Voltage-dependent channel structure reveals masterpiece responsible for all nerve, muscle activity
Scientists studying the tiny devices — called voltage-dependent ion channels — that are responsible for all nerve and muscle signals in living organisms for 50 years have been working like a bunch of blindfolded art critics.
Imagine evaluating sculptures, for example, without the privilege of sight. The task of analyzing visual art would shrink to partial, indirect descriptions that fail in conveying the object’s true character. Some aspects of the work of art and its surroundings would still be accessible, but ultimately the analysis would be unsatisfying, inaccurate and misleading.
Rockefeller’s Roderick MacKinnon, M.D., a Howard Hughes Medical Institute Investigator, Youxing Jiang, Ph.D., and their colleagues have removed the blindfold to reveal a masterpiece of nature’s engineering.
The masterpiece is a voltage-dependent potassium ion channel, the first published picture of which illustrates the cover of Nature’s May 1 edition, revealed by MacKinnon and his rainmaking group of biophysicist-crystallographers. Inside, Nature features two articles, co-authored by
Vanessa Ruta, Alice Lee, Jiayun Chen, Brian Chait, D. Phil.,
and Martine Cadene, Ph.D. that correct the proposals of
scientists who lacked the definitive picture showing a potassium channel with charge-triggered "paddles" responsible for opening and closing a passage for potassium ions to freely move through.
"While their research data was very good, their interpretation was incorrect," says MacKinnon. "Ambitious and gifted scientists had for years only a sketch, based on indirect experiments, for what the voltage-dependent channel actually looked like."
In 1998, MacKinnon and his research team published the very first potassium channel structure, which revealed the way that positively charged potassium ions flow easily through a protein’s pore spanning the cell membrane. In the five years following, the Rockefeller scientists have revealed the inner workings of sodium and potassium channels, or the why’s and how’s of ion movement through a cell’s membrane. What this earlier research did not do, however, was explain how a fundamental feedback loop worked.
Now, with the structure of the voltage-dependent ion channel, based on research carried out at The Rockefeller University, the National Synchrotron Light Source at Brookhaven National Laboratory and Cornell High Energy Synchrotron Source , MacKinnon’s group has answered the question of how this kind of channel functions as a voltage-dependent switch, driving muscle and nerve activity in all living organisms.
The May 1 findings not only portray an elusive ion channel structurally and mechanistically — the fifth such portrayal by MacKinnon’s group in as many years — but bring history full circle by showing, for the first time, the natural molecular mechanism that underlies the theory of the action potential demonstrated in a mathematical formulation by Alan Hodgkin and Andrew Huxley in 1952.
MacKinnon and colleagues have eclipsed their own already formidable contributions to the field of ion channel research — contributions that widen a foot trail into an avenue for an entire new area of medical study on ion channel diseases.
Still life, with electricity
Voltage dependent ion channels bring an explosive, and then restorative, burst of energy to an otherwise placid cell membrane. In a nerve cell, for example, the explosive burst begins when a neurotransmitter, called glutamate, hits a sensitive receptor on a cell’s surface that signals the need for a sea change. Glutamate triggers sodium-conducting channels on the cell’s surface to open up, allowing positively charged sodium to flow into the negatively charged interior of the cell. Almost instantaneously, neighboring voltage dependent sodium channels (Nav) open in response, allowing more sodium to enter, creating an upset in the normal negative inside, positive outside voltage common to all living cells.
As soon as the explosive cascade of sodium channeling begins, hyper-sensitive voltage-dependent potassium channels (Kv) along the same cell’s surface sense the catastrophic switching of the charge value inside the cell, and in their own domino effect, open up to allow positively charged potassium ions to quickly flow out of the cell. This movement restores a cell’s normal negative inside, positive outside charge value. The cell returns to its former calm.
The entire sequence of events takes only a few milliseconds, and occurs tens of thousands of times every day in human beings and organisms of all varieties. Without this hair trigger electrical system, life would be more than calm. There would be scant possibility of thinking, breathing or movement.
A picture is worth a thousand differential equations
The burst associated with the paired activities of sodium and potassium voltage-dependent ion channels underlies the theory of the action potential.
In 1952, scientists Hodgkin, Huxley and Bernard Katz, using the visible nervous system of squid, explained the action potential as the basis for the generation and propagation of the nerve impulse. Their landmark series of papers described the action potential in terms of mathematical equations (Hodgkin and Huxley won the Nobel prize in 1963). Their theory became a cornerstone of cellular biophysics, and set an entire field of scientists to work studying excitable membranes, or the outer coats of cells associated with all nerve and muscle activities, including the brain and the heart.
The three prescient scientists originally identified a feedback loop relating cell membrane permeability (ability of a cell’s coat to open up via a tiny device called an ion channel) and voltage (positive or negative charges): one condition determines the next, which in turn, creates the first condition all over again. After decades of studying the action potential, scientists have shown that the permeability sets the membrane voltage. In other words, ion channels’ opened or closed positions have an impact on the charge value of the cell membrane.
Scientists understand how membrane permeability to ions sets the voltage. Until now, however, no group of researchers had ever solved the riddle of how the membrane voltage determines the permeability, completing the feedback loop.
The combination of a picture of the voltage-dependent potassium channel at an atomic level and the functional and biochemical analysis by MacKinnon and his colleagues answers this question.
"We can now see how membrane voltage will change the shape of this kind of potassium channel," says MacKinnon, "causing it to open and close depending on the polarity of the membrane voltage."
MacKinnon’s group provides the first picture of a potassium voltage-dependent ion channel. Other kinds of voltage-dependent channels — sodium and calcium — will look very similar because scientists know the genetic sequence of all three types. In these sequences, high conservation, or duplication, across types, tells researchers that these channels share similar structure.
What’s more, while the Rockefeller scientists conducted their research using voltage-dependent potassium channels from archaea, a type of primitive organism, called Aeropyrum pernix that live in oceanic thermal vents, they found that tarantula and scorpion nerve toxins, whose purpose is to inhibit channels from higher organisms like humans and other mammals, also inhibit the archaea channels.
Vanessa Ruta, a graduate fellow at Rockefeller and first author on a March 2 letter in Nature described the results: "There is no reason for a South American tarantula or a Middle Eastern scorpion to inhibit an exotic, lower order creature that resides in a Japanese ocean thermal vent. This implies that voltage-dependent potassium channels are extremely similar throughout every branch of the evolutionary tree."
A colleague of MacKinnon, Rockefeller Professor David Gadsby, who heads a separate lab and also studies ion channels and their functioning, cites MacKinnon’s attentiveness to conserved parts as one of the keys to his canny leadership. "Rod pays close attention to evolution, noting which sequences are conserved. When he finds them he homes in to investigate the very essence of simple creatures’ ion channels that is consistently retained in more complex organisms."
Model scientists, not magic
"In almost every scientific forum," says MacKinnon, "someone always asks the question. ‘Do you have a magic wand?’"
The question arises because no other scientist has ever led a research team so successfully in solving elusive protein structures using x-ray crystallography, then using those structures as the centerpiece of a rigorous analysis of one of biology’s fundamental problems: how do voltage and permeability at the cell membrane form a reliable, instantaneous, feedback loop?
MacKinnon insists the secret to his success is the team of hardworking, exemplary researchers who seek him out.
"The researchers in my lab are extremely determined, and willing, for example, to purify the same protein not once, but hundreds of times, in order to verify it and produce enough of what we need for making crystals," says MacKinnon.
The voltage dependent potassium channel is but one example of the determination MacKinnon cites. It took postdoctoral associate Youxing Jiang, with the help of staff scientists Alice Lee and Jiayun Chen, five years to solve the structure of the voltage gated potassium channel. In desperation mid-way through, he and his colleagues in MacKinnon’s lab took up another project, the calcium-dependent potassium channel, which they solved and published in Nature in 2002.
"Many times I wanted to give up; it’s a balance between frustration and ambition," says Jiang. "But when we get very desperate in the lab, we always catch a hopeful glimpse of something."
MacKinnon adds: "Even when the research is discouraging, there’s always one door left slightly ajar."
Rockefeller University. April 2003.