Muscarinic toxin 2

The G-protein-coupled receptor (GPCR) family is one of the largest gene superfamilies with approx. 370 members responding to endogenous ligands in humans and a roughly equal amount of receptors sensitive to external stimuli from the surrounding. A number of receptors from this superfamily are well recognized targets for medical treatment of various disease conditions, whereas for many others the potential medical benefit of interference is still obscure. A general problem associated with GPCR research and therapeutics is the insufficient specificity of available ligands to differentiate between closely homologous receptor subtypes. In this context, venom peptides could make a significant contribution to the development of more specific drugs. Venoms from certain animals specialized in biochemical hunting contain a mixture of molecules that are directed towards a variety of membrane proteins. Peptide toxins isolated from these mixtures usually exhibit high specificity for their targets. Muscarinic toxins found from mamba snakes attracted much attention during the 1990s. These are 65-66 amino acid long peptides with a structural three-finger folding similar to the α-neurotoxins and they target the muscarinic acetylcholine receptors in a subtype-selective manner. Recently, several members of the three-finger toxins from mamba snakes as well as conotoxins from marine cone snails have been shown to selectively interact with subtypes of adrenergic receptors. In this review, we will discuss the GPCR-directed peptide toxins found from different venoms and how some of these can be useful in exploring specific roles of receptor subtypes.

Muscarinic toxin 2 from Dendroaspis angusticeps has been crystallized by vapour diffusion, in sodium acetate using sodium thiocyanate as a precipitant. Trigonal crystals (space group P3(1)21 or P3(2)21) have been obtained. The unit cell parameters are a = b = 64.2 A and c = 37.1 A. The presence of one molecule per asymmetric unit is estimated.

Toxins are of interest in drug design because the toxins provide three-dimensional templates for creating small molecular mimics with interesting pharmacological properties. Toxins are also useful in drug discovery because they can be used as pharmacological tools to uncover potential therapeutic targets. With their high potency and selectivity, toxins are often more useful in functional experiments than standard pharmacological agents. We have used two groups of neurotoxins, the dendrotoxins and the muscarinic toxins (MTs), to explore the involvement of subtypes of potassium ion channels and muscarinic receptors, respectively, in processes involved in cognition and the changes in neuronal properties with aging. From our current work, quantitative autoradiographic studies with radiolabelled dendrotoxins reveal widespread distribution of binding sites throughout rat brain sections, but few differences exist between young adult and aged rats. However, displacement studies with toxin K, which preferentially binds to the Kv1.1 subtype of cloned potassium channel, show the selective loss of such sites in regions of the hippocampus and septohippocampal pathway with aging. MTs have been tested for effects on performance of rats in memory paradigms. MT2, which activates m1 receptors, improves performance of rats in a step-down inhibitory avoidance test, whereas MT3, which blocks m4 receptors, decreases performance when given into the hippocampus. This is the first clear demonstration of a role for m4 muscarinic receptors in cognition.