Ion Channels as Receptors

One open ion channel lets about ten million ions cross a membrane every second. What can a cell do with a receptor like that?

Most receptors in this course hand their message to something else, such as a G protein or a kinase. A channel hands it to nothing. The signal opens a hole, ions pour through, and the voltage across the membrane changes in about a millisecond. So what decides which way the ions go, how does a hole tell one ion from another, and what opens the gate?

The idea: the signal opens a hole and the gradient does the rest

Chris Deppmann’s first point is that there are “Lots of ion channels” and that they are in “every cell type”, well beyond neurons. You have met two far from any nerve already: Orai in the T cell and the IP3 receptor on the calcium store.

Two kinds of protein move ions across a membrane (lesson membrane-structure-function). A transporter such as the Na+/K+ pump binds its ions and changes shape to carry them, uphill if it spends ATP, at a few hundred a second. A channel is a pore with a gate, and while the gate is open ions run downhill through it at around ten million a second. Pumps build the gradients slowly and channels spend them fast.

“Downhill” has two parts, and the phrase to remember is “Electro-chemical gradient”. An ion is pushed by its difference in concentration and pulled by the voltage across the membrane, and the inside of a resting cell sits near −70 mV. For each ion there is one voltage at which push and pull balance. That is its equilibrium potential, E, and the Nernst equation gives it:

E = (61.5 mV ÷ z) × log10 (concentration outside ÷ concentration inside), at 37 °C

Here z is the charge of the ion. With typical concentrations, E is about −90 mV for K+, about +60 mV for Na+ and above +120 mV for Ca2+. An open channel drags the membrane voltage toward the E of the ion it passes, so the direction of flow follows from one subtraction:

driving force = membrane voltage − E

At −70 mV, sodium is 130 mV away from its E, so it rushes in and the cell depolarises, meaning that its voltage becomes less negative. Potassium is only 20 mV from its E, so it leaks out gently and pulls the voltage back down. Try the calculator: change the concentrations on the two sides and watch E move.

Very few ions need to cross to change the voltage, so the concentrations barely shift and nothing has to be made. The voltage is the message, and when the ion is calcium the ion is a messenger too (lesson calcium-signaling).

One protein, watched opening and closing

How does anybody know what a single channel does? The current through one is a few picoamps, a few trillionths of an amp. In Göttingen, Erwin Neher and Bert Sakmann pressed the tip of a glass pipette against a frog muscle fibre and recorded the current through the small patch of membrane under it. The trace they published in 1976 sat at zero, stepped up to a fixed level and stepped back, as one acetylcholine-gated channel opened and shut.

The weak point was the seal between glass and membrane. Current that leaked around the rim reached the amplifier along with the signal and buried it in noise. By Neher’s account in his Nobel lecture, around 1980 they noticed by chance that slight suction on the pipette made the seal more than a hundred times tighter. Its resistance went from millions of ohms to billions, and single openings stood clear of the background. The method is the patch clamp.

To a good approximation a channel is either shut or open, and a stronger signal raises the share of time it spends open.

At the junction of nerve and muscle, Bernard Katz and his colleagues had found in the 1950s that acetylcholine is released in packets of fixed size. Each packet opens many channels at once.

How a pore passes potassium and keeps out the smaller sodium

A potassium channel can pass up to about a hundred million K+ ions a second, yet fewer than one Na+ gets through for every thousand K+. Sodium is the smaller ion, with a radius of 0.95 Å against 1.33 Å, so the pore cannot be a simple sieve.

Think of it as a trade. In water, every ion wears a coat of water molecules with their oxygen atoms turned toward it, and stripping that coat costs energy. An ion will enter a narrow pore only if the pore pays the cost back.

In 1998 Roderick MacKinnon’s laboratory showed how, with the structure of KcsA, a potassium channel from a soil bacterium. Four identical subunits surround one pore. Near the outer end it narrows to a selectivity filter 12 Å long, too tight for an ion to keep its water. The filter is lined with carbonyl oxygen atoms from the protein’s backbone, set in rings that surround a K+ ion much as the oxygens of water did. Potassium swaps water for protein and loses almost nothing. Sodium is too small to touch all the oxygens of a ring at once, and the filter is held too stiffly to close in on it. For Na+, leaving water costs more than the filter repays, so it stays outside.

The same structure explains the speed. Two K+ ions sit in the filter in single file, and the repulsion between them pushes one out as the next arrives. That is the answer for potassium channels; sodium and calcium channels build their filters differently.

Three keys for one gate: a ligand, a voltage, the outside world

The gate is what makes a channel a receptor, and gates answer to three kinds of key.

A ligand from outside. The acetylcholine receptor of muscle is the model (lesson receptor-field-guide): transmitter binds and the pore opens. Glutamate-gated channels let Na+ in and excite a neuron, while GABA-A receptors pass Cl− and hold its voltage down. Remember that “Both ionotropic and metabotropic can use ion channels.” An ionotropic receptor is the channel. A metabotropic receptor is a G-protein-coupled receptor that reaches a channel through a messenger inside the cell. You know four channels opened from inside: the heart’s potassium channel by Gβγ (lesson gpcrs-and-g-proteins), the rod’s channel by cGMP (lesson senses-and-cgmp), and the IP3 receptor and Orai (lesson calcium-signaling).

A voltage. Voltage-gated channels read the membrane itself. Each has four voltage sensors, and in each one a helix called S4 carries a positive charge at about every third position. A voltage across a membrane is an electric field, and a field pushes on charge. Depolarise the membrane and S4 shifts outward and pulls the gate open.

This drives the action potential, the nerve impulse. A small depolarisation opens some sodium channels, Na+ enters, the membrane depolarises further and more channels open. That positive feedback (lesson signaling-circuits) carries the voltage toward the E of sodium within about a millisecond. Then a loop on the inner side of each sodium channel shuts the pore (inactivation), while slower potassium channels open and bring the voltage back. Sodium that entered at one spot depolarises the next stretch of membrane, so the impulse travels along the axon, the nerve cell’s long fibre, and the inactivated channels behind it keep it from turning back.

A stimulus from the world. “How does the skin detect heat??” With a channel of the TRP family, which cells use for sensing their surroundings. Before you read its name, work out what it has to be.

Design problem

Design a detector for dangerous heat in a nerve ending in the skin. It must turn a temperature above about 43 °C into a nerve impulse within milliseconds. What kind of protein do you need, which ions flow and in which direction, and how does that small local change become an impulse that travels? Then explain why a chilli pepper feels hot.

A channel that needs two things at once, and what plugs a pore

The NMDA receptor is a glutamate-gated channel with a second lock, found at synapses, the junctions between nerve cells. At the resting voltage a magnesium ion from outside is drawn into the open pore and sticks there, so glutamate opens the gate and almost nothing flows. If the membrane is already depolarised, usually because AMPA-type glutamate receptors beside it have let Na+ in, the magnesium is driven out. The channel conducts only when two things happen together: the cell before the synapse has released glutamate, and the cell after it is already active. (Glycine or D-serine must also be bound.) What enters is Ca2+ along with Na+, and calcium is a messenger (lesson calcium-signaling). From there a path runs to the nucleus and to memory, the subject of a later lesson.

Receptors at a synapse are “Anchored in place by a bunch of different scaffolds.” At glutamate synapses the scaffold PSD-95 holds the tails of NMDA receptors with its PDZ domains (lesson protein-interaction-modules), which helps keep them opposite the site of glutamate release.

Because a channel is a hole, the simplest way to stop it is to plug it. “Pufferfish=tetrodotoxin (TTX).” Tetrodotoxin is a small molecule that lodges in the outer mouth of the voltage-gated sodium channels of nerve and skeletal muscle. Sodium cannot enter, no action potential starts, and the result is paralysis. His “UVa moment: Butch Brodie” belongs here: Edmund “Butch” Brodie of the University of Virginia studies newts that carry the toxin and the garter snakes that eat them. Resistant snakes have changed amino acids at the mouth of their sodium channel, so the toxin binds poorly.

Drugs plug pores as well. Ketamine, an anaesthetic that is also a drug of abuse, enters the open NMDA receptor channel and blocks it from within, and it also makes the channel open less often.

Channels can also be borrowed as tools: “Hacking Trp Channels”. Güler and colleagues (2012) made mice that express TRPV1 in chosen neurons, so that a dose of capsaicin switches those neurons on.