Sounds Guide Young Fish toward Home

Image: SCIENCE According to Dorothy, "there's no place like home." A new report suggests that for fish, no place sounds like home. Findings published today in the journal Science indicate that young fish, which can float out to sea during their larval stage, use sounds emanating from coral reefs to find their way back. Activities such as grinding fish teeth and snapping shrimp claws contribute to the din surrounding a coral reef, which can often be heard from distances up to a few kilometers away. To test whether the racket affected young fish, Stephen Simpson of the University of Edinburgh and his colleagues constructed 24 patches of artificial reef. The researchers outfitted half of them with speakers and broadcast recordings of reef noise, whereas the other half were kept silent. Of the two main types of fish attracted to the fake reefs, both cardinalfish and damselfish exhibited a preference for the louder reefs compared to the quiet ones. The two species did show differences in what types of noise they favored: damselfish were drawn more to higher-frequency sound, but cardinalfish exhibited no such preference.

The discovery that fish respond to reef sounds suggests a potentially valuable management tool, the authors say. "This is a significant step forward in our understanding of their behavior, which should help us to better predict how we should conserve or harvest populations of reef fishes in the future," Simpson remarks. "It should also alert policymakers to the damage that human activities like drilling and shipping may have on fish stocks because they drown out the natural clues given by animals." --Sarah Graham

Making Memories Stick

Some moments become lasting recollections while others just evaporate. The reason may involve the same processes that shape our brains to begin with

By R. Douglas Fields

Image: KAY CHERNUSH TO STIMULATE MEMORY FORMATION, an electrophysiological recording apparatus can both stimulate and record electrical signals from a 400-micron-thick slice of rat hippocampus. In the movie thriller Memento, the principal character, Leonard, can remember everything that happened before his head injury on the night his wife was attacked, but anyone he meets or anything he has done since that fateful night simply vanishes. He has lost the ability to convert short-term memory into long-term memory. Leonard is driven to find his wife's killer and avenge her death, but trapped permanently in the present, he must resort to tattooing the clues of his investigation all over his body. That disturbing story was inspired by the real case history of a patient known in the medical literature only as "HM." When HM was nine years old, a head injury in a bicycle accident left him with debilitating epilepsy. To relieve his seizures that could not be controlled in any other way, surgeons removed parts of HM's hippocampus and adjoining brain regions. The operation succeeded in reducing the brain seizures but inadvertently severed the mysterious link between short-term and long-term memory. Information destined for what is known as declarative memory--people, places, events--must pass through the hippocampus before being recorded in the cerebral cortex. Thus, memories from long ago that were already stored in HM's brain remained clear, but all his experiences of the present soon faded into nothing. HM saw his doctor on a monthly basis, but at each visit it was as if the two had never met.

This transition from the present mental experience to an enduring memory has long fascinated neuroscientists. A person's name when you are first introduced is stored in short-term memory and may be gone within a few minutes. But some information, like your best friend's name, is converted into long-term memory and can persist a lifetime. The mechanism by which the brain preserves certain moments and allows others to fade has recently become clearer, but first neuroscientists had to resolve a central paradox. Both long- and short-term memories arise from the connections between neurons, at points of contact called synapses, where one neuron's signal-emitting extension, called an axon, meets any of an adjacent neuron's dozens of signal-receiving fingers, called dendrites. When a short-term memory is created, stimulation of the synapse is enough to temporarily "strengthen," or sensitize, it to subsequent signals. For a long-term memory, the synapse strengthening becomes permanent. Scientists have been aware since the 1960s, however, that this requires genes in the neuron's nucleus to activate, initiating the production of proteins.

Memory researchers have puzzled over how gene activity deep in the cell nucleus could govern activities at faraway synapses. How does a gene "know" when to strengthen a synapse permanently and when to let a fleeting moment fade unrecorded? And how do the proteins encoded by the gene "know" which of thousands of synapses to strengthen? The same questions have implications for understanding fetal brain development, a time when the brain is deciding which synaptic connections to keep and which to discard. In studying that phenomenon, my lab came up with an intriguing solution to one of these mysteries of memory. And just like Dorothy, we realized that the answer was there all the time. Genetic Memory Early molecular biologists discovered that genes play a role in the conversion of a memory from short- to long-term. Their experiments with animals trained to perform simple tasks demonstrated that learning required new proteins to be synthesized in the brain within minutes of training, or else the memory would be lost [see "Memory and Protein Synthesis," by Bernard W. Agranoff; Scientific American, June 1967]. For a protein to be produced, a stretch of DNA inside the cell nucleus must be transcribed into a portable form called messenger RNA (mRNA), which then travels out to the cell's cytoplasm, where cellular machinery translates its encoded instructions into a protein. These researchers had found that blocking the transcription of DNA into mRNA or the translation of mRNA into a protein would impede long-term memory formation but that short-term memory was unaffected. How does a gene "know" when to strengthen a synapse permanently? Because one neuron can form tens of thousands of synaptic connections and there could not possibly be a gene dedicated to each one, cellular neuroscientists sought to explain how the cell nucleus was controlling the strength of these individual connections. They theorized that an unknown signaling molecule must be generated by a synapse when it was sufficiently stimulated. With its connection temporarily strengthened, this synapse could hold the memory for a short time while the signaling molecule departed, wending its way to the nucleus of the nerve cell. There this messenger molecule would activate appropriate genes needed to synthesize proteins that would permanently strengthen the synaptic connection. Yet a second problem was how this protein, once it was manufactured in the cell body of the neuron, could then find the one synapse among thousands that had called for it. By the mid-1990s, memory researchers had a more detailed picture of events. Several of them had shown that a transcription factor named CREB played a key role in converting short-term memory into long-term memory in animals as distantly related as flies and mice. Transcription factors are master proteins inside the cell nucleus that find and bind to specific sequences of DNA. They are thus the ultimate on/off switches that control a gene's transcription. So CREB activation within a neuron leads to gene activation, leading to manufacture of the mysterious synapse-strengthening proteins that transform a short-term memory into a long-term one. In 1997 elegant experiments by Uwe Frey of the German Federal Institute for Neurobiology, Gene Regulation and Plasticity and Richard G. M. Morris of the University of Edinburgh further showed that whatever these "memory proteins" were, they did not need to be addressed to a particular synapse. They could be broadcast throughout the cell but would only affect the synapse that was already temporarily strengthened and make that connection permanently stronger. These revelations still left at least one more burning question: What is the synapse-to-nucleus signaling molecule that determines when CREB should be activated and a memory preserved? Around this time, my colleagues and I found ourselves approaching the same problems as the memory researchers from a different perspective. In my laboratory at the National Institute of Child Health and Human Development, we study how the brain becomes wired up during fetal development. So while memory researchers were wondering how mental experience could affect genes, which could in turn affect certain synaptic connections, we were wondering how genes could specify all the millions of connections in the developing brain in the first place. Image: FROM FELEKE ESHETE AND R. DOUGLAS FIELDS IN JOURNAL OF NEUROSCIENCE, VOL. 21, NO. 17; SEPTEMBER 1, 2001. ©SOCIETY FOR NEUROSCIENCE CALCIUM IONS glow in a cross section of a neuron (left, with dark nucleus at center) filled with calcium-sensitive dye. The author used scanning laser confocal microscopy to track pulses of calcium influx following each action potential fired by the cell. Cross sections of the first view taken at two-millisecond intervals, then stacked (below left), provided a time-lapse representation of intracellular calcium density (green and red). We and other developmental neuroscientists already suspected that mental experience might have some role in honing the brain's wiring plan. The fetal brain could start out with a rough neural circuitry that was specified by genetic instructions. Then, as the young brain developed and tested those connections, it would preserve the most effective ones and eliminate the poor ones. But how, we wondered, does the brain identify which connections are worth keeping? Building a Brain As far back as 1949, a psychologist named Donald Hebb proposed a simple rule that could govern how experience might bolster certain neural circuits. Inspired by the famous Pavlovian dog experiments, Hebb theorized that connections among neurons that fired at the same time should become strengthened. For example, a neuron that fired when a bell sounded and a nearby neuron that fired when food was presented simultaneously should become more strongly connected to each other, forming a cellular circuit that learns that the two events are connected. Not every input to a nerve cell is strong enough to make that cell fire a signal of its own. A neuron is like a microprocessor chip in that it receives thousands of signals through its dendrites and constantly integrates all the input it receives from these connections. But unlike a microprocessor that has many output wires, a neuron has only one, its axon. Thus, a neuron can respond to inputs in only one way: it can either decide to send a signal on to the next neuron in the circuit by firing an impulse through its axon, or not.

When a neuron receives such a signal, the voltage of the membrane on its dendrite changes slightly in the positive direction. This local change in voltage is described as a "firing" of the neuron's synapse. When a synapse fires in brief, high-frequency bursts, the temporary strengthening observed in short-term memory formation occurs. But a single synapse firing briefly is generally not enough to make the neuron fire an impulse, technically termed an action potential, of its own. When many of the neurons' synapses fire together, however, their combined effort changes the voltage of the neuronal membrane enough to make the neuron fire action potentials and relay the message on to the next neuron in the circuit.

Hebb proposed that, like an orchestra player who cannot keep up, a synapse on a neuron that fires out of sync with the other inputs to the neuron will stand out as odd and should be eliminated, but synapses that fire together--enough so as to make the neuron fire an action potential--should be strengthened. The brain would thus wire itself up in accordance with the flow of impulses through developing neural circuits, refining the original general outline.

Moving from Hebb's theory to sorting out the actual mechanics of this process, however, one again confronts the fact that the enzymes and proteins that strengthen or weaken synaptic connections during brain wiring must be synthesized from specific genes. So our group set out to find the signals that activate those genes.

Because information in the nervous system is coded in the pattern of neural impulse activity in the brain, I began with an assumption that certain genes in nerve cells must be turned on and off by the pattern of impulse firing. To test this hypothesis, a postdoctoral fellow in my lab, Kouichi Itoh, and I took neurons from fetal mice and grew them in cell culture, where we could stimulate them using electrodes in the culture dish. By stimulating neurons to fire action potentials in different patterns and then measuring the amount of mRNA from genes known to be important in forming neural circuits or in adapting to the environment, we found our prediction to be true. We could turn on or off particular genes simply by dialing up the correct stimulus frequency on our electrophysiological stimulator, just as one tunes into a particular radio station by selecting the correct signal frequency.
Time Code
Once we observed that neuronal genes could be regulated according to the pattern of impulses the cell was emitting, we wanted to investigate a deeper question: How could the pattern of electrical depolarizations at the surface of the cell membrane control genes deep in the nucleus of the neuron? To do so, we needed to peer into the cell cytoplasm and see how information was translated on its way from the surface to the nucleus.

What we found was not a single pathway leading from the neuron's membrane to its nucleus but rather a highly interconnected network of chemical reactions. Like the maze of roads leading to Rome, there were multiple intersecting biochemical pathways crisscrossing as they carried signals from the cell membrane throughout the cell. Somehow electrical signals of varying frequencies on the membrane flowed through this traffic in the cytoplasm to reach their proper destination in the nucleus. We wanted to understand how.

finally, we began to appreciate that the important factor was time

The primary way that information about the neuronal membrane's electrical state enters this system of chemical reactions in the cytoplasm is by regulating the influx of calcium ions through voltage-sensitive channels in the cell membrane. Neurons live in a virtual sea of calcium ions, but inside a neuron the concentration of calcium is kept extremely low--20,000 times lower than the concentration outside. When the voltage across the neuronal membrane reaches a critical level, the cell fires an action potential, causing the calcium channels to open briefly. Admitting a spurt of calcium ions into the neuron with the firing of each neural impulse translates the electrical code into a chemical code that cellular biochemistry inside the neuron can understand.

In domino fashion, as calcium ions enter the cytoplasm, they activate enzymes called protein kinases. Protein kinases turn on other enzymes by a chemical reaction called phosphorylation that adds phosphate tags to proteins. Like runners passing the baton, the phosphate-tagged enzymes become activated from a dormant state and stimulate the activity of transcription factors. CREB, for instance, is activated by calcium-dependent enzymes that phosphorylate it and inactivated by enzymes that remove the phosphate tag. But there are hundreds of different transcription factors and protein kinases in a cell. We wanted to know how a particular frequency of action potential firing could work through calcium fluxes to reach the appropriate protein kinases and ultimately the correct transcription factors to control the right gene. By filling the neurons with dye that fluoresces green when the calcium concentration in the cytoplasm increases, we were able to track how different action-potential firing patterns translated into dynamic fluctuations in intracellular calcium. One simple possibility was that gene transcription might be regulated by the amount of calcium rise in a neuron, with different genes responding better to different levels of calcium. Yet we observed a more interesting result: the amount of calcium increase in the neuron was much less important in regulating specific genes than the temporal patterns of calcium flashes, echoing the temporal code of the neural impulse that had generated them.

Another postdoc in my lab, Feleke Eshete, followed these calcium signals to the enzymes they activate and the transcription factors those enzymes regulate, and finally we began to appreciate how different patterns of neural impulses could be transmitted through different intracellular signaling pathways. The important factor was time. We found that one could not represent the pathway from the cell's membrane to its DNA in a simple sequence of chemical reactions. At each step, starting from calcium entering the membrane, the reactions branched off into a highly interconnected network of signaling pathways, each of which had its own speed limits governing how well it could respond to intermittent signals. This property determined which signaling pathway a particular frequency of action potentials would follow to the nucleus.

Some signaling pathways responded quickly and recovered rapidly; thus, they could react to high-frequency patterns of action potentials but could not sustain activation in response to bursts of action potentials separated by long intervals of inactivity. Other pathways were sluggish and could not respond well to rapid bursts of impulses, but once activated, their slowness to inactivate meant that they could sustain signals between bursts of action potentials that were separated by long intervals of inactivity. The genes activated by this pathway would therefore respond to stimuli that are delivered repeatedly, but infrequently, like the repetition necessary for committing new information to memory. In other words, we observed that signals of different temporal patterns propagated through distinct pathways that were favorably tuned to those particular patterns and ultimately regulated different transcription factors and different genes. For instance, our measurements showed that CREB was rapidly activated by action potentials but sluggish in inactivating after we stopped stimulating the neuron. Thus, CREB would sustain its activation between repeated bursts of stimuli separated by intervals of 30 minutes or more, similar to the intervals of time between practice sessions required to learn new skills or facts. Given CREB's role in memory, we could not help but wonder if the signaling pathway we were studying to understand brain development might not also be relevant to the mechanism of memory. So we devised a test. Memory in a Dish If the part of the brain that was removed from the patient HM, the hippocampus, is dissected from a rat and kept alive in a salt solution, microelectrodes and electronic amplifiers can record the electrical impulses from individual synaptic connections on a neuron. By administering a burst of electrical shocks to a synapse, causing it to fire in a specific pattern, that synaptic connection can be strengthened. That is to say, the synapse produces about twice as much voltage in response to subsequent stimulations after it has received the high-frequency stimulus.

This increased strength, termed long-term potentiation (LTP), can be, despite its name, relatively short-lived. When test pulses are applied at a series of intervals after the high-frequency stimulus, the voltage produced by the synapse slowly diminishes back to its original strength within a few hours. Known as early LTP, this temporary synaptic strengthening is a cellular model of short-term memory. Remarkably, if the same high-frequency stimulus is applied repeatedly (three times in our experiments), the synapse becomes strengthened permanently, a state called late LTP. But the stimuli cannot be repeated one after the other. Instead each stimulus burst must be spaced by sufficient intervals of inactivity (10 minutes in our experiments). And adding chemicals that block mRNA or protein synthesis to the salt solution bathing the brain slice will cause the synapse to weaken to its original strength within two to three hours. Just as in whole organisms, the cellular model of short-term memory is not dependent on the nucleus, but the long-term form of memory is.

Indeed, Frey and Morris had used this technique to show that synapse-strengthening proteins would affect any temporarily strengthened synapse. First, they stimulated a synapse briefly to induce early LTP, which would normally last just hours. Then they fired a second synapse on the same neuron in a manner that would induce late LTP in that synapse: three bursts separated by 10 minutes. As a result, both synapses were permanently strengthened. The stronger stimulus sent a signal to the nucleus calling for memory-protein manufacture, and the proteins "found" any synapse that was already primed to use them. Based on our work showing how different patterns of impulses could activate specific genes, and recalling Hebb's theory that the firing of a neuron was critical in determining which of its connections will be strengthened, we asked whether a signaling molecule sent from the synapse to the nucleus was really necessary to trigger long-term memory formation. Instead we proposed that when a synapse fired strongly enough or in synchrony with other synapses so as to make the neuron fire action potentials out its axon, calcium should enter the neuron directly through voltage-sensitive channels in the cell body and activate the pathways we had already studied leading to CREB activation in the nucleus.

To test our theory, postdoc Serena Dudek and I administered a drug known to block synaptic function to the brain slice. We then caused neurons to fire action potentials by using an electrode to stimulate the neurons' cell bodies and axons directly. Thus, the neurons fired action potentials, but the synaptic inputs to these neurons could not fire. If a synapse-to-nucleus signaling molecule was necessary to trigger late LTP, our cellular model of long-term memory formation, then this procedure should not work, because the synapses were silenced by the drugs. On the other hand, if the signals to the nucleus originated from the neurons firing action potentials, as in our developmental studies, silencing the synapses should not prevent activation of the memory-protein genes in the nucleus. We next processed the brain tissue to determine if the transcription factor CREB had been activated. Indeed, in the small region of brain slice that had been stimulated to fire action potentials in the complete absence of synaptic activity, all the CREB had a phosphate molecule added to it, indicating that it had been switched to the activated state. We then checked for activity of the gene zif268, which is known to be associated with creation of LTP and memory. We found that it, too, was turned on by the hippocampal neuron firing, without any synaptic stimulation. But if we performed the same stimulation in the presence of another drug that blocks the voltage-sensitive calcium channels--which we suspected were the actual source of the signal from the membrane to the nucleus--we found that CREB phosphorylation, zif268 and a protein associated with late LTP called MAPK were not activated after the neurons fired.

These results clearly showed that there was no need for a messenger from the synapse to the nucleus. Just as in our developmental studies, membrane depolarization by action potentials opened calcium channels in the neuronal membrane, activating signaling pathways to the nucleus and turning on appropriate genes. It seems to make good sense that memory should work this way. Rather than each synapse on the neuron having to send private messages to the nucleus, the transcriptional machinery in the nucleus listens instead to the output of the neuron to decide whether or not to synthesize the memory-fixing proteins. Molecular Memento Perhaps undiscovered synapse-to-nucleus signaling molecules do participate in some way in the memory process, but our experiments indicate that they are not absolutely necessary. As predicted by Hebbian rules of learning, the firing of a neuron, resulting from the combined excitation of all synaptic input to the cell, is the necessary event for consolidating memory.

This understanding offers a very appealing cellular analogue of our everyday experience with memory. Like Leonard in Memento or any witness to a crime scene, one does not always know beforehand what events should be committed permanently to memory. The moment-to-moment memories necessary for operating in the present are handled well by transient adjustments in the strength of individual synapses. But when an event is important enough or is repeated enough, synapses fire to make the neuron in turn fire neural impulses repeatedly and strongly, declaring "this is an event that should be recorded." The relevant genes turn on, and the synapses that are holding the short-term memory when the synapse-strengthening proteins find them, become, in effect, tattooed.

R. DOUGLAS FIELDS is chief of the Nervous System Development and Plasticity Section of the National Institute of Child Health and Human Development and adjunct professor in the Neurosciences and Cognitive Science Program at the University of Maryland. His last article in Scientific American, " The Other Half of the Brain" (April 2004), described the importance of glial cells to thinking and learning.

MORE TO EXPLORE:

Regulated Expression of the Neural Cell Adhesion Molecule L1 by Specific Patterns of Neural Impulses. Kouichi Itoh, B. Stevens, M. Schachner and R. D. Fields in Science, Vol. 270, pages 1369–1372; November 24, 1995.

Synaptic Tagging and Long-Term Potentiation. Uwe Frey and Richard G.M. Morris in Nature, Vol. 385, pages 533–536; February 6, 1997.

Somatic Action Potentials Are Sufficient for Late-Phase LTP-Related Cell Signaling. Serena M. Dudek and R. Douglas Fields in Proceedings of the National Academy of Sciences USA, Vol. 99, No. 6, pages 3962–3967; March 19, 2002.

Memory Systems of the Brain: A Brief History and Current Perspective. Larry R. Squire in Neurobiology of Learning and Memory, Vol. 82, pages 171–177; November 2004.

Researchers Use X-Rays to 'See' Fingerprints

Television shows such as CSI dramatize the work of forensic investigators and glamorize their high-tech toys that help catch criminals. Now real-life criminologists might soon be adding a new weapon to their crime-fighting arsenal: a visualization technique for spotting fingerprints that uses x-ray vision. Results of early tests of the novel approach will be unveiled this week at the annual meeting of the American Chemical Society meeting in San Diego. In the standard approach to lifting fingerprints from a crime scene, known as contrast enhancement, a sample is treated with a substance--either vapor, liquid or powder--that adds color to a fingerprint and allows it to stand out from its background. Prints left on such surfaces as leather, plastic or fibrous textiles, can sometimes be difficult to detect, however. The technique developed by Chris Worley of the Los Alamos National Laboratory and his colleagues is a noninvasive one that relies on a process known as micro-x-ray fluorescence (MXRF). When a surface is exposed to a thin beam of x-rays, the MXRF instrument detects elements such as sodium, potassium and chlorine, which are present as salts in human sweat. Because the salts are deposited along the ridges present in a fingerprint, the fluorescence can be used to assemble a digital image of a print. "This process represents a valuable new tool for forensic investigators that could allow them to nondestructively detect prints on surfaces that might otherwise be undetectable by conventional methods," Worley says. "It won't replace traditional fingerprinting, but could provide a valuable complement to it."

MXRF cannot detect all the prints that conventional techniques do, because some prints won't contain enough of the necessary elements. But it might find some prints that would otherwise be missed: the researchers' tests illustrated that MXRF successfully identified prints from subjects whose hands were exposed to sunscreen, lotion or saliva, which could interfere with contrast enhancement. Currently, this method can only test samples that can physically be transported to a laboratory that has an MXRF machine. If further testing and refinement of the technique are successful, the team predicts it could be used commercially in two to five years, perhaps as a portable device.

Monkeys Pay for Prurient Pictures

For a monkey, not all images are created equal. A new report reveals that the animals value some pictures more than others and are willing to pay for the privilege of viewing the important ones. The results indicate that monkeys, like people, value information based on its social context. Robert Deaner of Duke University Medical Center and his colleagues studied male rhesus macaques that received juice rewards while looking at a variety of images of other macaques on a computer screen. The pictures included a neutral target, male monkeys that differed in social standing and the hindquarters of a female monkey, which reveal her sexual receptiveness. By systematically varying the amount of juice offered to the monkeys while changing the pictures they were seeing, the scientists determined how much the animals were willing to give up, or pay, in order to glimpse specific images. The team discovered that monkeys would give up a significant reward if it meant viewing high-ranking individuals or female behinds. But when given the chance to glance at images of low-ranking males, the subjects held out for additional juice.

The findings may help scientists understand the neural wiring that underlies social cognition. "At the moment, it's only a tantalizing possibility, but we believe that similar processes are at work in these monkeys and in people," says study co-author Michael Platt, also at Duke. "After all, the same kinds of social conditions have been important in primate evolution for both nonhuman primates and humans. So, in further experiments, we also want to try to establish in the same way how people attribute value to acquiring visual information about other individuals." The findings will appear in the March issue of Current Biology. --Sarah Graham

Secret of the Venus Fly Trap Revealed

Image: COURTESY OF FORTERRE AND MAHADEVAN Taking just a tenth of a second, the snapping mechanism that a Venus fly trap uses to capture its prey is one of the fastest movements in the plant kingdom. Scientists have long wondered how the plant manages such a feat without muscles or nerves. The answer, according to results published today in the journal Nature, is by shapeshifting. Lakshminarayanan Mahadevan of Harvard University and his colleagues used high-speed video to catch the Venus fly trap in action. The researchers first painted the plant's leaves with ultraviolet fluorescent dots and then filmed them shutting under ultraviolet light. By analyzing the images and modeling the movement using a mathematical formula, the team reconstructed the geometry of the leaves as they closed. When trigger hairs on the leaves are disturbed, the plant moves moisture in the leaf in response. This, in turn, affects the leaf's curvature. "In essence, a leaf stretches until reaching a point of instability where it can no longer maintain the strain," explains Mahadevan. "Like releasing a reversed plastic lid or part of a cut tennis ball, each leaf folds back in on itself, and in the process of returning to its original shape, ensnares the victim in the middle."

A better understanding of the Venus fly trap's impressive system could help researchers lean to emulate it. The team speculates that similar muscle-free movement could applied to valves or switches in microfluidic devices or sensors. --Sarah Graham

DNA Helps Nanoparticles Pull Themselves Together

	Image: MICHIGAN CENTER FOR BIOLOGICAL NANOTECHNOLOGY

A burgeoning area of nanotechnology research is the development of tiny drug delivery systems that can target diseased cells specifically, leaving healthy ones untouched. New results suggest a novel synthetic approach could cut the manufacturing time for one type of nanoscale delivery system in half.

Scientists at the University of Michigan have been working with branched polymers just nanometers long called dendrimers, which can carry many different types of molecules attached to their ends. Armed with contrast agents and drugs, a dendrimer can then locate and signal the presence of diseased tissue. But building a multifaceted dendrimer complex is labor intensive and requires separate, lengthy reaction steps for each additional molecule. In the current issue of the journal Chemistry and Biology, Youngseon Choi and his colleagues describe a different technique, which exploits the natural tendencies of DNA to speed up the process. The team first made separate batches of dendrimers, each carrying a single type of molecule as well as a small swatch of noncoding DNA. When solutions of these dendrimers were combined, the lengths of DNA formed complementary pairs, knitting the two dendrimer complexes together.

Using this approach, assembling a therapeutic dendrimer that could deliver five drugs to five different types of cells would require 10 steps. The traditional approach would require 25, each taking between two and three months. "With this approach, you can target a wide variety of molecules, drugs [and] contrast agents to almost any cell," comments study co-author James Baker of the University of Michigan. The results have proved the concept is feasible, the authors note, and could usher in a new age of self-assembling disease-fighters. --Sarah Graham

Chimps' Sense of Justice Found Similar to Humans'

Image: YERKES NATIONAL PRIMATE RESEARCH CENTER Inequities big and small can lead people to believe that life is indeed not fair. But how humans respond to unfair situations depends on the social circumstances: inequality among friends and family, for instance, is less disturbing than it is among strangers. The results of a new study indicate that the same is true for chimpanzees, a finding that sheds light on how our sense of fairness evolved. In the fall of 2003 Sarah Brosnan and Frans de Waal of the Yerkes National Primate Research Center in Atlanta determined that capuchin monkeys don't like being subjected to treatment they deem unjust. In the new work, the researchers tested the reactions of pairs of chimpanzees to exchanges of food that varied in quality. The animals received either a grape, which they coveted, or a less appealing cucumber, and they could see what their partner obtained. In pairs of chimps that had lived together since birth, the individual given the cucumber was less likely to react negatively to the situation than was the short-changed member of a pair that did not know each other as well. Indeed, chimps in the short-term social groups refused to work after their partner received a better reward for the same job. "Human decisions tend to be emotional and vary depending on the other people involved," Brosnan says. "Our findings in chimpanzees implies this variability in response is adaptive and emphasizes there is not one best response for any given situation, but rather it depends on the social environment at the time."

Further experiments to investigate reactions to unfair situations are ongoing at the center in the hopes of understanding why we humans make the decisions we do. "Identifying a sense of fairness in two closely related nonhuman primate species implies it could have a long evolutionary history," Brosnan remarks. The findings will be published in the February 7 edition of the Proceedings of The Royal Society B: Biological Sciences. --Sarah Graham

You, Robot

He says humans will download their minds into computers one day. With a new robotics firm, Hans Moravec begins the journey from warehouse drones to robo sapiens

By Chip Walter

When word got around that Hans Moravec had founded an honest-to-goodness robotics firm, more than a few eyebrows were raised. Wasn't this the same Carnegie Mellon University scientist who had predicted that we would someday routinely download our minds into robots? And that exponential advances in computing power would cause the human race to invent itself out of a job as robots supplanted us as the planet's most adept and adaptive species? Somehow, creating a company seemed ... uncharacteristically pragmatic. But Moravec doesn't see it that way. He says he didn't start Seegrid Corporation because he was backing off his predictions. He founded the company because he was planning to help fulfill them. "It was time," he says, slowly rubbing his hand across his bristle-short hair. "The computing power is here."

The 56-year-old Moravec should know. Born in Kautzen, Austria, and raised in Montreal, he has been pushing the envelope on robotics theory and experimentation for the past 35 years, first as the graduate student at Stanford University who created the "Stanford Cart," the first mobile robot capable of seeing and autonomously navigating the world around it (albeit very slowly), and later as a central force in Car-negie Mellon's vaunted Robotics Institute. His iconoclastic theories and inventive work in machine vision have both shocked his colleagues and jump-started research; Seegrid is just the next logical step. Moravec pulls an image up onto one of the two massive monitors that sit side by side on his desk, like great unblinking eyes. It's six o'clock in the evening, but an inveterate night owl, he's just starting his "day." "I have been drawing these graphs for years about what will be possible," he comments. His mouse roams along dots and images that plot and compare the processing power of old top-of-the-line computers with their biological equivalents. There is the ENIAC, for example, that in 1946 possessed the processing capacity of a bacterium and then a 1990 model IBM PS/2 90 that once harnessed the digital horsepower of a worm. Only recently have desktop computers arrived that can deliver the raw processing muscle of a spider or a guppy (about one billion instructions per second). "At guppy-level intelligence," he explains, "I thought we could manage 3-D mapping and create a robot that could get around pretty well without any special preparation of its environment."

But no one was creating that robot, so in the late 1990s Moravec says he began to grow "very antsy" about getting one built. In 1998 he wrote an ambitious grant proposal that outlined software for a robotic vision system. The Defense Advanced Research Projects Agency quickly funded the proposal, and three and a half years and $970,000 later, with PCs just reaching guppy smarts, a working demonstration was complete. "It proved the principle," Moravec says. "We really could map with stereo vision, if we did things just right." But doing things just right required more than prototype software. Robotic evolution, he adds, "has to be driven forward by a lot of trial and error, and the only way to get enough is if you have an industry where one company is trying to outdo another." To help things along, he and Pittsburgh physician and entrepreneur Scott Friedman founded Seegrid in 2003. Their focus: the unglamorous but potentially huge "product handling" market. Industrial robots already flourish in tightly constrained environments such as assembly lines. Where they fail is in locations loaded with unpredictability. So Seegrid concentrated on creating vision systems that enable simple machines to move supplies around warehouses without any human direction. Not exactly the stuff of science fiction, Moravec agrees, and a long way from superintelligent robots, but he says you have to start somewhere. Nearly everything sold has to be warehoused at some point, and at some point it also has to be rerouted and shipped. Right now human workers move millions of tons of supplies and products using dollies, pallet jacks and forklifts. Seegrid's first prototype devices automate that work, turning wheeled carts into seeing-eye machines that can be loaded and then walked through various routes to teach them how to navigate on their own. The technology is built on Moravec's bedrock belief that if robots are going to succeed, the world cannot be adapted to them; they have to adapt to the world, just like the rest of us. Other approaches can guide robots, but they typically rely on costly, precision hardware such as laser range finders or on extravagant arrangements that prewire and preprogram the machines to move through controlled spaces. Seegrid's system uses off-the-shelf CCD cameras and simple sonar and infrared sensors. Although these components gather imprecise information, the software compensates. It statistically compares the gathered data to develop a clean, accurate 3-D map. "If the same information keeps coming up, then the program decides that it's probably really there," Moravec explains. The robot then knows to stop or roll around it. This approach is how you might make your way through a dark room with a flashlight, in which you slowly build up a mental picture of what is around. Creating warehouse drones as a first step toward the startling robotic world Moravec foresees might seem an unlikely concession to reality. But those who know Moravec say it is no surprise: he is an unusual mix of whimsy, wild vision and rigorous pragmatism. He has been known to be so lost in thought during his daily walks to his office that he bumps into mailboxes, yet none of that eccentricity has tarnished his reputation as a first-rate engineer and programmer. "Some of Hans's ideas are pretty outrageous," admits Raj Reddy, who as director of the Robotics Institute brought Moravec to Carnegie Mellon in 1980, "but his work has always been very practical." Seegrid co-founder Friedman says it is exactly Moravec's vision and dogged persistence that separates him from the pack: "He's a genius, and he works hard." The same themes run through his view of the future of robotics. Evolution moves in tiny steps, Moravec notes, but accomplishes amazing things. Machine evolution will do the same as it incrementally nudges robots from their clumsy beginnings to the heights of human-level intelligence and mobility. "We don't need a lot of Einsteins to do this; we need a lot of engineers working diligently to make little improvements and then test them out in the marketplace," Moravec insists. And that, he says, will ultimately lead to robots becoming vastly more intelligent and adaptable than we are. That seems to leave us only one destination: the endangered species list. "Something like 99 percent of all species go extinct," Moravec observes. Why, he asks, should we be any different? Not that he sees us being destroyed by what he calls our "mind children" exactly. "It's not going to be like Terminator," he reassures. But children do often exceed the accomplishments of their parents. And in our evolutionary dotage, he is sure they will take good care of us, as parents' children often do. "They will create the perfect welfare state," he says. At least, we hope so.

Scientists Unravel How Geckos Keep Their Sticky Feet Clean

Image: COURTESY OF THE PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES The super sticky feet of geckos allow the animals to cling easily to nearly any surface. In fact, a single toe contains enough foot hairs, known as setae, to support the animal's entire body weight. Researchers are thus hoping to employ the gecko's secrets to manufacture adhesives with similar properties. Now scientists can add another impressive characteristic to the list: setae are self-cleaning. Previous research had hinted at a built-in cleaning process for gecko feet, but just how the creatures kept their toes tidy remained a mystery because they neither groom their footpads nor secrete fluids. Kellar Autumn and Wendy R. Hansen of Lewis and Clark College measured the amount of force between the setae and different surfaces both when they were dirt-free and in the presence of particulate contamination. They found that it takes only a few steps for setae to shed tiny silica spheres. "Self-cleaning in gecko setae may occur because it is energetically favorable for particles to be deposited on the surface rather than remain adhered to the spatulae," they write in the current issue of the Proceedings of the National Academy of Sciences.

The findings indicate that gecko foot cleaning occurs even under extreme exposure to clogging particles. To best imitate this property in synthetic adhesives the authors posit that an array of adhesive nanostructures should be made out of a relatively hard material having a small surface area and low surface energy for optimum performance. --Sarah Graham

A Glimpse of Supersolid

Solid helium can behave like a superfluid

By Graham P. Collins

Solids and liquids could hardly seem more different, one maintaining a rigid shape and the other flowing to fit the contours of whatever contains it. And of all the things that slosh and pour, superfluids seem to capture the quintessence of the liquid state--running through tiny channels with no resistance and even dribbling uphill to escape from a bowl. A superfluid solid sounds like an oxymoron, but it is precisely what researchers at Pennsylvania State University have recently witnessed. Physicists Moses Chan and Eun-Seong Kim saw the behavior in helium 4 that was compressed into solidity and chilled to near absolute zero. Although the supersolid behavior had been suggested as a theoretical possibility as long ago as 1969, its demonstration poses deep mysteries. Rotation is one way that superfluids reveal their peculiar properties. Take a bucket of ordinary liquid helium and rotate it slowly, then cool it down to about two kelvins, so that some of the helium becomes superfluid. The superfluid fraction will not rotate. Because part of the helium is motionless, the amount of force required to set the bucket and helium rotating is less than it would be otherwise. Technically, the helium's rotational inertia decreases.

Chan and Kim observed such a decrease of rotational inertia in a ring of solid helium. They applied about 26 atmospheres of pressure to liquid helium, forcing the atoms to lock in place and thereby form a fixed lattice. They observed the oscillations of the helium as it twisted back and forth on the end of a metal rod. The period of these torsional oscillations depended on the rotational inertia of the helium; the oscillations occurred more rapidly when the inertia went down, just as if the mass of the helium decreased. Amazingly, they found that about 1 percent of the helium ring remained motionless while the other 99 percent continued rotating as normal. One solid could somehow move effortlessly through another.

So how can a solid behave like a superfluid? All bulk liquid superfluids are caused by Bose-Einstein condensation, which is the quantum process whereby a large number of particles all enter the same quantum state. Chan and Kim's result therefore suggests that 1 percent of the atoms in the solid helium somehow form a Bose-Einstein condensate even while they remain at fixed lattice positions. That seems like a contradiction in terms, but the exchange of atoms between lattice sites might allow it. A characteristic of helium would tend to promote such an exchange--namely, its large zero-point motion, which is the inherent jiggling of atoms that represents a minimum amount of movement required by quantum uncertainty. (It is the reason helium ordinarily only occurs as a gas or a liquid: the extremely lightweight atoms jiggle about too much to form a solid.) Supporting the idea of condensation, the two researchers did not see superfluidity in solid helium 3, an isotope of helium that as a liquid undergoes a kind of condensation and becomes superfluid only at temperatures far below that needed by liquid helium 4. Another possibility is that the crystal of helium contains numerous defects and lattice vacancies (yet another effect of the zero-point motion). These defects and vacancies could be what, in effect, undergo Bose-Einstein condensation. But all those theories seem to imply that the superfluidity would vary with the pressure, yet Chan and Kim see roughly the same effect all the way from 26 to 66 atmospheres. Douglas D. Osheroff of Stanford University, the co-discoverer of superfluidity in helium 3, calls the lack of pressure dependence "more than a bit bewildering." He says that Chan and Kim have done "all the obvious experiments to search for some artifact." If they are correct, Osheroff adds, then "I don't understand how supersolids become super. I hope the theorists are thinking about it seriously."

Early Mammal Dined on Dinosaurs

Image: XI XIAPING New fossil finds from China are painting a different picture of the mammals that lived alongside the dinosaurs during the Mesozoic era. Most mammals known from this time are thought of as relatively small, nocturnal creatures--the hunted rather than the hunters. Findings published today in the journal Nature provide the first direct evidence that some mammals dined on their dinosaur contemporaries. Researchers from the American Museum of Natural History (AMNH) in New York City and the Chinese Academy of Sciences in Beijing recovered the 130-million-year-old remains of an opossum-size mammal from China's fossil-rich Liaoning province. While cleaning the fossil of Repenomamus robustus, the team discovered a small patch of bones within the rib cage, where the stomach of similarly sized living mammals would be. The stomach contents included the limbs, fingers and teeth of a juvenile herbivorous dinosaur known as a psittacosaur. Although adult psittacosaurs grew to a height of around six feet, the baby prey was just five inches long, about a third the size of Repenomamus robustus. From wear marks on the dinosaur's teeth, the researchers inferred that it was not an embryo. (This supports the notion that the mammal hunted the dinosaur, rather than snatching an unhatched egg out of a nest.) In addition, they surmise that it was swallowed in chunks because some of its bones were still connected.

The scientists also found remains of a larger mammal, about the size of a small dog, that was a close relative of the dinosaur eater. This fairly complete skeleton, Repenomamus giganticus, represents the largest mammal known so far from the Mesozoic era (spanning from about 250 million to 65 million years ago). Both creatures belong to a lineage that has no extant descendants. Remarks study co-author Jin Meng of AMNH, "This new evidence of larger size and predatory, carnivorous behavior in early mammals is giving us a drastically new picture of many of the animals that lived in the age of dinosaurs." --Sarah Graham

Cricket Courting Can Be a Deadly Deed

very funny...

Playing the field can be deadly, at least for crickets. Research published today in the journal Nature indicates that well-fed casanova crickets spend so much energy on mating calls to court female partners that they die sooner than malnourished males do. John Hunt of the University of New South Wales in Australia and his colleagues observed two groups of field crickets. One group ate a restrictive, low-protein diet whereas the other dined on protein-rich foods. The researchers monitored the creatures' size, mating behavior and how long they lived. For female crickets, those fed a robust diet lived longer than did their protein-starved counterparts. This pattern did not hold for the males, however. Instead, the well-fed males used their extra energy to woo female partners by calling more extensively during early adulthood and experienced shortened life spans as a result. "They literally knocked themselves out trying to impress female crickets," says study co-author Luc F. Bussiere, also at the University of New South Wales.

The findings demonstrate that the best reproductive strategy in the animal kingdom does not always coincide with living a long life. What is more, long-lived males are not necessarily those in the best condition, which indicates that longevity is not always a reliable measure of male quality. "One thing that consistently prolongs life span in a range of species is a restricted diet," remarks co-author Rob Brooks of the University of New South Wales. "Now we know a bit more about how this occurs in male crickets--by suppressing sexual advertisement." --Sarah Graham