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Receptors on Cell Surfaces Sense Environment
In our eyes, nose and mouth, we have sensors for light, odours and flavours. Within the body, cells have similar sensors for hormones and signalling substances, such as adrenalin, serotonin, histamine and dopamine.
As life evolved, cells have repeatedly used the same basic mechanism for reading their environment : G-protein – coupled receptors. But they remained hidden from researchers for a long time.
In a human, tens of thousands of billions of cells interact.
Most of them have developed distinct roles. Some store fat; others register visual impressions, produce hormones or build up muscle tissue.
In order for us to function, it is crucial that our cells work in unison, that they can sense their environment and know what is going on around them. For this, they need sensors.
Sensors on the cell surface are called receptors. Robert J. Lefkowitz and Brian K. Kobilka are awarded the 2012 Nobel Prize in Chemistry for having mapped how a family of receptors called G-protein–coupled receptors ( GPCRs ) work. In this family, we find receptors for adrenalin ( also known as epinephrine ), dopamine, serotonin, light, flavour and odour.
Most physiological processes depend on GPCRs. Around half of all medications act through these receptors, among them beta blockers, antihistamines and various kinds of psychiatric medications.
Knowledge about GPCRs is thus of the greatest benefit to mankind. However, these receptors eluded scientists for a long time.
An Elusive Enigma
At the end of the 19th Century, scientists began experimenting with adrenalin’s effects on the body. They soon realised that it does not work via nerves in the body and they concluded that cells must have some kind of receptor that enables them to sense chemical substances — hormones, poisons and drugs — in their environment.
But when researchers attempted to find these receptors, they hit a wall. They wanted to understand what the receptors look like and how they convey signals to the cell. The adrenalin was administered to the outside of the cell, and this led to changes in its metabolism that they could measure inside the cell.
Each cell has a wall : a membrane of fat molecules that separates it from its environment. How did the signal get through the wall? How could the inside of the cell know what was happening on the outside?
The receptors remained unidentified for decades. Despite this, scientists managed to develop drugs that specifically have their effect through one of these receptors.
In the 1940s, the American scientist Raymond Ahlquist examined how different organs react to various adrenalin – like substances. His work led him to conclude that there must be two different types of receptors for adrenalin. He called the receptors alpha and beta.
Such drugs undoubtedly produced effects in the cells, but how they did so remained a mystery. We now know why the receptors were so difficult to find: they are relatively few in number and they also are mostly encapsulated within the wall of the cell.
It was only at the end of the 1960s that Robert Lefkowitz enters the history of these receptors.
The young top student has his mind set on becoming a cardiologist. However, he graduates at the height of the Vietnam War, and he does his military service in the US Public Health Service at a federal research institution, the National Institutes of Health.
There he is presented with a grand challenge : Finding the receptors.
Lefkowitz’s supervisor already has a plan. He proposes attaching radioactive iodine to a hormone. Then, as the hormone binds to the surface of a cell, the radiation from the iodine should make it possible to track the receptor. Lefkowitz would also have to show that the hormone’s coupling to the cell’s outside actually triggers a process known to take place on the inside of the cell.
Lefkowitz begins working with adrenocorticotropic hormone, which stimulates the production of adrenalin in the adrenal gland. As the project enters its second year, Lefkowitz finally makes some progress. In 1970, he publishes articles in two prestigious journals where he outlines the discovery of an active receptor.
Using radioactively tagged substances, including beta blockers, his research group examines how these receptors work. And after fine-tuning their toolkits, they manage with great skill to extract a series of receptors from biological tissue.
Meanwhile, the knowledge about what happens inside cells has been growing. Researchers have found what they call G-proteins ( Nobel Prize in Physiology or Medicine 1994 ) that are activated by a signal from the receptor.
The G-protein, in turn, triggers a chain of reactions that alters the metabolism of the cell. By the beginning of the 1980s, scientists are starting to gain an understanding of the process by which signals are transmitted from the outside of the cell to its inside.
In the 1980s, Lefkowitz decides that his research group should try to find the gene that codes for the beta receptor.
This decision would prove to be crucial to this year’s Nobel Prize. The idea was that if the research group could isolate the gene and read the blueprint for the beta receptor, they could get clues as to how the receptor works.
At about the same time, Lefkowitz hires a young doctor, Brian Kobilka. Kobilka wanted to study the power of epinephrine in its smallest molecular detail.
Kobilka engages in the hunt for the gene. However, during the 1980s, trying to find a particular gene in the body’s enormous genome is a bit like trying to find a needle in a haystack.
However, Kobilka has an ingenious idea that makes it possible to isolate the gene. With great anticipation, the researchers begin to analyze its code; it reveals that the receptor consists of seven long and fatty ( hydrophobic ) spiral strings — so – called helices.
This tells the scientists that the receptor probably winds its way back and forth through the cell wall seven times.
Seven times. This was the same number of strings and same spiral shape as a different receptor that already had been found elsewhere in the body: the light receptor rhodopsin in the retina of the eye.
An idea is born: could these two receptors be related, even though they have completely different functions?
Robert Lefkowitz later described this as a “Real Eureka Moment”. He knew that both adrenergic receptors and rhodopsin interact with G-proteins on the inside of the cell. He also knew of about 30 other receptors that work via G-proteins.
There has to be a complete family of receptors that look alike and function in the same manner!
Since this groundbreaking discovery, the puzzle has been assembled bit by bit, and scientists now have detailed knowledge about GPCRs — how they work and how they are regulated at the molecular level.
Lefkowitz and Kobilka have been at the forefront of this entire scientific journey, and last year, in 2011, Kobilka and his team of researchers reported a finding that put the crown atop their work.
After successfully having isolated the gene, Brian Kobilka transferred to Stanford University School of Medicine in California. There he set out to create an image of the receptor — an unattainable goal in the opinion of most of the scientific community — and for Kobilka, it would become a long journey.
Imaging a protein is a process involving many complicated steps.
Scientists use a method called X-ray crystallography. The first image of a crystal structure of a protein was produced in the 1950s. Since then, scientists have X-rayed and imaged thousands of proteins.
However, a majority of them have been water – soluble, which facilitates the crystallization process.
Fewer researchers have managed to image proteins located in the fatty membrane of the cell.
In water, such proteins dissolve just as poorly as oil, and they are prone to form fatty lumps.
Furthermore, GPCRs are by nature very mobile ( They transmit signals by moving ), but inside a crystal they have to remain almost completely still. Getting them to crystallize is therefore a considerable challenge.
It took Kobilka over two decades to find a solution to all these problems. But thanks to determination, creativity and molecular biology sleight of hand, Kobilka and his research group finally achieved their ultimate goal in 2011 : they got an image of the receptor at the very moment when it transfers the signal from the hormone on the outside of the cell to the G-protein on the inside of the cell.
Life Needs Flexibility
The mapping of the over one hundred human receptors still presents challenges to scientists, as their purposes have yet to be figured out.
Researchers have also found that they are multifunctional; a single receptor can recognize several different hormones on the outside of the cell. The receptors’ number and flexibility enable the fine – tuned regulation of cells that life requires.