Each cell membrane has ion channels - protein pores that selectively conduct ions in and out of the cell. Due to their work membrane became electrically charged and has some transmembrane potential. At rest the potential kept on the same level. But under the influence of various signals certain ion channels may open or close, changing the flow of ions in and out of the cell and the charge of the membrane. As a result, certain cells (neurons, muscle cells, and gland cells) are activated and are able to respond to a signal.
However, sometimes channel-coding genes get damaged and cause inadequate response. For example, defects in certain domains of voltage-gated sodium channels Nav1.4 in muscle cells cause the membrane to "leak out" even when the channels are closed. Sodium ions penetrate the membrane and change the electric potential. In this case signals of the nervous system are not able to activate the muscle cells, and a patient develops paralysis. People with type 2 hypokalemic periodic paralysis suffer from muscle weakness up to total immobilization. Unfortunately, existing medicinal drugs against this condition are often inefficient.
"In our work we studied human voltage-gated ion channels, in particular the mutated forms of Nav1.4 channel from skeletal muscle. These mutations lead to a severe disease, type 2 hypokalemic periodic paralysis. We are the first to prove that there are natural chemical compounds able to block the leakage currents through mutated channels," explained Mikhail Petrovich Kirpichnikov, Doctor of Biological Sciences, member of the Russian Academy of Sciences, and dean of the Faculty of Biology, MSU.
Using a range of genetic methods, protein engineering, electrophysiology, NMR spectroscopy, and computer modeling, the scientists studied the reasons for abnormal activities in the channels damaged by mutation. For the first time they suggested a blocking agent for leakage currents - a toxin Hm-3 extracted from the venom of Heriaeus melloteei spider. According to the data obtained using site-directed mutagenesis, electrophysiology, NMR spectroscopy, and computer modeling, the toxin fixes the voltage-sensing domain of the channel in the position that prevents leakage of ions.
"The discovery of this toxin property gives us hope of developing efficient medicinal drugs for the treatment of patients with hypokalemic paralysis and other similar diseases. The model of interaction between the channel and the toxin gives us prospects for the development of new drugs," concludes Alexander Vasilevskiy, Candidate of Chemical Sciences, a lecturer at MSU, and the head of a laboratory at the Institute of Bioorganic Chemistry of the Russian Academy of Sciences.
The structural part of the work was commented on by Zakhar Shenkarev, Doctor of Physical and Mathematical Sciences, Professor of the Russian Academy of Sciences, a lecturer at Moscow Institute of Physics and Technology, and the group leader at the Institute of Bioorganic Chemistry of the Russian Academy of Sciences: "Studying the structure of ion channel complexes in the membrane is a difficult task. The main issue of our work was the instability of samples in the course of the NMR experiment. To solve it we had to come up with a range of new experimental methods that speeded up the process of structural data collection from several days to hours."
Männikkö R, Shenkarev ZO, Thor MG, Berkut AA, Myshkin MY, Paramonov AS, Kulbatskii DS, Kuzmin DA, Sampedro Castañeda M, King L, Wilson ER, Lyukmanova EN, Kirpichnikov MP, Schorge S, Bosmans F, Hanna MG, Kullmann DM, Vassilevski AA.
Spider toxin inhibits gating pore currents underlying periodic paralysis.
Proc Natl Acad Sci U S A. 2018 Apr 10. doi: 10.1073/pnas.1720185115.