct distribution pattern of HCN isoforms across the brain has been suggested to attribute to functional heterogeneity of native neuronal pacemaker currents (12). Nevertheless, considerable overlap was observed between HCN1 and HCN2 using in situ hybridization, namely in hippocampal pyramidal cells and some brainstem nuclei (12). Moreover, the coexistence of HCN1 and HCN2 mRNA has been demonstrated at...

Voltage-gated Na(+) channels are integral membrane proteins that function as a gateway for a selective permeation of sodium ions across biological membranes. In this way, they are crucial players for the generation of action potentials in excitable cells. Voltage-gated Na(+) channels are encoded by at least nine genes in mammals. The different isoforms have remarkably similar functional properties, but small changes in function and pharmacology are biologically well-defined, as underscored by mutations that cause several diseases and by modulation of a myriad of compounds, respectively. This review will stress on the modulation of voltage-gated Na(+) channels by scorpion alpha-toxins. Nature has designed these two classes of molecules as if they were predestined to each other: an inevitable 'encounter' between a voltage-gated Na(+) channel isoform and an alpha-toxin from scorpion venom indeed results in a dramatically changed Na(+) current phenotype with clear-cut consequences on electrical excitability and sometimes life or death. This fascinating aspect justifies an overview on scorpion venoms, their alpha-toxins and the Na(+) channel targets they are built for, as well as on the molecular determinants that govern the selectivity and affinity of this 'inseparable duo'.

Four novel peptide toxins that act on voltage-gated sodium channels have been isolated from tarantula venoms: ceratotoxins 1, 2, and 3 (CcoTx1, CcoTx2, and CcoTx3) from Ceratogyrus cornuatus and phrixotoxin 3 (PaurTx3) from Phrixotrichus auratus. The pharmacological profiles of these new toxins were characterized by electrophysiological measurements on six cloned voltage-gated sodium channel subtypes expressed in Xenopus laevis oocytes

No abstract