The human body is made up of many different types of tissues and many different kinds of cells. To co-ordinate body functions, cells signal to other cells in the same organ and in different organs by releasing chemical messengers that travel through the bloodstream. The chemical messengers control all of the vital body processes. For example, they determine the force of a heartbeat and the number of beats per minute, the height of the blood pressure, and the propulsive energy of the intestine. In the brain these chemicals profoundly influence our moods and our behavior, including our drives for food and sex. When Lefkowitz began his work in the late 1960's, scientists had already identified several chemical messengers but they did not know how these chemicals affected the target cells so as to alter their behavior. Over the subsequent 35 years Lefkowitz and his students painstakingly elucidated a family of molecules on the surface of target cells that receive the chemical messages. These receiving molecules are known as G protein-coupled receptors (GPCRs).
GPCRs are proteins embedded in the surface membrane of target cells with their receiving ends facing the outside fluid. Each cell produces many different GPCRs, each tuned to respond to different chemical messengers. For example, certain GPCRs called beta-adrenergic receptors located on heart muscle cells recognize adrenalin secreted by the adrenal gland and thereby control the heartbeat. When a human is physically threatened, the adrenal gland releases adrenalin which travels through the bloodstream and attaches to beta-adrenergic receptors on heart muscle. Once stimulated by the adrenalin, the receptors initiate a cascade of events that causes the heart to beat stronger and faster. This prepares the threatened person for "fight or flight".
In the late 1960's beta-adrenergic receptors were a theoretical concept. Scientists knew that adrenalin stimulated heart muscle cells, and they postulated that there must be a receptor that transmits this message. It is a long way from postulating a receptor to actually having a purified molecule in hand. Undaunted by the enormous challenge, in the late 1960's Lefkowitz began the tedious process of purifying the beta-adrenergic receptor. Purification required separating tiny amounts of the receptor protein from the thousands of other proteins that are much more abundant in cells. The problem was compounded because the beta-adrenergic receptors are designed to function in cell membranes which are composed of oily lipid molecules. The proteins do not dissolve in water and therefore they must be handled with special procedures that differ from those used for water soluble proteins. To overcome these obstacles, Lefkowitz and his colleagues had to employ ingenious technologies, including the use of artificial chemicals that mimic adrenalin and bind very tightly to the receptor. These synthetic chemicals contained radioactive atoms so that tiny amounts could be detected by radiation counting. Lefkowitz and colleagues used detergents to dissolve the receptor from cell membranes, and they devised methods to measure the amount of receptor by allowing it to bind to the radioactive chemical and then measuring the amount of chemical that was bound. Then the receptors were separated from other proteins using special techniques that separate proteins based on their individual properties, such as size, charge and hydrophobicity. The whole process took 15 years. Along the way, they learned much about the chemical properties of the beta-adrenergic receptor, but the ultimate goal was to determine the precise chemical makeup.
Like all other proteins, the beta-adrenergic receptor is composed of a linear chain of amino acids. Each position in this chain contains one of 20 possible amino acids. The sequence of amino acids is specified by the gene for the receptor according to the universal genetic code that was elucidated in the 1960's by Nobel Prize winner Marshall Nirenberg. A milestone was reached in 1986 when Lefkowitz finally had enough purified receptor to permit a molecular characterization. He collaborated with scientists at Merck Sharp and Dohme Research Laboratories to determine the partial sequence of amino acids in the protein using methods developed by Fred Sanger, another Nobel Prize winner. Lefkowitz and his Merck collaborators used this amino acid information to isolate a copy of the messenger RNA encoding the receptor. The messenger RNA consists of a string of chemicals called nucleotides. By determining the sequence of nucleotides in the messenger RNA and following the rules of the genetic code, the workers were able to deduce the sequence of all 418 amino acids in the receptor.
Inspection of the amino acid sequence of the beta-adrenergic receptor caused an immediate shock. The receptor sequence was not entirely unique. It strongly resembled a protein whose sequence was determined several years before. This protein was rhodopsin, the protein in the retina of the eye that acts as a receptor for photons of light and enables vision. Rhodopsin is a very abundant protein in the eye, and it had been relatively easy to isolate. A form of rhodopsin is also found in light-harvesting bacteria, another rich source of the protein. Scientists had shown that rhodopsin is sewn into the membrane of bacteria and photoreceptor cells. Like a sewing thread going through a fabric, the amino acid chain of rhodopsin crisscrosses the membrane seven times. The amino acid sequence of the beta-adrenergic receptor predicted that it also crosses the membrane seven times, and the overall sequence resemblance suggested that the beta-adrenergic receptor must function in the same way that rhodopsin functions.
The great value of this insight lay in the fact that scientists already knew a great deal about how rhodopsin functions. They knew that light triggers rhodopsin to initiate a cascade of chemical reactions relayed by specialized proteins within the cell. These intracellular relay proteins have the property that they bind intracellular chemicals called guanine nucleotides. Even before the beta-adrenergic receptor had been isolated, biochemical experiments by Lefkowitz and other scientists had shown that the beta-adrenergic receptor also acts by stimulating guanine nucleotide-binding proteins. In the genetic code guanine is abbreviated by the letter G and the intracellular relay proteins had been called “G proteins.” In 1994 the Nobel Prize was awarded to Alfred Gilman and Martin Rodbell for their discovery of G proteins. Since beta-adrenergic receptor activation is coupled to G proteins, the receprtor was designated as a G Protein-Coupled Receptor (GPCR). The parallels between rhodopsin and GPCRs allowed scientists to combine insights learned from studies of vision together with those emerging from study of beta-adrenergic receptors and other GPCRs to produce a complete picture of the mechanism by which GPCRs work, and more importantly, to reveal how the activity of the GPCRs is regulated so as to prevent too much or too little activity.
With the messenger RNA sequence of the beta-adrenergic receptor as a starting point, Lefkowitz and others soon found that the genome of animals encodes hundreds of related receptors, each specific for a different chemical messenger. The GPCRs respond not only to chemicals produced within the body, they also respond to chemicals in the environment. The nose has a family of GPCRs called odorant receptors that detect volatile chemicals. A Nobel Prize for this discovery was awarded to Richard Axel and Linda Buck. Certain GPCRs also respond to drugs. Many of the drugs in common use today, including some of those that treat Parkinson's Disease, schizophrenia and high blood pressure, act by binding to specific GPCRs and either increasing their activity or decreasing it. James Black received a Nobel Prize for discovering drugs that act upon GPCRs. He did this even before GPCRs were discovered. The discoveries of Lefkowitz and colleagues explain how Black's drugs work.
Even before the receptors were isolated, indirect experiments had shown that receptors are not always active. After they transmit their signals, the receptors are silenced by a feedback mechanism that prevents over-stimulation. In recent years, Lefkowitz demonstrated a remarkably sophisticated biochemical mechanism that is responsible for such down-regulation. In elucidating this mechanism, Lefkowitz discovered novel proteins that not only silence receptors, but also play diverse roles in physiology, controlling processes that include cell growth and differentiation. This work was aided immeasurably by the ability to compare and contrast discoveries made in the photoreceptor system with those made with the GPCRs.
As with any problem of central importance in biology and medicine, the elucidation of GPCRs was aided by efforts from many laboratories over the past three decades. Although many scientists contributed, Lefkowitz and his colleagues led the way throughout. As a result of their work, pharmaceutical companies now understand how many of their classic drugs control pathologic processes. Examples include β-blockers (such as propranolol) for high blood pressure and congestive heart failure, H2 antagonists (such as cimetidine) for peptic ulcers, H1 antagonists (such as chlorpheniramine) for allergy, and dopamine antagonists (such as clozapine) for schizophrenia. This knowledge now permits companies to search for even more powerful drugs that target a wide variety of G protein coupled receptors, thereby treating a wide assortment of diseases. The future of human health has been aided immeasurably by the work of Robert Lefkowitz, this year's recipient of the Shaw Prize for Life Science and Medicine.
Life Science and Medicine Selection Committee
The Shaw Prize
11 Septemer 2007, Hong Kong