Caltech Researchers Develop Gene Therapy to Boost Brain Repair for Demyelinating Diseases

PASADENA, Calif.—Our bodies are full of tiny superheroes—antibodies that fight foreign invaders, cells that regenerate, and structures that ensure our systems run smoothly. One such structure is myelin—a material that forms a protective, insulating cape around the axons of our nerve cells so that they can send signals quickly and efficiently. But myelin, and the specialized cells called oligodendrocytes that make it, become damaged in demyelinating diseases like multiple sclerosis (MS), leaving neurons without their myelin sheaths. As a consequence, the affected neurons can no longer communicate correctly and are prone to damage. Researchers from the California Institute of Technology (Caltech) now believe they have found a way to help the brain replace damaged oligodendrocytes and myelin.  

The therapy, which has been successful in promoting remyelination in a mouse model of MS, is outlined in a paper published February 8 in The Journal of Neuroscience.

"We've developed a gene therapy to stimulate production of new oligodendrocytes from stem and progenitor cells—both of which can become more specialized cell types—that are resident in the adult central nervous system," says Benjamin Deverman, a postdoctoral fellow in biology at Caltech and lead author of the paper. "In other words, we're using the brain's own progenitor cells as a way to boost repair."

The therapy uses leukemia inhibitory factor (LIF), a naturally occurring protein that was known to promote the self-renewal of neural stem cells and to reduce immune-cell attacks to myelin in other MS mouse models.

"What hadn't been done before our study was to use gene therapy in the brain to stimulate these cells to remyelinate," says Paul Patterson, the Biaggini Professor of Biological Sciences at Caltech and senior author of the study.

According to the researchers, LIF enables remyelination by stimulating oligodendrocyte progenitor cells to proliferate and make new oligodendrocytes. The brain has the capacity to produce oligodendrocytes, but often fails to prompt a high enough repair response after demyelination.

"Researchers had been skeptical that a single factor could lead to remyelination of damaged cells," says Deverman. "It was thought that you could use factors to stimulate the division and expansion of the progenitor population, and then add additional factors to direct those progenitors to turn into the mature myelin-forming cells. But in our mouse model, when we give our LIF therapy, it both stimulates the proliferation of the progenitor cells and allows them to differentiate into mature oligodendrocytes."  

In other words, once the researchers stimulated the proliferation of the progenitor cells, it appeared that the progenitors knew just what was needed—the team did not have to instruct the cells at each stage of development. And they found that LIF elicited such a strong response that the treated brain's levels of myelin-producing oligodendrocytes were restored to those found in healthy populations.

The researchers note, too, that by placing LIF directly in the brain, one avoids potential side effects of the treatment that may arise when the therapy is infused into the bloodstream. 

"This new application of LIF is an avenue of therapy that has not been explored in human patients with MS," says Deverman, who points out that LIF's benefits might also be good for spinal-cord injury patients since the demyelination of spared neurons may contribute to disability in that disorder.

To move the research closer to human clinical trials, the team will work to build better viral vectors for the delivery of LIF. "The way this gene therapy works is to use a virus that can deliver the genetic material—LIF—into cells," explains Patterson. "This kind of delivery has been used before in humans, but the worry is that you can't control the virus. You can't necessarily target the right place, and you can't control how much of the protein is being made."  

Which is why he and Deverman are developing viruses that can target LIF production to specific cell types and can turn it on and off externally, providing a means to regulate LIF levels. They also plan to test the therapy in additional MS mouse models.

"For MS, the current therapies all work by modulating or suppressing the immune system, because it's thought to be a disease in which inflammation leads to immune-associated loss of oligodendrocytes and damage to the neurons," says Deverman. "Those therapies can reduce the relapse rate in patients, but they haven't shown much of an effect on the long-term progression of the disease. What are needed are therapies that promote repair. We hope this may one day be such a therapy." 

The work done in this study, "Exogenous Leukemia Inhibitory Factor Stimulates Oligodendrocyte Progenitor Cell Proliferation and Enhances Hippocampal Remyelination," was funded by the California Institute for Regenerative Medicine, the National Institutes of Neurological Disorders and Stroke, and the McGrath Foundation.

Katie Neith
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Two Caltech Researchers Receive Frontiers of Knowledge Award

For their work in information and communication technologies, and biomedicine, Carver Mead, Moore Professor Emeritus of Engineering and Applied Science, and Alexander Varshavsky, Smits Professor of Cell Biology, have been honored by the BBVA Foundation as recipients of 2011 Frontiers of Knowledge awards. The BBVA Foundation—a social responsibility arm of the multinational Spanish banking group Banco Bilbao Vizcaya Argentaria (BBVA)—presents the 400,000 euro (approximately $520,000) awards to recognize world-class research and "contributions of lasting impact for their originality, theoretical significance, and ability to push back the frontiers of the known world." For more information on the awards, and to read profiles on the winners, click here.

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Alexander Varshavsky Receives King Faisal International Prize for Science

Alexander Varshavsky, Caltech's Howard and Gwen Laurie Smits Professor of Cell Biology, has been awarded the 2012 King Faisal International Prize (KFIP) for Science. The winners of the prize, which also includes awards for medicine, Arabic language and literature, Islamic studies, and service to Islam, were announced in Riyadh, Saudi Arabia, on January 16.

Presented by the King Faisal Foundation, the prize seeks to honor "scholars and scientists who have made significant contributions and advances in areas that benefit developing and Islamic countries, and humanity at large." According to the organization, many winners of the KFIP have gone on to win Nobel prizes for their work.

Varshavsky was recognized for his groundbreaking work in cell biology, including advances that have "created a new realm of biology and have been essential for progress in research on human cancer, neurodegeneration, immune responses, and other fundamental biological processes."

His main recognized contribution was the fundamental discovery, in the 1980s, of biological regulation by intracellular protein degradation and its central importance in cellular physiology. This discovery by the Varshavsky laboratory involved the understanding, through genetic and biochemical insights, of the biological functions of the ubiquitin system, a major proteolytic circuit in living cells. Ubiquitin is a small protein that is present in cells either as a free protein or as a part of complexes with many other proteins. The association of ubiquitin with cellular proteins marks them for degradation or other metabolic fates. Through its ability to destroy specific proteins, the ubiquitin system plays a major role in cell growth and differentiation, DNA repair, regulation of gene expression, and many other biological processes.

"Alex has been a true pioneer in cell biology and this well-deserved award is further recognition of the significance of his work and its broad impact across the biological sciences," says Stephen Mayo, Bren Professor of Biology and Chemistry and chair of the Division of Biology.

Varshavsky is a member of the National Academy of Sciences, the American Academy of Arts and Sciences, the American Philosophical Society, and the Academia Europaea. He has received many international prizes in biology and medicine, including the 2011 Otto Warburg Prize (Germany); the 2008 Gotham Prize in Cancer Research; the 2006 Gagna Prize (Belgium); the 2006 Griffuel Prize (France); the 2005 Stein and Moore Award; the 2001 Horwitz Prize; the 2001 Merck Award; the 2001 Wolf Prize in Medicine (Israel); the 2000 Lasker Award in Basic Medical Research; and the 1999 Gairdner International Award (Canada).

Nominations for the KFIP are accepted from organizations and universities throughout the world, and winners are selected through peer review and a committee of experts in the given field. A total of 47 scholars from 11 different countries have won the prize for science, which was first awarded in 1984. The winners will receive their awards, which include a cash prize of 750,000 Saudi riyals ($200,000), in March during a special ceremony held in Riyadh under the auspices of the king of Saudi Arabia. 

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Making Sense of Sensory Connections

Caltech Researchers Identify Mechanism Behind Associative Memory by Exploring Insect Brains

PASADENA, Calif.—A key feature of human and animal brains is that they are adaptive; they are able to change their structure and function based on input from the environment and on the potential associations, or consequences, of that input. For example, if a person puts his hand in a fire and gets burned, he learns to avoid flames; the simple sight of a flame has acquired a predictive value, which in this case, is repulsive. To learn more about such neural adaptability, researchers at the California Institute of Technology (Caltech) have explored the brains of insects and identified a mechanism by which the connections in their brain change to form new and specific memories of smells.   

"Although these results were obtained from experiments with insects, the components of the mechanism exist also in vertebrate, including mammalian, brains which means that what we describe may be of wide applicability," says Stijn Cassenaer, a Broad Senior Research Fellow in brain circuitry at Caltech and lead author of a paper—published in the journal Nature on January 25—that outlined the findings. The study focused on insects because their nervous systems are smaller, and thus likely to reveal their secrets sooner than those of their vertebrate counterparts.

To home in on sensory memories, the researchers concentrated on olfaction, or the sense of smell. When a person encounters a favorite food or the perfume of a loved one, she will typically experience a recall, usually positive, based on the memories evoked by those smells. Such a recall—to a smell, sound, taste, or any other sensory stimulus—is evidence of "associative" learning, says Gilles Laurent, a former professor of biology at Caltech and senior author of the study, as learning often means assigning a value, such as beneficial or not, to inputs that were until then neutral. The original, neutral stimulus acquires significance as a result of being paired, or associated, with a reinforcing reward or punishment—in this case, the pleasant emotion recalled by a smell.

"When we learn that a particular sensory stimulus predicts a reward, there is general agreement that this knowledge is stored by changing the connections between particular neurons," explains Cassenaer. The problem, however, is that the biological signals that represent value (positive or negative) are broadcast nonspecifically throughout the brain. How then, are they assigned specifically to particular connections, so that a certain sensory input, until then neutral, acquires its new, predictive value? "In this study, we carried out experiments to investigate how the brain identifies exactly which connections, out of an enormously large number of possibilities, should be changed to store the memory of a specific association."

To get a closer look at these connections, Cassenaer and Laurent—who is now director at the Max Planck Institute for Brain Research in Germany—measured neural activity in an area of the locust brain where olfactory memories are thought to be stored. They found that what allows the brain to identify which synapses should be modified, and thus where the nonspecific reward signal should act, is a very transient synchronization between pairs of connected neurons.

"When pairs of connected neurons fire in quick succession, the strength of their connection can be altered. This phenomenon, called spike-timing dependent plasticity, has been known for many years. What is new, however, is recognizing that it also makes these connections sensitive to an internal signal released in response to a reward,” says Cassenaer. "If no reward is encountered, the cells' sensitivity fades. However, if the sensory stimulus is followed by a reward within a certain time window, then these connections are the only ones altered by the internal reward signal. All other connections remain unaffected."

Laurent says that the molecular underpinnings of this phenomenon, as well as the process by which the stored memories are later read out, are an area of much-needed exploration.

"We are currently developing the necessary tools to examine this with sufficient specificity, which will allow us to evaluate animals' behavior as they learn," says Cassenaer.

The study, "Conditional modulation of spike-timing-dependent plasticity for olfactory learning," was funded by the Lawrence Hanson Chair at Caltech, the National Institutes on Deafness and other Communication Disorders, Caltech's Broad Fellows Program, the Office of Naval Research, and the Max Planck Society.

Katie Neith

Worm Seeks Worm: Caltech Researchers Find Chemical Cues Driving Aggregation in Nematodes

PASADENA, Calif.— Scientists have long seen evidence of social behavior among many species of animals, both on the earth and in the sea. Dolphins frolic together, lions live in packs, and hornets construct nests that can house a large number of the insects. And, right under our feet, it appears that nematodes—also known as roundworms—are having their own little gatherings in the soil. Until recently, it was unknown how the worms communicate to one another when it's time to come together. Now, however, researchers from the California Institute of Technology (Caltech) and the Boyce Thompson Institute at Cornell University have identified, for the first time, the chemical signals that promote aggregation. 

"We now have an expanded view of a very fundamental type of communication, which is recognizing other members of the same species and getting together with them," says Jagan Srinivasan, a senior research fellow in biology at Caltech and lead author of the study detailing this process, which was published in the January issue of PLoS Biology.

The researchers looked at the lab-friendly Caenorhabditis elegans worm—a relatively safe version of the phylum, whose parasitic cousins include hookworms, whipworms, and trichinas, which cause trichinosis—to gather data.

According to Paul Sternberg, Thomas Hunt Morgan Professor of Biology at Caltech and a corresponding author on the paper, nearly 25 percent of the world's human population is infected with some type of parasitic nematode; animals and plants can fall prey to the nasty worms, too. Since nematode parasites live inside a host and attack it internally, knowing how the worms communicate via chemicals could be very important to the fields of biomedicine and agriculture.

"One of the ways to eradicate them would be to have some sort of a chemical that can attract them in order to kill them more efficiently," explains Srinivasan.

Sternberg and Srinivasan are not new to the idea of chemical signaling among C. elegans. In 2008, their research showed how the worms secrete chemicals as a sexual attractant. This time, they worked to find chemical cues that control the social behavior of aggregation. What they found is a complex "language," in which the worms combine different chemicals into compounds, building a molecular library of signals that regulate behavior. They did this by testing a previously identified family of chemicals in mutant worms—made to not produce the chemicals on their own—to measure the behavioral effects of the different chemical combinations. 

"We're starting to get a hold on the chemical 'alphabet' that makes up these words, which have different meanings in different social contexts," says Srinivasan. "It's a modular code that tells us that within the physiology of the organism, there is a lot going on in terms of how the environment is interpreted and read out for social communication."

For example, one class of chemicals the researchers found encourages worm-to-worm company, while a different class of compounds being expressed at the same time keeps other worms away. This suggests that the worms release different amounts of each compound based on what each worm is trying to communicate. If the worm is starting a new colony, it probably just wants a certain number of worms around to find and share food—too many and the colony may not thrive. However, if there is a big piece of fruit, the worm may call on a large group to help access the food source.

"The amazing thing here is that for one chemical, if it's modified even just a little bit, the meaning is changed," says Sternberg, who is also an investigator with the Howard Hughes Medical Institute. "That's what makes it more like a language. If I say a Chinese word, and my intonation is wrong, the word has a different meaning."

Next, the team will explore whether or not the same chemical compounds are made by other nematodes. They will also work to figure out how the worms' nervous system senses and sorts the different compounds.

"Understanding the worm's language is just a first step," says Srinivasan. "We hope that by learning more about how social recognition occurs in the worm nervous system, we can eventually provide insights into how the human brain encodes social information, too."

The study, "A Modular Library of Small Molecule Signals Regulates Social Behaviors in Caenorhabditis elegans," was funded by the National Institutes of Health and the Howard Hughes Medical Institute. Other members of Sternberg's lab who contributed to the study were postdoc Alon Zaslaver and graduate student Margaret C. Ho. Cornell researchers Stephen H. von Reuss, Neelanjan Bose, Parag Mahanti, Oran G. O'Doherty, and Frank Schroeder, along with Arthur Edison of the University of Gainesville in Florida, were also coauthors on the paper. 


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Eric Davidson Awarded the International Prize for Biology

Eric Davidson, Caltech's Norman Chandler Professor of Cell Biology, has been awarded the 2011 International Prize for Biology by the Japan Society for the Promotion of Science. On November 28, Davidson received a medal at a ceremony in Tokyo and an imperial gift, a silver vase from Emperor Akihito. The award also includes ten million yen (more than $125,000 USD).

Davidson was recognized for pioneering the concept of gene regulatory networks, which are networks of hundreds of interacting regulatory genes that control the development of an animal from embryo to adult. Regulatory genes are segments of DNA that determine when and where other genes are turned on or off. Ultimately, these regulatory networks dictate the expression of all the genes that encode for the proteins responsible for an animal's biological structure and function. Each year's International Prize for Biology is awarded in a different area of biology; this year, the prize recognized achievements in developmental biology.

"Eric has been a true pioneer in recognizing that complexity is a fundamental attribute of biological systems," says Stephen Mayo, the Bren Professor of Biology and Chemistry at Caltech, and chair of the Division of Biology. "His recognition for achievements in developmental biology is both timely and well deserved."

Davidson is the fourth winner from Caltech since the inception of the prize in 1985. Previous Caltech recipients are Masakazu Konishi, the Bing Professor of Behavioral Biology; Elliot Meyerowitz, the George Beadle Professor of Biology; and the late Seymour Benzer.

The prize was established to commemorate the 60-year reign of Emperor Akihito's late father, Hirohito, who was long interested in biological research. Akihito himself has published papers on fish evolution, and his son, Prince Fumihito, has a PhD in evolutionary bioscience. Crown Prince Naruhito attended the ceremony and was joined by Empress Michiko for the reception. A two-day symposium in Kyoto, called "Genetic Control of Development," followed the ceremony. "The week was an unforgettable experience," Davidson says.

Marcus Woo
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Biologists Deliver Neutralizing Antibodies that Protect Against HIV Infection in Mice

Process represents novel approach to HIV prevention

PASADENA, Calif.—Over the past year, researchers at the California Institute of Technology (Caltech), and around the world, have been studying a group of potent antibodies that have the ability to neutralize HIV in the lab; their hope is that they may learn how to create a vaccine that makes antibodies with similar properties. Now, biologists at Caltech led by Nobel Laureate David Baltimore, president emeritus and Robert Andrews Millikan Professor of Biology, have taken one step closer to that goal: they have developed a way to deliver these antibodies to mice and, in so doing, have effectively protected them from HIV infection. 

This new approach to HIV prevention—called Vectored ImmunoProphylaxis, or VIP—is outlined in the November 30 advance online publication of the journal Nature.

Traditional efforts to develop a vaccine against HIV have been centered on designing substances that provoke an effective immune response—either in the form of antibodies to block infection or T cells that attack infected cells. With VIP, protective antibodies are being provided up front.

"VIP has a similar effect to a vaccine, but without ever calling on the immune system to do any of the work," says Alejandro Balazs, lead author of the study and a postdoctoral scholar in Baltimore's lab. "Normally, you put an antigen or killed bacteria or something into the body, and the immune system figures out how to make an antibody against it. We've taken that whole part out of the equation."

Because mice are not sensitive to HIV, the researchers used specialized mice carrying human immune cells that are able to grow HIV. They utilized an adeno-associated virus (AAV)—a small, harmless virus that has been useful in gene-therapy trials—as a carrier to deliver genes that are able to specify antibody production. The AAV was injected into the leg muscle of mice, and the muscle cells then put broadly neutralizing antibodies into the animals' circulatory systems. After just a single AAV injection, the mice produced high concentrations of these antibodies for the rest of their lives, as shown by intermittent sampling of their blood. Remarkably, these antibodies protected the mice from infection when the researchers exposed them to HIV intravenously.

The team points out that the leap from mice to humans is large—the fact that the approach works in mice does not necessarily mean it will be successful in humans. Still, the researchers believe that the large amounts of antibodies that the mice were able to produce—coupled with the finding that a relatively small amount of antibody has proved protective in the mice—may translate into human protection against HIV infection.

"We're not promising that we've actually solved the human problem," says Baltimore. "But the evidence for prevention in these mice is very clear."

The paper also notes that in the mouse model, VIP worked even in the face of increased exposure to HIV. To test the efficacy of the antibody, the researchers started with a virus dose of one nanogram, which was enough to infect the majority of the mice who received it. When they saw that the mice given VIP could withstand that dose, they continued to bump it up until they were challenging them with 125 nanograms of virus.

"We expected that at some dose, the antibodies would fail to protect the mice, but it never did—even when we gave mice 100 times more HIV than would be needed to infect 7 out of 8 mice," says Balazs. "All of the exposures in this work were significantly larger than a human being would be likely to encounter."

He points out that this outcome likely had more to do with the properties of the antibody that was tested than the method, but adds that VIP is what enabled the large amount of this powerful antibody to circulate through the mice and fight the virus. Furthermore, VIP is a platform technique, meaning that as more potent neutralizing antibodies are isolated or developed for HIV or other infectious organisms, they can also be delivered using this method.

"If humans are like mice, then we have devised a way to protect against the transmission of HIV from person to person," says Baltimore. "But that is a huge if, and so the next step is to try to find out whether humans behave like mice."

He says the team is currently in the process of developing a plan to test their method in human clinical trials. The initial tests will ask whether the AAV vector can program the muscle of humans to make levels of antibody that would be expected to be protective against HIV.

"In typical vaccine studies, those inoculated usually mount an immune response—you just don't know if it's going to work to fight the virus," explains Balazs. "In this case, because we already know that the antibodies work, my opinion is that if we can induce production of sufficient antibody in people, then the odds that VIP will be successful are actually pretty high."

The study, "Antibody-based Protection Against HIV Infection by Vectored ImmunoProphylaxis," was funded by the Bill and Melinda Gates Foundation, the National Institutes of Health, and the Caltech-UCLA Joint Center for Translational Medicine. Caltech biology researchers Joyce Chen, Christin M. Hong, and Lili Yang also contributed to the paper, as well as Dinesh Rao, a hematologist from the University of California, Los Angeles. 

Katie Neith

Caltech Scientists Point to Link between Missing Synapse Protein and Abnormal Behaviors

PASADENA, Calif.—Although many mental illnesses are uniquely human, animals sometimes exhibit abnormal behaviors similar to those seen in humans with psychological disorders. Such behaviors are called endophenotypes. Now, researchers at the California Institute of Technology (Caltech) have found that mice lacking a gene that encodes a particular protein found in the synapses of the brain display a number of endophenotypes associated with schizophrenia and autism spectrum disorders.

The new findings appear in a recent issue of The Journal of Neuroscience, with Mary Kennedy, the Allen and Lenabelle Davis Professor of Biology at Caltech, as the senior author. 

The team created mutations in mice so that they were missing the gene for a protein called densin-180, which is abundant in the synapses of the brain, those electro-chemical connections between one neuron and another that enable the formation of networks between the brain's neurons. This protein sticks to and binds together several other proteins in a part of the neuron that's at the receiving end of a synapse and is called the postsynapse. "Our work indicates that densin-180 helps to hold together a key piece of regulatory machinery in the postsynaptic part of excitatory brain synapses," says Kennedy.

In mice lacking densin-180, the researchers found decreased amounts of some of the other regulatory proteins normally located in the postsynapse. Kennedy and her colleagues were especially intrigued by a marked decrease in the amount of a protein called DISC1. "A mutation that leads to loss of DISC1 function has been shown to predispose humans to development of schizophrenia and bipolar disorder," Kennedy says.

In the study, the researchers compared the behavior of typical mice with that of mice lacking densin. Those without densin displayed impaired short-term memory, hyperactivity in response to novel or stressful situations, a deficit of normal nest-building activity, and higher levels of anxiety. "Studies of mice with schizophrenia and autism-like features have reported similar behaviors," Kennedy notes.  

"We do not know precisely how the molecular defect leads to the behavioral endophenotypes. That will be our work going forward," Kennedy says. "The molecular mechanistic links between a gene defect and defective behavior are complicated and, as yet, mostly unknown. Understanding them goes to the very heart of understanding brain function."

Indeed, she adds, the findings point to the need for a better understanding of the interactions that occur between proteins at synapses. Studies of these interactions could provide information needed to screen for new and better pharmaceuticals for the treatment of mental illnesses. "This study really reinforces the idea that small changes in the molecular structures at synapses are linked to major problems with behavior," Kennedy says.

Caltech coauthors of the paper, "Deletion of Densin-180 Results in Abnormal Behaviors Associated with Mental Illness and Reduces mGluR5 and DISC1 in the Postsynaptic Density Fraction," include Paul Patterson, the Anne P. and Benjamin F. Biaggini Professor of Biological Sciences; Holly Carlisle; Tinh Luong; Andrew Medina-Marino; Leslie Schenker; Eugenia Khorosheva; Keith Gunapala; and Andrew Steele. The paper's other authors are Tim Indersmitten and Thomas O'Dell of the David Geffen School of Medicine at the University of California, Los Angeles. The work was supported by the National Institutes of Health, the Gordon and Betty Moore Foundation Center for Integrative Study of Cell Biology, the Howard Hughes Medical Institute, the National Science Foundation, the McGrath Foundation, and the Broad Fellows in Brain Circuitry program.

Kimm Fesenmaier

Caltech Researchers Find Pulsating Response to Stress in Bacteria

PASADENA, Calif.—If the changing seasons are making it chilly inside your house, you might just turn the heater on. That's a reasonable response to a cold environment: switching to a toastier and more comfortable state until it warms up outside. And so it's no surprise that biologists have long thought cells would respond to their environment in a similar way.

But now researchers at the California Institute of Technology (Caltech) are finding that cells can respond using a new kind of pulsating mechanism, instead of just shifting from one steady state to another and staying there. The principles behind this process are surprisingly simple, the researchers say, and could drive other cellular processes, revealing more about how the cells—and ultimately life—work.

In their experiment, the researchers studied how a bacterial species called B. subtilis responds to a stressful environment—for example, one without food. In such conditions, the single-celled organism activates a large set of genes that help it deal with hardship, by aiding cell repair for instance. Previously, biologists had thought the bacteria would handle stress by turning on the relevant genes and simply leaving them on until the stress goes away.

Instead, the researchers found that B. subtilis continuously flips these genes on and off. When faced with more stress, it increases the frequency of these pulses. The pulsating action is like switching your heater on full blast for a brief period every few minutes, and turning it on and off more frequently if you want the house to be warmer.

"It's a very different view of how a cell can respond to a particular stress," says James Locke, a postdoctoral scholar at Caltech. Locke and graduate student Jonathan Young are the lead authors on a paper describing this work, which was published in the October 21 issue of Science.

To make their finding, the researchers introduced a chemical to B. subtilis that inhibits the production of ATP, the energy-carrying molecules of cells. The team found that the stress induced by this chemical triggers interactions within a set of genes—collectively called a genetic circuit. This circuit, which contains a set of positive and negative feedback loops, generates sustained pulses of activity in a key regulatory protein called σB  ("sigma B"). The researchers attached fluorescent proteins to the circuit, causing the cells to glow green when σB was activated. By making movies of the flashing cells, the team could then study the dynamics of the circuit.

The key to this pulsating mechanism is the variability inherent in how proteins are made, the researchers say. The number of copies of any specific protein in a given cell fluctuates over time. The bacterial gene circuit amplifies these molecular fluctuations, also called noise, to generate discrete pulses of σB activation. The stress also activates another key protein that modulates the pulse frequencies.

By turning a steady input (the stress) into an oscillating output (the activation of σB) the genetic circuit is analogous to an electrical inverter, a device that converts direct current (DC) into alternating current (AC), explains Michael Elowitz, professor of biology and bioengineering at Caltech, Howard Hughes Medical Institute investigator, and coauthor of the paper. "You might think you need some kind of elaborate circuitry to implement that, but the cell can do it with just a few proteins, and by taking advantage of noise."

This work provides a blueprint for how relatively simple genetic circuits can generate complex and dynamic behaviors in individual cells, the researchers say. "We're excited to think that similar mechanisms may occur in other cellular processes," Locke says. "It'd be interesting in the future to see which aspects of this circuit architecture also appear in more complex systems, such as mammalian cells.”

"With this work and recent work in other systems, we're starting to get a glimpse of just how dynamic cellular control systems really are," Elowitz adds. "That's something that was very difficult to see in the past."

The other authors of the Science paper, "Stochastic pulse regulation in bacterial stress response," are research technicians Michelle Fontes and Maria Jesus Hernandez Jimenez. The research was funded by the National Institutes of Health, the National Science Foundation, the Packard Foundation, the International Human Frontier Science Organization, and the European Molecular Biology Organization.

Marcus Woo

Switching Senses

Caltech biologists find that leeches shift the way they locate prey in adulthood

PASADENA, Calif.—Many meat-eating animals have unique ways of hunting down a meal using their senses. To find a tasty treat, bats use echolocation, snakes rely on infrared vision, and owls take advantage of the concave feathers on their faces, the better to help them hear possible prey. Leeches have not just one but two distinct ways of detecting dinner, and, according to new findings from biologists at the California Institute of Technology (Caltech), their preferred method changes as they age.

Medicinal leeches, like many aquatic animals, use water disturbances to help them find a meal. Juvenile leeches eat the blood of fish and amphibians, while adults opt for blood meals from the more nutritious mammals. Since it was known that leeches change their food sources as they develop, the Caltech team wanted to know if the way they sense potential food changed as well. Their findings are outlined in a paper now available online in the Journal of Experimental Biology.

The group set up experiments to test how much leeches rely on each of the two sensory modalities they use to find food: hairs on their bodies that can note disturbances in the water made by prey moving through it and simple eyes that can pick up on the passing shadows that those waves make. They monitored both juvenile and adult leeches as they reacted to mechanical waves in a tank of water or to passing shadows, as well as to a combination of the two stimuli. The leeches in both age groups responded in similar ways when only one stimulus was present. But when both waves and shadows existed, the adult leeches responded solely to the waves. 

"We knew that there was a developmental switch in what kind of prey they go after," says Daniel Wagenaar, senior author of the paper and Broad Senior Research Fellow in Brain Circuitry at Caltech. "So when we saw a difference in the source of disturbances that the juveniles go after relative to the adults, we thought 'great—it's probably matching what we know.'"

However, the team was very surprised to see that the individual sensory modalities aren't modified during development to help decipher different types of prey. The leech's visual system doesn't really change as the animal matures; neither does the mechanical system. What does change, however, is the integration of the visual and mechanical cues to make a final behavioral decision.

"As they mature, the animals basically start paying attention to one sense more than the other," explains Cynthia Harley, lead author of the study and a postdoctoral scholar in biology at Caltech. She says that the team will now focus their studies on the adult leeches to learn more about how this sensory information is processed both at the behavioral and cellular levels.

Paper coauthor Javier Cienfuegos, now a freshman at Yale, contributed to the study while a high school student at the Polytechnic School, which is located next to Caltech's campus. He ran about half of the experimental trials and was "instrumental in the success of the study," says Harley.

The research outlined in the paper, "Developmentally regulated multisensory integration for prey localization in the medicinal leech," was funded by the Burroughs Wellcome Fund and the Broad Foundations.

Katie Neith