Why do we age? It’s more than just a philosophical question; it’s a puzzle that has frustrated scientists for decades. Currently, the most accepted hypothesis is that aging is the result of accumulated damage to our cells during our lifetime. “Accumulated damage” encompasses a variety of things that can go wrong, including DNA mutations, problems in the way molecules such as proteins are built, or abnormal interactions between molecules. Over time, the damage interferes with the body’s ability to maintain itself the way it used to, and the process we call “aging” occurs.
Click on the picture for a bigger version. Multiple external and internal factors can lead to the formation of free radicals and cell damage, which accelerates aging.
It sounds like a great hypothesis, but how can we prove it, and how can we measure accumulated damage? Damage can be caused by a number of factors, and the types of damage that can occur varies wildly between species and even between individuals of the same species. To make matters worse, exposure to certain environmental influences or toxins can accelerate aging. For example, overexposure to UV light from the sun can increase signs of aging in skin cells, and studies have shown that smoking can also accelerate aging. How can scientists study something with so much variability and uncertainty? A recent fruit fly paper published in eLife by the Gladyshev lab describes a new way to study damage accumulation. Instead of measuring the damage itself, they measure the byproducts of cellular metabolism as a proxy.
You see, even if we were able to reduce exposure to environmental toxins, we’d still get older. That’s because unfortunately, even our bodies are working against us. Our cells have metabolic processes, which are the life-sustaining chemical reactions that are needed to keep them alive and reproducing. The small molecules that are produced by these reactions (called metabolites) are usually important for the cells; they could be fuel for energy, signals for growth or reproduction, or even necessary for defenses and interactions with the environment. Unfortunately, these processes also create toxic byproducts such as reactive oxygen species (also known as free radicals). These byproducts cause damage and have often been associated with many age-related diseases such as cancer, heart disease, and Alzheimer’s disease.
To test the idea that accumulating metabolic byproducts can lead to aging, the authors of this study used a new method called “metabolite profiling” to measure the amount of metabolites in flies as they aged. (If you’re interested, the technique they used is called liquid chromatography mass spectrometry). They first found that the diversity of metabolites increases with age. This suggests that mistakes were being made during the metabolic reactions, causing new types of byproducts to appear that can damage cells. Additionally, a subset of metabolites accumulated with age, which may indicate that these byproducts are not being sufficiently cleaned up by maintenance processes in the cells. The authors found that many of these had previously been identified as damaging, directly confirming that accumulating toxic byproducts is correlated with aging.
Finally, the authors also used a calorie-restricted diet to extend the lifespan of a group of flies (although it’s not yet understood why, a severely restrictive diet can increase lifespan in many model animals, including mice and rats. It’s not recommended for humans, however, because there are unpleasant side-effects). Most interestingly, when the authors compared metabolite accumulation in normal flies versus the lifespan-extended flies, they showed that the metabolite accumulation was slower in the longer-lived flies, corresponding with the slower progression of aging.
So how can these findings help us? This paper supports the hypothesis that accumulation of damage leads to aging by showing that metabolites accumulate at a rate that corresponds with relative age in fruit flies. Because most species share the same cellular metabolic processes, these results are relevant to mammals. The next step will be to identify particular types of metabolites and determine how they contribute to aging in flies and mammalian models. The authors of this paper already did some of the legwork. They found that many of the metabolites that differed between the lifespan-extended group and the normal group of flies were associated with processes for using and storing energy from fats and proteins (not surprising considering the flies were on a strict diet). The authors suggest that changing these metabolic processes through diet may have compensated somehow for the accumulation of toxic byproducts. Future research may be able to expand upon these findings, and perhaps even figure out a way to interfere with these processes to slow or alter aging. I just hope I live long enough to see it!
Reference:
Avanesov A.S., Kerry A Pierce, Sun Hee Yim, Byung Cheon Lee, Clary B Clish & Vadim N Gladyshev (2014). Age- and diet-associated metabolome remodeling characterizes the aging process driven by damage accumulation, eLife, 3 DOI: http://dx.doi.org/10.7554/elife.02077
A study in 2012 found that approximately 7.2% of adults in the United States have an alcohol use disorder (a term that covers any person for whom their drinking causes distress or harm). That adds up to approximately 17 million Americans! Treatments for alcoholism, such as behavioral therapies or medications, can often be ineffective in the long-term due to changes that happen in the brain as a result of addiction. Improved treatment for alcoholism therefore requires an understanding of how and why addiction forms.
Figure 1. Genes can account for about 50% of the risk for alcoholism. Image by Addiction Blog
Research has shown that genes are responsible for about 50% of the risk for alcoholism, which can explain why alcoholism seems to run in families. Although genes alone don’t determine whether someone will become an alcoholic, they can influence a person’s sensitivity and tolerance for alcohol. Environmental or social interactions account for the remainder of the risk. A person with increased sensitivity to the stimulant effects of alcohol (increased energy, lowered inhibition) and/or decreased sensitivity to its depressant effects (motor impairment, sedation) might have a more pleasant experience with it, making them more likely to use it regularly. On the other hand, someone who finds the taste or side effects more aversive is unlikely to become addicted. For example, some people of Asian descent carry a gene variant that causes them to metabolize alcohol too fast, and they experience symptoms like flushing, nausea, and rapid heartbeat when they drink.
Studying the genes that increase the risk of alcoholism in animal models could lead to better preventive measures in at-risk families. Alcohol addiction also causes neuronal changes in the brain that can be studied in animal models to gain a better understanding of how the brain is changing and how to reverse it. Such an understanding will inevitably lead to better treatments for alcohol addiction.
Figure 2. Fruit flies can serve as a model for alcohol addiction. Drawing by the Heberlein Lab
Wait, isn’t this a fruit fly blog? How can they possibly have anything to do with alcohol addiction? You may have heard the old adage, “You catch more flies with honey than vinegar”, but in truth, fruit flies are more attracted to the smell of vinegar and fermenting fruit. Fruit flies like to feed off of the microbes (such as yeast) that are found in fermenting fruit, and they have evolved to develop a tolerance for the alcohol produced by fermentation. In fact, researchers have found that fruit flies even prefer to feed and lay their eggs on substances with about 4-5% alcohol—the concentration of an average beer.
One of the reasons fruit flies have learned to love alcohol is because it protects them from parasitic wasps, one of their most dangerous predators. Parasitic wasps inject their eggs into fruit fly larva. Although the larva’s immune system can sometimes resist the wasp growing inside it, most often the wasp larva eats the fly from the inside out, later bursting from it as a mature wasp. But while fruit flies have evolved a resistance to alcohol, the parasitic wasps have not. When a larva has been injected by one of these predators, it will consume almost toxic levels of alcohol in an attempt to kill off the parasite within it. In fact, female fruit flies that have detected the presence of parasitic wasps nearby will seek out the most alcohol-laden foods she can find before laying her eggs, giving her offspring their greatest chance to survive the predators.
But what happens when fruit flies are exposed to levels of alcohol not normally found in their natural environments? Stronger alcohol concentrations than what they have evolved to tolerate can actually be harmful to fruit flies. Does that mean fruit flies are smart enough to avoid such concentrations?
Figure 3. A fruit fly drinking alcohol from a capillary tube during a test for whether flies prefer food laced with alcohol. Photo by G. Shohat-Ophir, UCSD
Researchers have found that fruit flies seem to treat intoxicating levels of alcohol in the same way that some humans do—as a reward they just can’t get enough of. Fruit flies that have never been exposed to alcohol before will initially show a slight preference for it over a non-alcoholic food source, likely because it instinctively knows that alcohol signals food and safety. But with each exposure their preference for the alcohol-laden food gets stronger and stronger, despite its bitter taste and aversive side effects such as motor incoordination and sedation (sound familiar?). Eventually, these fruit flies show signs of alcoholism: they regularly drink until they’re “drunk”, build up a tolerance over time and drink more and more to compensate, and continue drinking despite increasingly dangerous side effects. They’ll keep drinking even if researchers add an aversive stimulus to the alcohol-laden food, such as a repulsive chemical or an electric shock. Fruit flies will also experience symptoms of withdrawal when the alcohol is taken away, and relapse to previous levels of drinking when it’s returned. Perhaps even more surprising is that drunk fruit flies lower their standards when looking for suitable sexual mates, and flies that have been sexually rejected will turn to alcohol to cope. These parallels between human and fruit fly drinking behavior are amazing! Scroll down to see a breakdown of the many ways fruit flies show signs of alcohol addiction.
These findings have led scientists to develop fruit flies as a model organism for alcohol addiction, because although fruit flies and humans may seem very different, many genes and cellular processes are shared between them (and in fact among most species!). Fruit flies are fantastic for genetic research, and could tell us a lot about genetic risks for alcoholism and why alcohol addiction forms. They can even be used to study the changes that occur in the brain as a result of addiction, since fruit flies exhibit many “alcoholic” behaviors.
What have fruit flies taught us so far? Fly scientists have already identified several genes that contribute to the risk of alcoholism (listed below). Despite their funny names, they could provide some very serious information about why some individuals have higher sensitivity or tolerance for alcohol and how these genes can be targeted by drugs to prevent or treat alcoholism.
Krasavietz – Researchers have found that fruit flies with a mutation in this gene are completely uninterested in alcohol. They showed that mutations in krasavietz reduced flies’ sensitivity to the “sedation” effect of alcohol, causing flies to quickly become sedated after intoxication, skipping all the fun stimulating side-effects.
Cheapdate – Flies with a mutation in this gene experienced increased sensitivity to alcohol, such that much lower doses were able to cause “intoxicated” behaviors.
Happyhour – A mutation in this gene causes flies to be less sensitive to the sedative effects of alcohol while maintaining normal sensitivity for the stimulating effects. A mutation in this gene in humans might increase the risk of addiction, because they will have a more pleasant experience when drinking and are more likely to drink regularly.
Hangover – Flies with a mutation in the hangover gene don’t develop a tolerance for alcohol. Instead of needing increased doses to achieve the same behavioral effects over time, the same dose always affects them the same way. Because increased consumption over time leads to dependence and addiction, human with a mutation in this gene may be at lower risk of alcoholism.
Researchers have also learned that a desire for alcohol in flies depends upon a certain chemical in the brain called neuropeptide F (NPF), which is very similar to neuropeptide Y (NPY) found in mammals. NPY signaling in mammals has been linked to stress, alcohol consumption, sexual motivation, and sugar satiety, among other things. In flies, reduced NPF levels led to increased alcohol intake. Sexual fulfillment was found to increase NPF levels, while sexual rejection decreased them. NPF levels could therefore indicate general “reward satisfaction”, and reduced levels causes flies to seek out something rewarding, whether it’s sex, drugs, or rock and roll. It is very likely that NPY in mammals plays the same role, which means manipulating NPY levels in humans could be a possible treatment for addiction.
Finally, fruit fly researchers have found that dopamine, a chemical that some neurons in the brain use for communication, was also involved in alcohol addiction. Dopamine has also been implicated in addiction in mammalian research because of the role it plays in the mammalian “reward system”, a brain region that gets hijacked in addiction. Further research in this area in flies may provide clues as to how dopamine signaling can lead to changes in “reward” structures.
No animal model will ever be a perfect model for alcoholism, because it is a largely a human phenomenon influenced by social, cultural, and cognitive factors. But animal models can be used to model important physical and behavioral facets of addiction, and to determine the genetic basis for withdrawal and tolerance. These findings will lead to the development of better medications for the prevention and treatment of alcoholism.
Fruit flies exposed to high levels of alcohol show behaviors that indicate they could be “tipsy” and/or “drunk”
Figure 4. The “booze-o-mat” is where fruit flies are placed in tubes with alcohol vapors to ensure consumption. Photo by the Heberlein Lab
While researchers sometimes provide fruit flies with alcohol by mixing it into their food, they can ensure consistency in their flies’ level of “drunkenness” by putting them in tubes filled with alcohol vapors so they breathe it in (Figure 4). This allows them to assess the behavioral effects of alcohol with less variation. At lower doses, the alcohol acts as a stimulant and increases their level of activity (comparable to increased energy and lowered inhibitions in humans). This is thought to be the rewarding effects of the alcohol. But increase the dose, and flies start showing signs of motor incoordination. They actually seem to be tipsy—they fall over, bump into each other and the walls, and have difficulties climbing. Even higher doses have a sedating effect.
Fruit flies prefer to consume alcohol, even if they don’t need it for nutritional purposes
Figure 5. Fruit flies are given a choice of capillary tubes containing either regular food or alcohol-laden food. Modified from Devineni & Heberlein, 2009
When fruit flies that have never experienced alcohol before are given a choice between non-alcoholic food and alcohol-laden food, they initially show only a slight preference for it. But the next time they are given a choice, they will overwhelming choose the food laced with alcohol. The flies will even consume enough alcohol to cause intoxication and alter its behavior as previously described. They will consume an intoxicating amount of alcohol time and time again with increased frequency, much like an alcoholic might.
Fruit flies will continue to consume alcohol even if they don’t like the way it tastes
Researchers have shown that while fruit flies like the way alcohol smells (it signals food and safety, after all), they don’t like the way it tastes. Nevertheless, just as humans consume alcohol despite its initially aversive bitter taste (and eventually develop a “taste for it”) fruit flies drink the alcohol anyway. Even when researchers associate alcohol with an aversive stimulus, such as giving the flies an electric shock whenever they drink or lacing the alcohol with a repulsive chemical called quinine, they will continue to drink up. This shows that flies are willing to overcome an aversive stimulus in order to consume alcohol.
Fruit flies develop a tolerance to alcohol and consume more and more each time to compensate
Like humans, fruit flies build up a resistance to the effects of alcohol after repeated exposure, which is known as tolerance. Flies that have been previously exposed to alcohol show an increasing tolerance to its effects over time, and will drink more and more each time to produce the same behavioral effects.
Fruit flies develop a physiological dependence on alcohol and experience symptoms of withdrawal
In humans, withdrawal symptoms include dysphoria, anxiety, cognitive impairment, and seizures. Clinically, these symptoms are a sign of alcohol dependence. After flies were chronically exposed to alcohol, they showed some of these same symptoms when it was taken away.
In one study, researchers showed that flies experiencing withdrawal had a lowered threshold for seizures and had more seizures than flies that had never been exposed to alcohol.
Researchers have also shown that fruit fly larva experience cognitive impairment as a symptom of withdrawal. They found that larva who drank too much had difficulties learning at first, but after chronic exposure they adapted and were able to learn almost as well as larva that were not exposed to alcohol. When the alcohol was taken away, their cognitive abilities were lost, and when the alcohol was returned, the larva also regained their learning abilities. These results suggest that the animals were dependent on alcohol not just physiologically, but also cognitively.
Fruit flies “relapse” after abstinence from alcohol
One characteristic of alcohol addiction is relapse, in which an individual will return to similar or greater consumption levels after a period of abstinence. In one study, fruit flies were chronically exposed to alcohol, followed by a period of abstinence. When the researchers provided them with alcohol again, the flies immediately began drinking at the same levels as before, without the gradual increase in preference that is seen when fruit flies are exposed for the first time.
Male fruit flies who have been sexually rejected turn to alcohol to cope
Researchers found that male fruit flies who had been unsuccessful in attempting to mate with a female drank more alcohol afterwards than males who were successful. They found that the desire to drink was dependent on levels of neuropeptide F (NPF), which were decreased after rejection but increased after successful mating. Alcohol consumption increased NPF levels again, suggesting that NPF acts as a “reward signal”. NPF is similar to mammalian neuropeptide Y (NPY), which has been linked to stress, anxiety, sexual motivation, alcohol consumption, and sugar satiety in mammals.
“Drunk” male fruit flies become hypersexual and lower their standards for suitable sexual mates
Figure 6. Drunk male fruit flies sometimes form a “courtship chain”, where male flies follow each other around trying to mate. Photo by Kyung-An Han laboratory, Penn State
Normally, male flies will only try to mate with females, but when they have been exposed to alcohol, they will not only step up their courting of females, but also even try to mate with other males. This effect got worse and worse with each exposure to alcohol. Unfortunately, just as humans have learned over the years, rates of successful mating actually decreased after getting tipsy. Even for fruit flies, getting drunk doesn’t necessarily lead to good sex.
References:
Milan N. & Todd A. Schlenke (2012). Alcohol Consumption as Self-Medication against Blood-Borne Parasites in the Fruit Fly, Current Biology, 22 (6) 488-493. DOI: http://dx.doi.org/10.1016/j.cub.2012.01.045
Kacsoh B.Z., N. T. Mortimer & T. A. Schlenke (2013). Fruit Flies Medicate Offspring After Seeing Parasites, Science, 339 (6122) 947-950. DOI: http://dx.doi.org/10.1126/science.1229625
Shohat-Ophir G., R. Azanchi, H. Mohammed & U. Heberlein (2012). Sexual Deprivation Increases Ethanol Intake in Drosophila, Science, 335 (6074) 1351-1355. DOI: http://dx.doi.org/10.1126/science.1215932
Lee H.G., Jennifer S. Dunning & Kyung-An Han (2008). Recurring Ethanol Exposure Induces Disinhibited Courtship in Drosophila, PLoS ONE, 3 (1) e1391. DOI: http://dx.doi.org/10.1371/journal.pone.0001391
Robinson B., Anna Kuperman & Nigel S. Atkinson (2012). Neural Adaptation Leads to Cognitive Ethanol Dependence, Current Biology, 22 (24) 2338-2341. DOI: http://dx.doi.org/10.1016/j.cub.2012.10.038
Devineni A.V. (2009). Preferential Ethanol Consumption in Drosophila Models Features of Addiction, Current Biology, 19 (24) 2126-2132. DOI: http://dx.doi.org/10.1016/j.cub.2009.10.070
… and all you ever wanted to know about fruit flies!
Have you ever wondered how fruit fly scientists perform experiments with these tiny insects every day? Since I work with fruit flies, I want to provide a glimpse of the day-to-day life of a fly researcher (imagine you’re a “fly on the wall” in our lab!). My project is to figure out how neurons in the memory system communicate with each other to store long-term memories. I dissect fruit fly brains and use fluorescence imaging to watch what neurons are doing in real-time, and I collaborate with other graduate students and postdocs in the lab who perform behavioral experiments. But what are the day-to-day activities that every fly scientist has to take care of?
Our stocks of flies are kept in incubators at a stable temperature and light cycle.
My day usually doesn’t start until 10am when the lights in our flies’ incubators turn on. Most of our flies are kept in these refrigerator-sized incubators at a tightly-controlled temperature of 25°C (77°F), which is a fruit fly’s favorite temperature. They’re also kept on a strict light cycle—12 hours of light starting at 10am followed by 12 hours of darkness, which is important for reducing variation in behavioral experiments.
Fly lines are kept in vials, which are stored on racks for easy stacking.
There are thousands of different fly lines, but I only have about 200 of them. Each fly line is kept in a small vial with food at the bottom (the yellow stuff, which is a thick gel of yeast, sugar, and cornmeal) and a cotton plug at the top to keep them from escaping. Each fly line has to be “flipped” to a new vial every couple of weeks after the hundreds/thousands of flies have finished making a mess of their current home.
A fly vial in use.
If you look closely, you can see pupal cases stuck to the walls of the vials. An adult fly can live for about a month at its optimal temperature, during which time a female can lay hundreds of eggs. The eggs hatch into larvae after approximately 24 hours, which spend about 6 days chewing through food and gaining strength. When the time is right, the larvae climb the sides of the vials and turn into pupae. After 4 motionless days, a new
Fruit flies go through four life stages: egg, larva, pupa, and adult. source
fly emerges from its pupal case. In total, it takes about 10 days after an egg is laid before a fly emerges. We can tell when a fly is about to come out because the pupa gets darker as the fly develops inside it. Within hours after emerging, an adult female fly is ready to mate and lay eggs.
Knocking out flies with CO2 in the vial prevents them from escaping
Flies are dumped onto a CO2 pad to keep them unconscious while they are being sorted.
Now I need to get flies out of some of those vials so I can use them for my experiments. Carbon dioxide (CO2) knocks flies unconscious, so we slide a syringe needle in through the cotton top of the vial and pump in some CO2 to knock them out. Then we can dump the flies out onto CO2 pads, which have a porous styrofoam top and CO2 flow underneath to keep the flies unconscious.
Wild flies (bottom) have bright red eyes, but lab flies (top) usually have white eyes until a gene with an eye color feature is added. source
For our experiments, we need to make sure our flies have the right genes or mutations. In my case, I need flies with a fluorescent marker in memory-specific regions of the brain so I can see them under a microscope. This usually means I need to collect flies with two added genes in their DNA: a “marker” gene and a “location” gene. I will need to sort through the flies to make sure they have both genes. Luckily, when a scientist makes a fly line with a new gene, they attach some DNA code for a physical feature to the gene. Usually, the feature is a change in the flies’ eye color. Although wild flies have bright red eyes, our lab flies are bred to have a mutation in the eye color gene so that they have white eyes.
Lab flies with an extra gene inserted into their DNA usually have orange eyes. The shade of orange varies, but darker colors can often mean more than one new gene is present. Photo by Weiwu Xie, source
Because our flies have no eye color, adding a gene with an eye-color feature makes these flies stand out among their white-eyed brethren. This way, we can sort through the flies and pick out the ones with colored eyes, most commonly orange. Flies with two genes have darker orange eyes because the color adds up, which is great for me.
We use a fine-tipped paintbrush to carefully sort through the flies without harming them.
I sort through the flies under a microscope using a paintbrush, and then brush the flies I need for my experiments into smaller vials to separate them.
“Fly morgues” are used to dispose of flies. They also conveniently double as fly traps.
There are, of course, tons of flies I don’t need. We can’t possibly save all the unnecessary flies, so they get dumped into a “fly morgue”. These are bottles filled with a mixture of water, ethanol, apple cider vinegar, and a drop of dish detergent. The funnel keeps them from escaping after we dump them in, and the detergent breaks the surface tension of the water so the flies can’t just skim across the surface. It’s a quick and painless death for our noble subjects, and at least they die swimming in their beloved vinegar. These also make great traps in the home to take care of fruit fly infestations. If you don’t have any vinegar, a splash of wine should do it.
A dissection microscope. A dish with wells for dissecting is shown underneath.
Now it’s time to prepare the flies for experiments. For some people in our lab, this means loading the live flies into machines that track their activity for sleep or memory studies. For others, such as myself, it means dissecting out the brains from individual flies (or in some studies, the larvae instead), and performing experiments on the brains instead of live animals. As you can imagine, it takes a pretty long time to learn how to dissect out the tiny brain from these insects. We use a microscope to see what we’re doing, and very carefully remove the shell of the fly head using fine-tipped tweezers.
A close-up “action shot” of a fly head being dissected. The head is being held with a pair of fine-tipped tweezers.
If you’re interested in seeing a dissection in action, check out this video from Jove, the Journal of Visualized Experiments.
A graphical representation of an adult fruit fly brain. Modified from Chiang et al, 2011
The final brain is too small to see by eye (though the most experienced among us can point out the tiny white dot in a drop of water). Fun fact: did you know that fruit flies don’t have blood like we do? They have an open circulatory system filled with “hemolymph”, which is a clear liquid filled
Click on the picture for a bigger version. The brain (circled in red) is floating in a solution. Can you spot it?
with nutrients for cells. We dissect brains in a solution that’s designed to mimic this hemolymph and keep the brain alive as though it was still inside the fly.
Close-up of the imaging set-up for dissected fly brains. Fresh hemolymph-like solution is being perfused in from the right to replace the old solution being vacuumed out on the left. A 60x magnification objective is shining a tiny beam of fluorescent light into the dish.
A structure in the fly brain called the “mushroom bodies” is fluorescing under a microscope. Photo by Jenett et al / CC-BY-2.0
Once I’ve dissected my brain, I pin it down in a dish for imaging. The brain-in-a-dish is placed under a microscope with a fluorescent light so that the fluorescent marker in the brain glows. During the experiments, I make sure that fresh hemolymph-like solution is flowing over the brain and being vacuumed out on the other side to keep the brain healthy, and sometimes I use this system to add in drugs which can affect activity in the neurons. This allows me to manipulate specific neurons and study how it affects communication between them.
In future “Fly Life” articles, I will go into detail about how specific experiments are performed, such as my imaging technique or the behavioral experiments used by my labmates. Until then…
Got any questions or want more details? Feel free to ask in the comments section!
Discussions about whether schools for children should start later have been making headlines recently, highlighting the importance of getting enough sleep at night. We all know how important sleep is for day-to-day performance—you’ve likely experienced firsthand how hard it can be to think and focus after a bad night’s sleep. Luckily, these effects are reversible: just get enough sleep for the next couple of nights and you’ll feel refreshed again. But can sleep deprivation have long-term, irreversible consequences in children?
Table 1. Human children require more sleep than adults. Data obtained from the National Sleep Foundation
Across multiple species, young animals need more sleep than adults. Although the purpose of sleep is not fully understood, researchers believe that the brain may use this time to repair itself, store new memories, and modify itself to stay current with recently learned skills or adapt to changes in the environment (a process known as plasticity). The brains of young animals are very plastic and are undergoing a lot of changes as they develop, and scientists have always suspected that increased sleep is necessary for normal brain development. In humans, the majority of brain growth occurs before the age of two, which is also the period of life with the highest amount of sleep. In a recent paper published in Science, the Sehgal lab studied the link between sleep and brain development using Drosophila melanogaster and found that loss of sleep in immature flies led to abnormal development in a fast-growing area of the brain and consequent behavioral problems in adult flies.
Just like humans and other mammals, fruit flies need a good night’s sleep to function normally during the day. The authors began their study by confirming that in flies, young animals also sleep more than adults. They then sleep deprived a group of flies by placing them in a shaking machine for two nights and measured their behavior three days later (a long time for a humble fruit fly!). Fly researchers prefer to study innate behaviors because they are instinctual instead of learned, suggesting that their underlying brain structure develops genetically rather than from experience (in these cases, it is thought that the “nature versus nurture scale” is tipped toward nature.) In this case, the authors studied “courtship behavior”, a measure of how well male flies can solicit female flies and successfully pass on his genes (so to speak). They found that flies that were sleep deprived when they were younger didn’t perform as well as flies that had gotten enough sleep. The authors showed that this behavioral abnormality was specifically caused by loss of sleep in young flies, because flies that were sleep deprived as adults performed normally three days later.
Figure 1. Sleep is required in young flies for normal development of fast-growing brain regions. Sleep deprivation during youth causes lack of growth in the VA1v, an important region for courtship behavior. Image modified from Murakami and Keene, 2014.
What happened in the brains of young sleep-deprived flies that led to their courtship inadequacy as adults? Previous research had already located the brain regions responsible for this innate behavior, so the authors knew where to start looking. They found that one of the regions (a structure called the VA1v) had not grown as much in flies that had been sleep-deprived when they were young. The VA1v is a very fast-growing region during development, and the authors showed that loss of sleep irreversibly slowed its growth. On the other hand, structures that do not undergo fast growth during development were normal. The authors concluded that sleep deprivation in young animals impairs brain development in fast-growing areas, resulting in irreversible behavioral abnormalities.
These results demonstrate just how important it can be to get enough sleep, especially as children. Research with human children already indicate that loss of sleep can have long-term effects on behavior, and the findings from this paper suggest that the consequences may not always be reversible if developing regions of the brain are affected. But what if children have a developmental or genetic disorder—such as a pediatric sleep disorder—that causes loss of sleep during this critical time period?
The authors also figured out the difference in young fly brains that caused them to sleep more than adults. Using the impressive set of genetic tools that Drosophila are famous for, they identified a small set of dopaminergic (DA) cells that behaved differently in young versus adult flies. Dopamine is a chemical in the brain that some neurons use to communicate with each other, and is already known to be important for the “be awake!” signal in several mammalian species and humans. The authors found that this particular set of DA neurons was less active in young flies than adults, while other DA neurons had the same activity level regardless of age. The neurons communicated with a sleep-related structure known as the dorsal fan-shaped body (dFSB). The authors found that in young flies, reduced activity in these DA neurons allowed the dFSB to be more active, causing the flies to sleep more. When the authors artificially activated the DA neurons, they found that young flies were unable to sleep and had behavioral problems three days later, while adult flies did not experience any long-term effects. This result matched the one they obtained when they sleep deprived young flies by shaking them, confirming that reduced activity in these neurons was responsible for the extra sleep in young flies.
How can knowing the circuit responsible for extra sleep in young flies help us humans? Dopamine plays the same role in causing wakefulness in mammals as it does in flies, and the dFSB is similar to known mammalian sleep-related structures (such as the VLPO). Researchers can use these findings as a starting point for identifying similar DA neurons in mammals. Eventually, scientists may be able to develop a treatment that acts on these neurons in children with sleep disorders, allowing them to get more sleep and ensure normal brain development during this critical period.
Figure 2. The circuit responsible for extra sleep in young flies. In young flies, a set of dopaminergic (DA) neurons is less active, allowing a sleep-related structure known as the dFSB to be more active and promote extra sleep. In adult flies, the DA neurons are more active and suppress dFSB activity, leading to relatively less sleep.
Original reference:
Kayser M.S., Yue Z. & A. Sehgal (2014). A Critical Period of Sleep for Development of Courtship Circuitry and Behavior in Drosophila, Science, 344 (6181) 269-274. DOI: http://dx.doi.org/10.1126/science.1250553
Click on the picture for a bigger version. Multiple external and internal factors can lead to the formation of free radicals and cell damage, which accelerates aging.
Figure 1. Genes can account for about 50% of the risk for alcoholism. Image by 
Figure 2. Fruit flies can serve as a model for alcohol addiction. Drawing by 
Figure 3. A fruit fly drinking alcohol from a capillary tube during a test for whether flies prefer food laced with alcohol. Photo by G. Shohat-Ophir, UCSD
Figure 4. The “booze-o-mat” is where fruit flies are placed in tubes with alcohol vapors to ensure consumption. Photo by 
Figure 5. Fruit flies are given a choice of capillary tubes containing either regular food or alcohol-laden food. Modified from 
Figure 6. Drunk male fruit flies sometimes form a “courtship chain”, where male flies follow each other around trying to mate. Photo by Kyung-An Han laboratory, Penn State
Our stocks of flies are kept in incubators at a stable temperature and light cycle.
Fly lines are kept in vials, which are stored on racks for easy stacking.
A fly vial in use.
Fruit flies go through four life stages: egg, larva, pupa, and adult. 
Knocking out flies with CO2 in the vial prevents them from escaping
Flies are dumped onto a CO2 pad to keep them unconscious while they are being sorted.
Wild flies (bottom) have bright red eyes, but lab flies (top) usually have white eyes until a gene with an eye color feature is added. 
Lab flies with an extra gene inserted into their DNA usually have orange eyes. The shade of orange varies, but darker colors can often mean more than one new gene is present. Photo by Weiwu Xie, 
We use a fine-tipped paintbrush to carefully sort through the flies without harming them.
“Fly morgues” are used to dispose of flies. They also conveniently double as fly traps.
A dissection microscope. A dish with wells for dissecting is shown underneath.
A close-up “action shot” of a fly head being dissected. The head is being held with a pair of fine-tipped tweezers.
A graphical representation of an adult fruit fly brain. Modified from 
Table 1. Human children require more sleep than adults. Data obtained from the 
Figure 1. Sleep is required in young flies for normal development of fast-growing brain regions. Sleep deprivation during youth causes lack of growth in the VA1v, an important region for courtship behavior. Image modified from 
Figure 2. The circuit responsible for extra sleep in young flies. In young flies, a set of dopaminergic (DA) neurons is less active, allowing a sleep-related structure known as the dFSB to be more active and promote extra sleep. In adult flies, the DA neurons are more active and suppress dFSB activity, leading to relatively less sleep.