Introduction of sodium channel
The sodium channel is a channel present on the membrane that allows a small amount of Na+ to enter the cell along its electrochemical gradient, as discovered by British scientists Hodgkin and Huxley. It can be divided into two types, voltage-gated and ligand-gated. The sodium ion channel is the primary activation bond for electrical signals in all animals, while the electrical signal is the basis for a series of physiological processes such as neural activity and muscle contraction. In the human body, there are nine known voltage-gated sodium ion channel subtypes that play roles in different organs and physiological processes. Abnormalities in the sodium channel can lead to a range of neurological, muscular, and cardiovascular-related diseases, particularly epilepsy, arrhythmia, and persistent pain, or inability to sense pain, etc. So far, one thousand in the nine sodium channel proteins in the human body have been found. Multiple point mutations are associated with known diseases. In addition, sodium channel is also a direct target of many local anesthetics and many neurotoxins in nature. Many snake venoms, scorpion venoms, and spider toxins all have adverse effects on sodium channels. In 2017, the Yan Ning research group of Tsinghua University Medical School took the first “3D photo” for the sodium channel using a “camera” called a cryo-electron microscope. In its paper "Three-dimensional Structure of Near-Atomic Resolution of Voltage-Controlled Sodium Ion Channels in Eukaryotes", the 3.8-resolution cryo-electron microscopy structure of voltage-gated sodium ion channels in eukaryotes was first reported. It has greatly helped humans understand the structure of sodium channels.
Sodium channel family
The sodium channel is an ion channel formed by an intrinsic membrane protein that allows sodium ions to pass through the cell membrane. The sodium channel gene family contains 11 members and encodes 10 different sodium channels. The sodium channel is a transmembrane glycoprotein composed mainly of the α-subunit and beta-subunits. The alpha-subunit contains four homologous repeat domains (DI-DIV), and each domain contains six helical transmembrane segments (S1-S6). The six transmembrane segments can be divided into two categories based on their function: S1-S4 is a voltage sensor; S5 and S6 form a channel for selective transport of sodium ions. The alpha-subunit alone has the function of a sodium channel. The β-subunit (β1-β4) is a single transmembrane protein that alters the density of sodium channels and cell excitability by regulating the expression of sodium channels on the cell surface. In addition, the β-subunit can also regulate the transport of intracellular α-subunits and cell adhesion. In 2011, Payandeh et al. reported the crystal structure of the sodium ion channel of Arcobacter butzleri. Based on the crystal structure of the sodium ion channel obtained, they speculated how the sodium ion channel works: when the sodium channel is activated, the cells are depolarized, and the electric field on both sides of the cell membrane changes, resulting in a positive charge in DI-DIII. The voltage sensor S4 moves quickly outward. Then, the outwardly moving connector between S4 and S5 pushes S5-S6 outward as a structural unit. During this time, S5−S6 always maintains a tight coupling structure, which forces S5−S6 and its surrounding subunits to move, eventually leading to the opening of the tunnel. The sodium channel is inactivated within a few milliseconds after opening, mainly due to the slowing of the DIV voltage sensor, and eventually, the sodium channel is closed. The electron density map of the sodium channel shows that when the channel is closed, S6 is superimposed on the closed structure around the channel. Typically, the sodium channel will remain in an inactive state until the cell membrane potential is depolarized again. The main function of the sodium channel is to regulate the excitability of neurons. The inherent properties of distribution, density, transport, activation threshold, and reactivation can greatly affect the electrical activity of neurons. In addition, electrophysiological experiments show that sodium channels play an important role in the generation and transmission of pain. In recent years, Vastani et al. have studied the role of sodium channels in chronic pain and neuropathic pain and pointed out that the confirmation of voltage-gated sodium channels affects the efficacy of local anesthesia and neuroelectric modulation techniques. Therefore, sodium channel inhibitors are often used clinically to inhibit the electrical excitability of neurons by blocking the continuous sodium ion current, and finally achieve the purpose of analgesia.
Sodium channel-related diseases
After mutations in the genes encode the α- and β-subunits of the sodium channel, the structure and biophysical properties of the sodium channel are affected, eventually resulting in a sodium channel disease. Sodium channel diseases can be classified into four types according to the organs involved, peripheral nerve sodium channelopathies, brain sodium channelopathies, skeletal muscle sodium channelopathies, and cardiac sodium channelopathies. In recent years, many reports have reported that certain natural small molecule compound neurotoxins and polypeptide neurotoxins in nature can block nerve conduction by inhibiting sodium ion current. As a result, more and more pharmaceutical companies have developed these natural sodium channel inhibitors into sodium channel drugs for the treatment of clinical diseases. Tetrodotoxin (TTX) and Saxitoxin (STX) are two well-studied compound neurotoxins with molecular formulas C11H17N3O8 and C10H17N7O4, respectively. The toxicity of TTX and STX is very strong. After eating, the first is sensory nerve paralysis, followed by motor nerve paralysis, which eventually leads to limb weakness or even inability to exercise. This is mainly because both are sodium channel inhibitors, which can specifically bind to the neurotoxin binding site 1 on the sodium channel (located on the ring formed between S5 and S6 in DI-DIV), thereby inhibiting nerve conduction, eventually leading to limb paralysis. Therefore, TTX and STX are often studied extensively as potential analgesics and local anesthetics. Clinical studies have shown that the analgesic effect of TTX is 3200 times than thatof morphine; the local anesthetic effect is 4000 times than that of procaine hydrochloride. In Canada, the use of TTX (formula Tecti) for analgesic treatment in patients with chronic cancer has entered Phase III clinical trials with a subcutaneous TTX concentration of 15 μg/mL. As early as 1975, Southcott has demonstrated that STX can inhibit nerve conduction for a long time, and its local anesthetic effect is 100,000 times as that of procaine. However, both TTX and STX are more toxic, and more than a certain dose can cause toxic side effects. In 2011, when Berde et al. used Tectin in combination with commercially available bupivacaine (0.25% concentration) and epinephrine (5 μg/mL), the duration of local anesthesia was prolonged. Therefore, a single injection of drugs can relieve pain for a long time, and ultimately reduce the need for addictive drugs such as opioids after surgery. In 2013, Sahadev et al. also encapsulated STX with the glucocorticoid dexamethasone in liposomes and injected them subcutaneously into rats with mechanical pain due to sciatic nerve injury. The results showed that the thermal pain block duration of the rats continued to be (6.9 ± 1.2) d after a single injection of liposomes; if liposomes were injected on days 0, 5 and 12 after injury, the lag time can last (18.1 ± 3.4) d. Therefore, when TTX or STX is combined with other anesthetic analgesic drugs (morphine, cocaine, bupivacaine, dexamethasone, etc.), not only the duration of analgesia is prolonged, but also the side effects are reduced by lowering the dose. Certain local anesthetics and analgesics circulating on the market are amines containing aromatic groups like tetrodotoxin and saxitoxin. It can suppress or reduce pain at very low concentrations and has a good effect on the treatment of neuropathic pain. When a spider preys or defies an enemy, its venom gland can paralyze or even kill prey or natural enemies by secreting spider toxins. This is mainly since spider toxins contain a variety of molecularly-sized, disulfide-rich peptide neurotoxins that affect the normal activities of ion channels in neurons, including sodium channels, potassium channels, and calcium channels. These neurotoxins play an important role in medical research, and some of them are clinically used to relieve pain or treat diseases such as arrhythmia. According to the literature, nearly one-third of the total number of spider toxins is targeted at sodium channels. It is conservatively estimated that spider venom contains at least 10 million biologically active peptides. However, the peptides with ion channel regulation function reported in the literature are only about 0.01%, and many components have not been isolated and identified. Therefore, there are many new spider toxins in spider venom that need to be discovered, characterized and studied, which will greatly promote the development and utilization of analgesic and local anesthetic drugs in the future. The members of the Cysteine-rich secretory protein (CRISP) family are mostly single-stranded secreted proteins with a molecular mass between 20 and 30 kDa. Their primary structure contains multiple highly conserved Cys residues. It has been confirmed that CRISP distributed in the venom gland of reptiles has a blocking effect on a variety of ion channels. In 2009, Xiaoet al. found that Lampetra japonica cysteine-rich buccal gland protein (Lj-CRBGP) has high homology (60%) with 16 of the venom-rich cysteine-secreting proteins and contains highly conserved cysteine residue. In addition, electrophysiological experiments have shown that natural Lj-CRBGP can attenuate neuronal excitability by blocking sodium currents in hippocampal neurons and dorsal root neurons. Therefore, Xiao et al. speculated that the “vampire” in the water may inhibit the local pain response of the host fish through LJ-CRBGP in its oral gland to ensure its successful adsorption on the fish body. However, the specific mechanism still requires further animal experiments.