Molecule of the Month: Voltage-gated Sodium Channels

As I sit here writing this article, a flurry of signals is coursing through my nervous system, helping me sort out how to get this story started. The same thing is happening in your brain, as you read these words and make sense of them. Nerve impulses travel throughout nervous system at breakneck speed, carrying messages from cell to cell that together make up our thoughts and control our actions. Several different methods are used to transmit these messages. A complex machinery of neurotransmitters and receptors, such as AMPA receptors, pass messages across synapses from one cell to the next. Propagation of signals along the long axons of nerve cells is somewhat simpler, performed by voltage-gated sodium channels and a few helpers.

Axon membranes are filled with channels that open and close based on the voltage difference across the membrane. If the voltage difference is high, these channels are tightly closed, but when the voltage drops, they open briefly and allow sodium ions to pass across the membrane. A few simple steps are needed to send a signal. First, ion pumps build up an excess of sodium outside the cell, creating the voltage difference that keeps the channels shut. The signal is then triggered by other types of channels that allow sodium to cross back across the membrane and equalize the amounts on either side. This process reduces the voltage difference in the local area and causes nearby sodium channels to open. The voltage lowers around these channels, causing more channels further down the axon to open. Much like a stadium wave at a sporting event, a wave of channel opening spreads down the axon, ultimately reaching the next cell. Of course (as is often the case with biology), things aren’t quite this simple, and potassium channels and other proteins also help to refine the wave.

Our voltage-gated sodium channels, such as the one shown here from PDB entry 6j8j, are composed of a long protein chain that folds to form a pore through the membrane, with four voltage-sensing elements arrayed around the pore. A few additional subunits, shown here in green, tune the action of the channel. The voltage-sensing elements include a helical segment (much of which is in an unusual 3₁₀ conformation) that has several positively-charged arginine and lysine amino acids (shown in magenta), that respond to changes in local voltage and open and close the pore.

Sodium channels transmit all the messages between our brain and our muscles, so as you can imagine, they are susceptible targets for attack. Many animals make small toxins that block the action of these channels, causing paralysis. Two are shown here, in PDB entry 6a95. Tetrodotoxin, the lethal toxin found in pufferfish and a variety of other poisonous animals, binds in the middle and physically blocks the pore. On the other hand, the spider toxin (shown in yellow) binds to one of the voltage-sensing elements and corrupts its action, allowing the spider to paralyze its prey. These toxins aren't all terrible, however: they are giving us hints about how we might modify the action of sodium channels to block pain or other disorders.