The Bitter and the Sweet: Fruit Flies Reveal a New Interaction Between the Two

Fruit flies have a lot to teach us about the complexity of food. Like these tiny little creatures, most animals are attracted to sugar but are deterred from eating it when bitter compounds are added.

A new study conducted by UC Santa Barbara’s Craig Montell, Duggan Professor of Neuroscience in the Department of Molecular, Cellular and Developmental Biology, explains a breakthrough in understanding how sensory input impacts fruit flies’ decisions about sweet taste. The findings were published today in the journal Neuron.

It is generally well known that the addition of bitter compounds inhibits attraction to sugars. However, until now the cellular and molecular mechanisms underlying an important aspect of this ubiquitous animal behavior were poorly understood.

When animals encounter bitterness in foods, two factors cause them to stop eating. First, bitter compounds bind to proteins called bitter gustatory receptors (GRs), which inhibits feeding. The second –– and more elusive –– factor involves inhibition of the sugar response. This is the focus of Montell’s research.

At the center of the team’s discovery is the function of an odorant-binding protein (OPB) in the gustatory system. These proteins are usually but not exclusively resident in the olfactory system. Montell’s team found definitive evidence that an OBP, synthesized and released from non-neuronal cells, not only binds bitter tastants, but also moves and binds to the surface of nearby gustatory receptor neurons (GRNs) that contain sugar-activated GRs.

This unanticipated process inhibits the activity of these GRNs and reduces the fruit flies’ attraction to sugars. These results not only reveal an unexpected role for an OBP in taste, but also identify the first molecular player (OBP49a) involved in the integration of opposing attractive and aversive gustatory stimuli in fruit flies.

The researchers used two different fruit flies, wild-type and mutants missing the OBP49a protein, to demonstrate that bitter compounds suppress feeding behavior by binding to the OBP49a protein. As expected, wild-type flies find bitter aversive and prefer the lower concentration of sucrose when the higher concentration of sucrose is laced with bitter tastants such as quinine.

The same was not true of the mutant flies, which do not express OBP49a. Their avoidance behavior was impaired because the bitter compounds did not inhibit the sweet response by binding to OPB49. However, loss of OBP49a did not affect gustatory behavior or action potentials in sugar- or bitter-activated GRNs when the GRNs were presented with just one type of tastant.

"We showed that the OBP49a protein was in very close proximity or even touching the sugar GRs," said Montell. "If the bitter compound weren’t present, there would be normal sugar activation. We found that decreased behavioral avoidance to a sucrose/aversive mixture in the mutant flies was due to a deficit in the sugar-activated GRNs and not due to effects on GRNs activated by bitter compounds."

OBP49a is the first molecule shown to promote the inhibition of the sucrose-activated GRNs by aversive chemicals in fruit flies. The findings demonstrate at least one important cellular mechanism through which bitter and sweet taste integration occurs in the taste receptor neurons. However, the findings do not exclude the possibility that suppression of sweet by bitter compounds could also take place through the integration of separate bitter and sweet inputs in the brain.

"As we get a better understanding of aversive and attractive chemosensory behaviors in flies, it helps us understand how insect pests can be controlled," said Montell. "This is a step toward understanding the behaviors of related insects that spread disease. Molecules related to the OBPs and GRs in fruit flies are also in ticks and mosquitos that spread parasites and viruses."

Motional layers in the brain

Recognising movement and its direction is one of the first and most important processing steps in any visual system. By this way, nearby predators or prey can be detected and even one’s own movements are controlled. More than fifty years ago, a mathematical model predicted how elementary motion detectors must be structured in the brain. However, which nerve cells perform this job and how they are actually connected remained a mystery. Scientists at the Max Planck Institute of Neurobiology in Martinsried have now come one crucial step closer to this “holy grail of motion vision”: They identified the cells that represent these so-called “elementary motion detectors” in the fruit fly brain. The results show that motion of an observed object is processed in two separate pathways. In each pathway, motion information is processed independently of one another and sorted according to its direction.

Ramón y Cajal, the famous neuroanatomist, was the first to examine the brains of flies. Almost a century ago, he thus discovered a group of cells he described as “curious elements with two tufts”. About 50 years later, German physicist Werner Reichardt postulated from his behavioural experiments with flies that they possess “elementary motion detectors”, as he referred to them. These detectors compare changes in luminance between two neighbouring photoreceptor units, or facets, in the fruit fly’s eye for every point in the visual space. The direction of a local movement is then calculated from this. At least, that is what the theory predicts. Since that time, the fruit fly research community has been speculating about whether these “two-tufted cells” described by Cajal are the mysterious elementary motion detectors.

The answer to this question has been slow in coming, as the tufted cells are extremely small – much too small for sticking an electrode into them and capturing their electrical signals. Now, Alexander Borst and his group at the Max Planck- Institute of Neurobiology have succeeded in making a breakthrough with the aid of a calcium indicator. These fluorescent proteins are formed by the neurons themselves and change their fluorescence when the cells are active. It thus finally became possible for the scientists to observe and measure the activity of the tufted cells under the microscope. The results prove that these cells actually are the elementary motion detectors predicted by Werner Reichardt.

As further experiments have shown, the tufted cells can be divided into two groups. One group (T4 cells) only reacts to a transition from dark to light caused by motion, while the other group (T5 cells) reacts oppositely – only for light-to-dark edges. In every group there are four subgroups, each of which only responds to movements in a specific direction – to the right, left, upwards or downwards. The neurons in these directionally selective groups release their information into layers of subsequent nerve tissue that are completely separated from one another. There, large neurons use these signals for visual flight control, generating the appropriate commands for the flight musculature, for example. This could be impressively proven by the scientists: When they blocked the T4 cells, the neurons connected downstream and the fruit flies themselves were shown in behavioural tests to be blind to motions caused by dark-to-light edges. When the T5 cells were blocked, light-to-dark edges could no longer be perceived.

In discussions about their research results, which have just been published in the scientific journal Nature, both lead authors, Matt Maisak and Jürgen Haag, were very impressed with the “cleanly differentiated, yet highly ordered” motion information within the brains of the fruit flies. Alexander Borst, head of the study, adds: “That was real teamwork – almost all of the members in my department took part in the experiments. One group carried out the calcium measurements, another worked on the electrophysiology, and a third made the behavioural measurements. They all pulled together. It was a wonderful experience.” And it should continue like this, since the scientists are already turning to the next mammoth challenge: they would now like to identify the neurons that deliver the input signals to the elementary motion detectors. According to Reichardt, the two signals coming from neighbouring photoreceptors in the eye have to be delayed in relation to one another. “That is going to be really exciting!” says Alexander Borst.

Hunger affects decision making and perception of risk

Hungry people are often difficult to deal with. A good meal can affect more than our mood, it can also influence our willingness to take risks. This phenomenon is also apparent across a very diverse range of species in the animal kingdom. Experiments conducted on the fruit fly, Drosophila, by scientists at the Max Planck Institute of Neurobiology in Martinsried have shown that hunger not only modifies behaviour, but also changes pathways in the brain.

Animal behaviour is radically affected by the availability and amount of food. Studies prove that the willingness of many animals to take risks increases or declines depending on whether the animal is hungry or full. For example, a predator only hunts more dangerous prey when it is close to starvation. This behaviour has also been documented in humans in recent years: one study showed that hungry subjects took significantly more financial risks than their sated colleagues.

Also the fruit fly, Drosophila, changes its behaviour depending on its nutritional state. The animals usually perceive even low quantities of carbon dioxide to be a sign of danger and opt to take flight. However, rotting fruit and plants – the flies’ main sources of food – also release carbon dioxide. Neurobiologists in Martinsried have now discovered how the brain deals with this constant conflict in deciding between a hazardous substance and a potential food source taking advantage of the fly as a great genetic model organism for circuit neuroscience.

In various experiments, the scientists presented the flies with environments containing carbon dioxide or a mix of carbon dioxide and the smell of food. It emerged that hungry flies overcame their aversion to carbon dioxide significantly faster than fed flies – if there was a smell of food in the environment at the same time. Facing the prospect of food, hungry animals are therefore significantly more willing to take risks than sated flies. But how does the brain manage to decide between these options?

Avoiding carbon dioxide is an innate behaviour and should therefore be generated outside the mushroom body in the fly’s brain: previously, the nerve cells in the mushroom body were linked only with learning and behaviour patterns that are based on learned associations. However, when the scientists temporarily disabled these nerve cells, hungry flies no longer showed any reaction whatsoever to carbon dioxide. The behaviour of fed flies, on the other hand, remained the same: they avoided the carbon dioxide.

In further studies, the researchers identified a projection neuron which transports the carbon dioxide information to the mushroom body. This nerve cell is crucial in triggering a flight response in hungry, but not in fed animals. “In fed flies, nerve cells outside the mushroom body are enough for flies to flee from the carbon dioxide. In hungry animals, however, the nerve cells are in the mushroom body and the projection neuron, which carries the carbon dioxide information there, is essential for the flight response. If mushroom body or projection neuron activity is blocked, only hungry flies are no longer concerned about the carbon dioxide,” explains Ilona Grunwald-Kadow, who headed the study.

The results show that the innate flight response to carbon dioxide in fruit flies is controlled by two parallel neural circuits, depending on how satiated the animals are. “If the fly is hungry, it will no longer rely on the ‘direct line’ but will use brain centres to gauge internal and external signals and reach a balanced decision,” explains Grunwald-Kadow. “It is fascinating to see the extent to which metabolic processes and hunger affect the processing systems in the brain,” she adds.

The discerning fruit fly: Linking brain-cell activity and behavior in smell recognition

Behind the common expression “you can’t compare apples to oranges” lies a fundamental question of neuroscience: How does the brain recognize that apples and oranges are different? A group of neuroscientists at Cold Spring Harbor Laboratory (CSHL) has published new research that provides some answers.

In the fruit fly, the ability to distinguish smells lies in a region of the brain called the mushroom body (MB). Prior research has demonstrated that the MB is associated with learning and memory, especially in relation to the sense of smell, also known as olfaction.

CSHL Associate Professor Glenn Turner and colleagues have now mapped the activity of brain cells in the MB, in flies conditioned to have Pavlovian behavioral responses to different odors. Their results, outlined in a paper published today by the Journal of Neuroscience, suggest that the activity of a remarkably small number of neurons — as few as 25 — is required to be able to distinguish between different odors.

They also found that a similarly small number of nerve cells are involved in grouping alike odors. This means, for instance, that “if you’ve learned that oranges are good, the smell of a tangerine will also get you thinking about food,” says Robert Campbell, a postdoctoral researcher in the Turner lab and lead author on the new study.

These intriguing new findings are part of a broad effort in contemporary neuroscience to determine how the brain, easily the most complex organ in any animal, manages to make a mass of raw sensory data intelligible to the individual — whether a person or a fly — in order to serve as a basis for making vital decisions.

Looking closely at Kenyon cells

The neurons in the fly MB are known as Kenyon cells, named after their discoverer, the neuroscientist Frederick Kenyon, who was the first person to stain and visualize individual neurons in the insect brain. Kenyon cells receive sensory inputs from organs that perceive smell, taste, sight and sound. This confluence of sensory input in the MB is important for memory formation, which comes about through a linking of different types of information.

Kenyon cells make up only about 4% of the entire fly brain and are extremely sensitive to inputs triggered by odors, in which only two connections between neurons, called synapses, separate them from the receptor cells at the “front end” of the olfactory system.

But in contrast to other regions of the brain, such as the vertebrate hippocampus, the sensory responses in the MB are few in number and relatively weak. It is the sparseness of the signals in the Kenyon cell neurons that makes studying memory formation in flies so promising to Turner and his team. “We set out to learn if these signals were really informative to the animal’s learning and memory with regard to smell,” Turner says.

In particular, Turner’s group wanted to see if they could link these signals with actual behavior in flies. The team used an imaging technique that allowed them to view the responses of over 100 Kenyon cells at a time and, importantly, quantify their results. They found that even the very sparse responses in these cells that are triggered by odors provide a large amount of information about odor identity. Turner suspects the very selectiveness of the response helps in the accurate formation and recall of memories.

When the researchers used two odors blended together in a series of increasingly similar concentrations, they found that two very similar smells could be distinguished as a result of the activity of as few as 25 Kenyon cells. This correlated well with the behavior of the flies: when brain activity suggested the flies had difficulty discerning the odors, their behavior also showed they could not choose between them.

The activity of these cells also accounts for flies’ ability to discern novel odors and group them together. This was determined in a “generalization” test, in which the degree to which flies learned a generalized aversion to unfamiliar test odors could be predicted based upon the relatively similar activity patterns of Kenyon cells that the odors induced.

“Being able to do this type of ‘mind-reading’ means we really understand what signals these activity patterns are sending,” says Turner. Ultimately, he and colleagues hope to be able to relate their findings in the fly brain with the operation of the brain in mammals.

Gustatory Tug-of-war Key To Whether Salty Foods Taste Good

Fruit fly’s salt taste sensation strategy may apply to other animals, including humans

As anyone who’s ever mixed up the sugar and salt while baking knows, too much of a good thing can be inedible. What hasn’t been clear, though, is how our tongues and brains can tell when the saltiness of our food has crossed the line from yummy to yucky — or, worse, something dangerous.

Now researchers at the Johns Hopkins University School of Medicine and the University of California, Santa Barbara report that in fruit flies, at least, that process is controlled by competing input from two different types of taste-sensing cells: one that attracts flies to salty foods, and one that repels them. Results of their research are described in the June 14 issue of Science.

“The body needs sodium for crucial tasks like putting our muscles into action and letting brain cells communicate with each other, but too much sodium will cause heart problems and other health concerns,” explains Yali Zhang, Ph.D., who led the recent study as part of his graduate work at Johns Hopkins. To maintain health, Zhang says, humans and other animals perceive foods with relatively low salt concentrations as tasty, but avoid eating things with very high salt content.

To find out how the body pulls off this balancing act, Zhang worked with his adviser, Craig Montell, Ph.D., a leading scientist in the field of sensory biology and now a professor at UC Santa Barbara, and graduate student Jinfei Ni to get an up-close view of the fly equivalent of a tongue: its long, curly proboscis. They zoomed in on the proboscis’ so-called sensilla, hair-like structures that serve as the fly’s taste buds.

Previous research had identified several distinct types of sensilla, one of which attracts flies to a taste, while another repels them. Zhang loaded an electrode with a mixture of water and different concentrations of salt, and touched it to each type of sensilla, using the same electrode to detect the electrical signals fired by the sensilla in response to the salt. He found that up to a point, increasing salt concentrations would produce increasingly strong electrical signals in the attractive sensilla, but after that point, the electrical signals dropped off as the concentration continued to rise. In contrast, the repellant sensilla gave off stronger and stronger electrical signals as the salt concentration rose.

Zhang said the team realized that the taste receptor cells in the attractive and repellant sensilla were likely locked in a tug-of-war over whether the fly would continue eating or go off in search of better food. At lower concentrations, the attractive signal would dominate the repellant signal, sending a cumulative message of “yum!” But at high concentrations, the repellant signal would overwhelm the attractive signal, sending the signal “yuck!”

To further test this conclusion, the team mutated a gene called Ir76b that codes for a protein they suspected was involved in the action of the attractive sensilla. To their great surprise, Zhang found that loss of Ir76b function caused flies to avoid the otherwise attractive low-salt food. The reason for this, he found, was that mutating Ir76b only impaired the responses of the attractive sensilla, leaving the repellant sensilla to win the day. Looking further into the action of the protein produced by Ir76b, the team found that it is a channel with a pore that lets sodium pass into the taste cells of the sensilla. Unlike most pores of this type, which have gates that must be opened by certain key chemical or voltage changes in their environment, this gate is always open, meaning that at any time, sodium can flood into the cell and spark an electrical signal. “It’s an unusual setup, but it makes sense because the local sodium concentration outside taste receptor cells appears to be a lot lower than that surrounding most cells. The taste receptor cells don’t need to keep the gate closed to protect themselves from that excess sodium,” Zhang says.

Long before we humans started worrying about regulating our sodium intake, it was a problem all animals had to deal with, Zhang says, and thus his research has implications for other animals, including humans. Although animal taste buds and insect sensilla have different makeups, he suspects that the tug-of-war principle may apply to salt-tasting throughout the animal kingdom, given that different species behave similarly when it comes to salty foods. Identifying the low-salt sensor in humans could be particularly useful, he says, as it could lead to the development of better salt substitutes to help people control their sodium intake.

Targeting an aspect of Down syndrome

University of Michigan researchers have determined how a gene that is known to be defective in Down syndrome is regulated and how its dysregulation may lead to neurological defects, providing insights into potential therapeutic approaches to an aspect of the syndrome.

Normally, nerve cells called neurons undergo an intense period of extending and branching of neuronal protrusions around the time of birth. During this period, the neurons produce the proteins of the gene called Down syndrome cell-adhesion molecule, or Dscam, at high levels. After this phase, the growth and the levels of protein taper off.

However, in the brains of patients with Down syndrome, epilepsy and several other neurological disorders, the amount of Dscam remains high. The impact of the elevated Dscam amount on how neurons develop is unknown.

Bing Ye, a faculty member at U-M’s Life Sciences Institute, found that in the fruit fly Drosophila, the amount of Dscam proteins in a neuron determines the size to which a neuron extends its protrusions before it forms connections with other nerve cells. An overproduction of Dscam proteins leads to abnormally large neuronal protrusions.

Ye also identified two molecular pathways that converge to regulate the abundance of Dscam. One, dual leucine zipper kinase (DLK), which is involved in nerve regeneration, promotes the synthesis of Dscam proteins. Another, fragile X mental retardation protein (FMRP), which causes fragile X syndrome when defective, represses Dscam protein synthesis. Because humans share these genes with Drosophila, the DLK-FMRP-Dscam relationship presents a possible target for therapeutic intervention, Ye said.

Many genes are involved in neurological disorders like Down syndrome, and how molecular defects cause the disease is complex.

"But because of the important roles of Dscam in the development of neurons, its related defect is very likely to be an aspect of Down syndrome and it may be an aspect of the syndrome that can be treated," said Ye, an assistant professor in the Department of Cell and Developmental Biology at the U-M Medical School.

Ye’s next step is to test the effects of overexpression of Dscam in mice to see how it changes the development of the nervous system and the behavior of the animal.

Down syndrome occurs in about one in 830 newborns; an estimated 250,000 people in the U.S. have the condition, according to the National Library of Medicine’s Genetics Home Reference.

A new strategy required in the search for Alzheimer’s drugs?

In the search for medication against Alzheimer’s disease, scientists have focused – among other factors – on drugs that can break down Amyloid beta (A-beta). After all, it is the accumulation of A-beta that causes the known plaques in the brains of Alzheimer’s patients. Starting point for the formation of A-beta is APP. Alessia Soldano and Bassem Hassan (VIB/KU Leuven) were the first to unravel the function of APPL – the fruit-fly version of APP – in the brain of healthy fruit flies. (PLoS Biology)

Alessia Soldano (VIB/KU Leuven): “We have discovered that APPL ensures that brain cells form a good network. We now have to ask ourselves the question whether this function of APPL is also relevant to Alzheimer’s disease.”

Bassem Hassan (VIB/KU Leuven): “Since we show that APP and APPL show similar activities in cultured cells, we suspect that APP in the human brain functions in the same manner as APPL in the brain of fruit flies. Hopefully we can use this to ask and eventually answer the question whether A-beta or APP itself is the better target for new drugs.”

Plaques in the brain: cause or effect
The brain of a person with Alzheimer’s disease is very recognizable due to the so-called plaques. A plaque is an accumulation of proteins that are primarily made up of Amyloid beta (A-beta), a small structure that splits off from the Amyloid Precursor Protein (APP). We have been dreaming for a long time of a drug that can break down A-beta, but we should be asking ourselves whether this is really the best strategy. After all, it is not yet clear whether the plaques are a cause or effect of Alzheimer’s disease. In order to answer this question, it is important to determine the function of APP in healthy brains.

Optimum communication between brain cells
Alessia Soldano and Bassem Hassan study APPL, the fruit-fly version of APP. APPL is found throughout the fruit-fly brain, but primarily in the so-called alpha-beta neurons that are vital to learning processes and memory. The alpha-beta neurons must form functional axons for optimum functioning. Axons are tendrils projecting from the neuron, which are essential for communication between neurons. The VIB scientists had previously shown that APPL is important for memory in flies. Now, they have discovered that – in the developing brain of a fruit fly – APPL ensures that the axons are long enough and grow in the correct direction. APPL is therefore essential in the formation of a good network of neurons. The question is whether or not it is a good strategy to target a protein with such an important function in the brain in order to combat Alzheimer’s disease. (PLoS Biology)