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Roderick MacKinnon, MD, Honored by 2003 Nobel Prize in Chemistry for Work on Elucidating the Structure of Ion Channels

2004; American Society of Nephrology; Volume: 15; Issue: 4 Linguagem: Inglês

10.1097/01.asn.0000118342.72706.69

1533-3450

Steven C. Hebert, Gerhard Giebisch,

Ion channel regulation and function

This past year, Rod Mackinnon shared the Nobel Prize in Chemistry for his groundbreaking discoveries in defining the structures of potassium and chloride channels. This work sheds new light on the mechanisms by which channels determine which ion to permit passage (called selectivity) and open or close (gate) in response to a variety of extracellular and intracellular molecules or ligands (e.g., voltage). Dr. MacKinnon is the third recipient in a Nobel trilogy that has defined the modern era of ion channels. In 1963, Sir John Carew Eccles, Sir Alan Lloyd Hodgkin, and Sir Andrew Fielding Huxley shared the Nobel Prize in Medicine for their discoveries of the ionic mechanisms underlying the action potential—the idea that ion fluxes across a cell membrane generate electrical impulses. In 1991, Erwin Neher and Bert Sakmann shared the Nobel Prize in Medicine for their work providing the means for directly visualizing ion currents through single channels (i.e., the invention of patch clamping). This work demonstrated that pores through biologic membranes provide a pathway for the rapid flux (up to millions of ions per second) of ions that underlie the electrical properties of cells. This technology subsequently allowed identification and classification of the various types of channels based on the ion that permeates the pore (e.g., ionic selectivity for K+, Na+, Cl−, or Ca2+), the electrical conductance, and the mechanism for opening and closing of the pores (channel gating). The third component of the trilogy is the work of Rod MacKinnon and collaborators, which provides the first x-ray crystallographic snapshots of channels. These latter seminal discoveries elucidated the structural basis for the K+ and Cl− channel selectivity and gating, that is, the fundamental properties that make an ion channel. Figure 1Roderick MacKinnon, MDRoderick MacKinnon is the John D. Rockefeller Jr. Professor and Head of the Laboratory of Molecular Neurobiology and Biophysics at Rockefeller University and an Investigator of the Howard Hughes Medical Institute. In addition to the Nobel Prize in Chemistry, Rod MacKinnon also received the Lasker Award in 1999, the Rosenstiel Award in 2000, and the Gairdner Award in 2001. In 2000, he was also honored by the Hodgkin-Huxley-Katz Prize of the British Physiologic Society and was elected to the US National Academy of Sciences. After receiving a BA in Biochemistry from Brandeis in 1978 and the MD from Tufts, he returned to Brandeis for postdoctoral studies in the laboratory of Christopher Miller, PhD, where he began his scientific journey on ion channel function. He joined the faculty at Harvard Medical School in 1999, where he used electrophysiological, biochemical, and molecular biologic tools to identify regions in K+ channels that control ion selectivity and gating. Realizing that an in-depth understanding of these cardinal channel properties would require visualization of the K+ channel protein structure, he began his pursuit of this goal after joining the faculty at Rockefeller University. This was a lofty but difficult goal, as very few membrane proteins, in contrast to soluble proteins, had been crystallized. Many thought he was embarking on a fool’s errand. He succeeded! Over the past 40 years, we have come to understand the fundamental importance of ion channels in cell biology (including renal physiology and pathophysiology). Ion channels set the cell membrane potential, generate electrical signals in excitable cells, regulate cell volume and cell movement, as well as mediate net transport of certain ions in renal epithelia (e.g., K+ secretion in distal nephron segments). Ion channels are also common targets for pharmaceutical agents. Moreover, mutations in ion channel genes have been identified in a number of inherited diseases–the ROMK K+ channel gene (KCNJ1) and the Cl− channel gene (CLC-KB) in Bartter syndrome and the Na+ channel gene (ENaC) in Liddle syndrome, to name only a few. Rod MacKinnon’s work has unveiled the beauty and economy of nature in defining the structural basis of ion selectivity and gating by K+ channels. The movement of ions through channels is governed by conformational changes resulting in channel opening or closing (i.e., gating) that respond to membrane voltage and/or a variety of extracellular and intracellular molecules or ligands. The means by which ion channels are able, on the one hand, to discriminate between anions and cations (and even between cations like K+ and Na+) and, on the other, to allow rapid passage of the ions has been a fundamental question in biology. It turns out that a short and narrow region of the outer pore, the selectivity filter, provides oxygen atoms on carbonyl side chains of amino acids that interact with K+ ions in a way that is virtually identical to water oxygen molecules that form the hydration shell around the K+ ion in free solution. This narrow passageway is just the right size to coordinate best with K+, but not Na+, ions—accounting for selectivity. In addition, the portion of the transmembrane pore beyond the outer selectivity filter is a water-filled cavity so that the majority of the transit through the greasy membrane is facilitated. This beautiful design allows for both discrimination among ions and the rapid transit of ions across the membrane. In electrically active cells like neurons, channels have a gatekeeper function that can shut down ion conduction, a process called inactivation. Inactivation is critical for defining the shape of action potential that mediates transmission of electrical signals in nerve and muscle. In 2001, MacKinnon and co-workers used x-ray crystallography to solve the puzzle of how the inactivation gate works. They found that the protein tail at the end of the channel can enter the cytoplasmic part of the channel pore as an extended peptide to plug the channel—putting a cork in the bottle. Another gatekeeping puzzle being solved by the MacKinnon group is elucidation of the gating mechanism. Gating can be seen as the transduction of voltage or ligand binding to the switching on or off of K+ channels. This is an area of active investigation that is crucial to understanding channel function in cells (e.g., nerves and muscle) and how certain pharmacologic agents work. In a landmark series of papers, MacKinnon and co-workers provided the first structural views of how gating works. Their observations have not only shown that the gate is separate from the selectivity filter, but they also provided new ideas of how unexpected conformational changes can account for channel opening or closing. An important consequence of the separation of the selectivity filter and gates permits conformational changes in the protein to open or close the channel without altering its ability to discriminate among ions. All is, however, not so simple. In Cl− channels where gating is closely tied to Cl− ion conduction. MacKinnon and co-workers sought to determine the structural basis of gating in ClC Cl− channels using the same type of x-ray crystallographic approach that had been successful with K+ channels. ClC channels permit passage of Cl− ions across biologic membranes that govern the electrical activity of excitable cells like muscle and certain neurons. ClC channels also play critical roles in Cl− absorption in the loop of Henle and distal tubule (mutations in ClC-Kb or its accessory subunit, barttin, cause Bartter syndrome) and are important to endocytic processes in the proximal tubule (e.g., mutations in ClC5 cause Dent disease with a low molecular weight proteinuric and Fanconi-like phenotype). MacKinnon and colleagues provided the first 3-D view of a ClC channel from the bacterium, Escherichia coli. This has provided a fascinating look at how this channel selects and conducts Cl− ions. When comparing the selectivity filters of K+ and Cl− channels, MacKinnon recognized a fundamental conserved principle from a chemical point of view. In both channels, charges on alpha-helices (oxygen in K+ channels and nitrogen in Cl− channels) are used to stabilize the respective ion in the pore. These very exciting and fundamental discoveries by Roderick MacKinnon and colleagues have not only provided a new molecular understanding of channel function, but they should also promote discovery of new channel-altering drugs and how they work. Moreover, we will begin to understand how mutations that alter the amino acid sequence of these channel proteins modify channel and cell function in inherited disorders.