Leaning back in an office busy with mysterious images of rotating, luminescent molecules suspended in a jet-black vacuum, she laughingly recounts that her colleagues have at times called her a dreamer. Her goal, "to try to understand the principles of life from the inside by unraveling the detailed structure of ribosomes," has taken Prof. Ada Yonath of Weizmann's Structural Biology Department on a long uphill road strewn with technological and conceptual barriers.

Ribosomes are essential to life. Upon receiving genetically encoded instructions from the cell nucleus, the ribosomal factory churns out proteins - the body's primary component and the basis of all enzymatic reactions. Understanding protein biosynthesis is therefore the gateway to grasping life itself, and its darker side - the emergence of disease when production goes haywire.

This explains why ribosomes have been the target of biochemical, biophysical, and genetic studies. However, throughout nearly four decades of research, these pivotal biological units have resisted scientific attempts to reveal their functional design. To examine microscopic structures, scientists expose crystals of the material in question to high-intensity X-ray beams, a method known as X-ray crystallography. Yet the ribosome represents a daunting crystallographic challenge. Notoriously unstable, this giant protein complex also lacks the internal symmetry and repetitions that have eased the way to understanding the structure of other biological entities, such as viruses.

Indeed, back in the late 1970s when Yonath began her research, most scientists viewed her quest as a mission impossible. "Top teams around the world, such as those at UCLA and MIT in the United States and the Medical Research Council in England, had been trying to crystallize ribosomes since the 1960s - with no success. I thought, this is such a delightful group of "unsuccessful" people - they were all Nobel prizewinners - I would like to be among them," Yonath recalls, smiling.

And her determination paid off. In 1980, having tried over 25,000 conditions for growing ribosomal crystals, Yonath, who divides her time between Weizmann and the Max Planck Research Unit for Ribosomal Structure in Hamburg and Berlin, produced the first ribosomal crystal ever.

Now she has reached another scientific landmark: she has determined the structure of the small ribosomal subunit of the bacterium Thermus thermophilus at the highest resolution ever achieved. Her study recently appeared in the Proceedings of the National Academy of Sciences (PNAS).

The uniqueness of Yonath's approach lies in phasing - designing heavy atoms as markers that stand out like flares in the ribosomal map due to their high electron density. These markers significantly enhance the ability to pinpoint functional units within the ribosome.

Ribosomes consist of two independent subunits of unequal size. Yonath set her sights on 30S, the smaller subunit. More specifically, she wanted to capture "snapshots" of 30S in its active form - during the precise moment that protein biosynthesis begins. To do this, her team introduced an analogue of messenger RNA - a molecular go-between arriving from the nucleus. "The messenger attaches itself to a specific site thereby opening the gate to protein biosynthesis, which is essentially kept under lock and key," Yonath explains. Once activated and bound, it was possible to catch the 30S subunit in the act, by flash freezing the crystals at cryo-temperature (-185C or -365F).

Yonath's findings are the result of almost twenty years of experimentation. Along the way, she became the first scientist to create ribosomal crystals that diffract to high resolution, around 3 angstrom (1A = 39.37-10 inches). "My belief in the possibility of obtaining these crystals was partially inspired by natural phenomena," she says. "In nature, stressful biological conditions trigger "packing" of ribosomes into condensed crystalline structures that help prevent ribosomal damage. It is this structure that enables bears to resume protein production in the spring, following hibernation. The same thing happens in fertilized chicken and lizard eggs when shock-cooled, and in the brain cells of people suffering from dementia." Ironically, Yonath's quest for durable crystals to explore protein biosynthesis, fundamental to life, eventually led to the shores of the Dead Sea. There, two hardy strains of thermophilic (heat-loving) and halophilic (salt-loving), bacteria proved ideal candidates. "After all, they've been around almost unchanged for five million years," she explains. Yonath also pioneered cryo-cystallography, today a standard research procedure in structural biology. This approach is based on exposing crystals to cryo-temperature during X-ray measurement to minimize their disintegration.

The goal of near-atomic resolution of ribosomal structures is closer than ever before, following Yonath's latest achievements. Her approach and procedures have been repeated by a growing contingent of international researchers - all racing to unravel the mystery of ribosomal functioning. Superior strategies for fighting the pathogenic protein biosynthesis characterizing cancer cells, and improved antibiotics that target bacterial agents at the ribosomal level might yet be among the legacies of these hard-won ribosomal revelations.

Prof. Ada Yonath holds the Martin S. Kimmel Chair. Her research is supported by the Helen and Milton Kimmelman Foundation, New York.

Organic molecules could shape the future of electronics, say Weizmann Institute scientists who have recently placed common semiconductor-based devices, for the first time ever, under these molecules" control.

The inclusion of organic molecules in electronics would provide an extensive range of possibilities. However, attempts to do so have been impeded by "pinholes" - small defects in the layers of organic molecules used in semiconductor research. Pinholes are very difficult to detect and yet radically sway conductance. Scientists were unable to determine whether electric current measurements resulted from the passage of the current through these pinholes or through the organic molecule itself.

Ayelet Vilan, a graduate student working with Prof. David Cahen of the Weizmann Institute's Materials and Interfaces Department, decided to skirt the problem. Using newly synthesized organic molecules, she constructed a one-molecule-thick layer that is so thin the electric current generally passes by the molecules without interacting with them. The problem of assessing whether the current passed via an organic molecule or via a pinhole was thus eliminated. This enabled the accurate analysis of these molecules" effect on the semiconductor.

The scientists also found that changing the organic molecules used in the monolayer led to a predictable change in electrical characteristics  meaning that they could control the semiconductor's properties.

To work with the very fragile monolayers, Vilan developed a new method for preparing semiconductor devices. The technique is founded on a widely used semiconductor device (diode), which is comprised of a semiconductor (called GaAs) connected to a metal. She inserted the organic monolayer between these two components. Since it was essential to ensure that the monolayer would not be crushed, Vilan, building on the findings of Ellen Moons, one of Cahen's former students, used a thin gold leaf as the metal sheet and gently floated it onto the monolayer.

The study, published in Nature, introduces a feasible way to incorporate organic molecules into electronic devices. "But mainly," says Vilan, "it provides new insights into the emerging field of molecular electronics. So little is known about the interactions that occur between organic molecules and the electric conductors we normally use. This approach may provide a basis for designing novel types of semiconductor-based devices, from improvements in relatively simple applications, such as solar cells, to new computer chips."

Prof. David Cahen's research is supported by the Fusfeld Research Fund, Pennsylvania.

Despite more than a century of research on inhibitory neurons, very little is known about how this small population (10-20% of brain neurons) exerts its controlling effect on the brain.

Inhibitory neurons are pivotal for normal brain development, learning, and memory, so it is not surprising that they are involved in most neurological disorders. A recent Weizmann study, published in the January 2000 issue of Science, reveals key principles underlying the design and function of this inhibitory system.

By repressing the level of activity in neighboring neurons, inhibitory neurons (I-neurons) prevent the brain from quickly spinning out of control into hyperexcited states or full-blown epilepsy. One of the signs in children with autism and attention deficit hyperactivity disorder (ADHD) is I-neuron malfunction: their inhibitory system does not effectively suppress unwanted information, impeding their ability to make choices. I-neuron malfunction is involved in the majority of neurological disorders: Alzheimer's disease, neural trauma, addictions, and a wide range of psychiatric problems such as depression, obsessive-compulsive behavior, and schizophrenia.

In the past, researchers basically thought that I-neurons just spray an inhibitory neurotransmitter called GABA onto their neighbors. But this did not explain how they inhibit the right neurons at exactly the right time and to the right degree. The new study carried out in the laboratory of Prof. Henry Markram of the Neurobiology Department shows how this is achieved.

The research team found new types of I-neurons, revealing that this tiny population is much more diverse than previously thought. Further, using new methods that they developed, the researchers succeeded in recording directly how individual inhibitory neurons control their neighbors. They found that I-neurons build complex synapses (connections) onto their target neurons. The synapses selectively filter inhibitory messages, enabling I-neurons to shut down the activity in neighbors as required. These synapses act as fast-switching "if-then" filtering gates that allow inhibition to be applied only at the exact millisecond and only to the extent necessary.

Each I-neuron fixes complex if-then gates on thousands of neighboring neurons and is therefore "in charge" of controlling their activity. The gates allow I-neurons to rapidly switch their focus onto any neuron to which they are connected. This ingenious design principle is what enables the small group of I-neurons to exert such a sophisticated effect, simultaneously giving personal attention to the activity of each of the neurons to which they are linked.

The researchers showed that a "discussion" between I-neurons and target neurons is involved in deciding which type of if-then gate should be set up to filter the inhibitory message. This decision-making process could allow each neuron in the brain to be inhibited in a unique way. Dubbed the "interaction principle," this process generates maximal diversity of if-then gates, allowing more complex and more refined control over large numbers of neurons.

The researchers went on to reveal a remarkable ability of I-neurons: they can sense neurons that share the same functions in the brain. I-neurons "select" groups of target neurons to construct the same type of if-then gates, possibly enabling the I-neurons to control groups of neurons collectively. It also means that I-neurons can "sniff out" brain neurons that collaborate in the most elementary functions even if they seem different in almost every other way (i.e., they can identify neurons descended from the same "ancestors").

"I-neurons can trace the family trees of neurons. In other words, they could help us to work out how neurons are related to one another. This could one day enable us to map the functional aspect of the brain according to the genealogy of its neurons  an organizing principle we never dreamed would be possible, says Markram. The researchers believe that the ability to detect functionally related groups in the brain, called "the homogeneity principle," results from common signal molecules released by target cells. I-neurons may use the signal molecules to determine what kind of if-then gates to build. Future research designed to identify the nature of these molecules could yield a potent tool for mapping the functional structure of the brain.

Prof. Markram's research is supported by the Minna James Heineman Stiftung, Germany; the Abramson Family Foundation, North Bethesda, Maryland, and the Nella and Leon Benoziyo Center for Neurosciences.

 

 

While crucial to warding off disease, immune cells have traditionally been thought to be potentially damaging to the central nervous system (CNS) - the brain and the spinal cord. However, a team of Weizmann Institute scientists has now found that immune cells can be recruited to treat partial spinal cord injuries.

Severing the spinal cord causes complete paralysis of the organs innervated by the central nervous system, from the point of injury downward. In fact, even a partial injury of the spinal cord may cause complete paralysis, due to the "hostile environment" created by damaged fibers causing harm to the undamaged fibers. As a result, even in cases of partial spinal cord injury, the damage continues to spread, intensifying the paralysis. Blocking the spread of damage may therefore save the nerve cells undamaged by the initial trauma - and with them, at least some of the patient's motor activity.

Several years ago, a team of Weizmann researchers led by Prof. Michal Schwartz of the Neurobiology Department found that following neuronal injury, immune cells known as macrophages may be recruited to encourage repair and renewed growth of damaged nerve fibers. Schwartz now hopes to take this research one step further. In a study recently published in The Lancet, she proposes adding additional immune cells, known as T-cells, to the damage-control battalion aimed at blocking the spread of damage.

At first glance, this idea seems to oppose the widespread view of immune cells as potentially damaging to the central nervous system. Indeed, while macrophages normally help to heal damaged tissue, previous research by Schwartz revealed that the mammalian CNS actually suppresses an immune response following injury. This suppression may be the result of an evolutionary trade-off. In contrast to fish and other lower life forms capable of repairing damaged CNS fibers, humans and other mammals can repair only peripheral nerves, while injuries to the brain or spine leave them permanently paralyzed or otherwise handicapped.

To get smart, higher animals may have had to pay a price, Schwartz suggests. Along with the asset of complex brains capable of continuous learning came a disadvantage - loss of the self-healing ability existing in lower vertebrates. "The need to prevent immune cells from "remodeling" the brain may have dictated losing the tissue-repair capacity since the immune cells could disrupt the complex and dynamic neuronal networks that build up during a lifetime," says Schwartz.

T-cells prevent infection by seeking out and destroying pathogenic "enemies" that infiltrate the body. But the body also contains T-cells that are directed against its own components. The accepted notion is that these anti-self T-cells may cause autoimmune diseases, such as multiple sclerosis and diabetes. However, the Institute scientists have shown that a controlled amount of these autoimmune cells, when directed against specific components, can assist in curbing injury-induced neuronal damage.

Following treatment with anti-self T-cells, rats with partial injuries of the spinal cord regained some motor activity in their previously paralyzed legs while untreated rats developed increasing and sometimes even total paralysis. These findings may lead to an innovative clinical treatment for preventing total paralysis after partial spinal cord injury.

Immune cells have a double-edged sword potential in treating neuronal injuries, Schwartz explains. The key to using them effectively is to extract the cells from the patients blood and increase their amount and activity in such a way that their healing effect is maximized while their potential risk is minimized. The cells are then reintroduced into the damaged neuronal area. Schwartz: "The concept is to work together with the body's existing self-repair mechanism, which apparently requires encouragement and monitoring."

Other scientists participating in this study were Weizmann Profs. Irun Cohen of the Immunology Department, Michal Neeman of the Biological Regulation Department, and Prof. Solang Akselrod of Tel Aviv University. Working with Prof. Michal Schwartz were Dr. Eti Yoles, Dr. Eugenia Agranov, Ehud Hauben, Uri Nevo, and Gila Moalem, all of the Neurobiology Department.

Prof. Michal Schwartz holds the Maurice and Ilse Katz Chair of Neuroimmunology. Her research is funded Proneuron Ltd., the Alan T. Brown Foundation to Cure Paralysis, New York, the Glaucoma Research Foundation, San Francisco, California, and the Jerome and Binette Lipper Award.