Wisconsin Researchers Identify Sleep Gene
04/27/05 -- Zeroing in on the core cellular mechanisms of sleep, researchers at University of Wisconsin Medical School have identified for the first time a single gene mutation that has a powerful effect on the amount of time fruit flies sleep.
In its normal state, the Drosophila (fruit fly) gene, called Shaker, produces an ion channel that controls the flow of potassium into cells, a process that critically affects, among other things, electrical activity in neurons. A handful of recent studies suggest that potassium channels are also involved in the generation of sleep in humans.
Reported in the April 28 issue of Nature, the finding points to novel approaches to treating sleep irregularities in humans-from promoting restorative sleep to prolonging wakefulness.
"This research offers the possibility of developing a new class of compounds that could affect potassium channels in the brain rather than other brain chemical systems targeted currently," says lead author Dr. Chiara Cirelli, assistant professor of psychiatry at UW Medical School.
UW-Madison genetics professor Barry Ganetsky, likely the world expert on the Shaker gene, was a collaborator on the study. Dr. Giulio Tononi, UW Medical School professor of psychiatry, was the senior author on the paper.
Most people sleep seven to eight hours a night, and if they are deprived of sleep, their cognitive performance suffers greatly. However, a few people do well with just three or four hours of sleep-a trait that seems to run in families.
"We wanted to determine which genes underlie this phenomenon in order to shed light on the mechanisms and functions of sleep," Tononi says.
The Wisconsin study focuses on factors that control sleep duration as opposed to the timing of when sleep occurs, which is regulated by the circadian system, Tononi notes. "The key molecular mechanisms controlling the circadian timing of sleep are well understood, but details about the homeostatic mechanism that regulates the amount of sleep have been unclear," he says.
In a four-year, round-the-clock search, researchers screened 9,000 mutated fruit flies, many of them supplied by Ganetsky's lab, and found one line of them that slept one-third the amount of normal flies. Put through a series of tests, the short-sleeping flies, named minisleep (mns), were found to perform normally and did not appear to be impaired by sleep deprivation. The mns flies, however, did have shorter life spans.
Following the testing, the researchers noticed shaking in the flies' legs as the insects recovered from anesthesia. The observation led the team to focus on the Shaker gene, which produces this effect. Nevertheless, Shaker's main job in flies-and in its equivalent in humans--is to control the excitability of cell membranes.
Genetic analysis of the mns flies, conducted by Daniel Bushey, a post-doctoral fellow working with the Tononi team, revealed that their Shaker genes contained a single amino-acid mutation. Because of the mutation, a functional ion channel could not be formed on the cell membrane and potassium therefore could not flow through it.
When the researchers first tested flies with the Shaker gene, they found that some of them with other mutations were normal sleepers. But these flies became short sleepers when the researchers removed genetic modifiers from their genome.
"This told us that genetic forces push hard against this phenotype to make it ineffective," Cirelli says. "Being a short sleeper is probably not a good thing. We know that the mns mutation affects mortality, but we're not sure how."
In earlier studies, Tononi's team discovered that fruit flies do, in fact, sleep.
"The more behaviors we look at, in terms of sleep, the more we find that sleep in fruit flies is very, very similar to sleep in mammals," Cirelli says.
Like humans, fruit flies generally are quiet and immobile for between six and 12 hours each night and lose most of their ability to respond to stimuli, the researchers found. When deprived of sleep, humans and their winged counterparts rebound on the following night by sleeping longer and more deeply. Flies also sleep more in their youth than later in life, when their sleep is fragmented, as with humans.
In other studies, the scientists also observed that caffeine has the same stimulating effects on human and fly sleep, and that similar genes are expressed in both species when they are awake and asleep. Tononi's team also conducted EEGs on the flies and found evidence of the same electrophysiological changes occurring during sleep and wakefulness as in humans.
"The electrical changes in humans look different that they do in flies because our brains are organized differently," Cirelli says. "But the EEGs showed electrophysiological changes signifying that the flies were asleep and awake."
In mammals the changes produce hallmark waves, or oscillations of groups of neurons, easily detected by EEG. The waves are slower during deep sleep and faster during waking times. One way of getting from the faster to the slower state is by opening ion channels, allowing potassium to flow through them.
"Our hypothesis is that if you don't have potassium channels, you won't get slow waves," Cirelli says. "The cell membrane will remain activated, preventing long periods of deep, non-REM sleep."
The researchers say that the fly research translates to humans even more than they thought it would. "Humans have the same kind of genes and potassium channels. And we know that slow waves must be generated by changes in the excitability of neuron cell membranes," Cirelli says.
"Potassium changes may have a huge affect on sleep in humans."
Sleep is a highly complex activity and probably involves many genes, some of which are more influential than others, says Cirelli. "We believe this gene is very powerful because it acts on the final common pathway and has the ability to change the excitability of neurons."
Source: University of Wisconsin-Madison
美国科学家在28日出版的《自然》杂志上报告说，他们发现如果果蝇的某个基因发生变异，就会导致其睡眠减少。由于果蝇基因构成和睡眠特征与人相似，该发现将有助于研究和解决人类睡眠问题。
美国威斯康星大学科学家在报告中说，他们对果蝇的1．4万多个基因进行了研究。结果发现，如果果蝇的“沙克尔”基因发生变异，其睡眠时间至少比正常果蝇少三分之一。通常果蝇每天睡眠10到12小时，而基因变异的果蝇每天只需要3到4小时的休息。实验证明，睡眠减少并不会立即对基因变异果蝇造成任何影响，在24小时无休息的情况下，它们在遇到袭击时的躲避反应与平常相当，而正常果蝇的反应明显放慢。不过，变异果蝇的寿命比普通果蝇短三分之一。
科学家进一步发现，“沙克尔”基因控制果蝇体内产生一种蛋白质，这种蛋白质能够促使钾离子进入神经细胞。“沙克尔”基因发生变异，果蝇体内的钾离子就无法进入神经细胞，这可能是导致睡眠减少的原因。
果蝇的基因构成与人类相似，一直被当作研究人体的生物模型。它的睡眠特征，比如睡眠不足导致反应迟钝等都与人类相似。而人体内也存在“沙克尔”基因，它的作用也与果蝇的类似。因此，研究人员认为，该研究结果有助于更好地理解和解决人类睡眠问题。
生物网4月27日报道，美国威斯康辛医学院的科研工作者致力于研究动物细胞的睡眠机理。他们发现基因突变可以影响果蝇睡眠时间的长短。
正常状态下，果蝇的特有基因将控制细胞中碳酸氢钾的活动，这种活动主要发生在果蝇的神经元细胞中。科学家们发现人类的睡眠过程也存在这种活动机理。此发现可能为失眠患者找到能延长睡眠的方法。
华盛顿大学医学院的精神病学助理教授Chiara Cirelli说，该研究结果表明，科学家可以制造出一种化合物来影响碳酸氢钾在人脑神经元中的活动，由此帮助失眠患者延长睡眠时间。
华盛顿大学遗传学教授Barry Ganetsky也参加了研究。他说，世上大多数人每天睡眠时间为七到八个小时，如果他们正常的睡眠时间被剥夺，其认知能力就会受到影响。然而，某些人每天仅睡眠三到四个小时也能达到正常人的认知水平。科学家们目前研究的重点是弄清楚基因控制人类睡眠的机制并且改造这些基因使之符合人们的需要。
威斯康辛医学院的研究方向集中在找出调节人体生理节奏的方法。目前科学家对控制人体生理节奏的化学机制已经清楚，但是对于人体调节其生理节奏的适应性机能还不十分了解。
在长达四年的时间里，科学家总共对9000只果蝇进行了试验。他们发现其中一些果蝇的睡眠时间仅为其它果蝇的三分之一。通过一系列的试验表明，这些果蝇并没有由于睡眠时间的减少而在认知能力上比其它果蝇差。但是，此类果蝇的生命周期却比一般果蝇要短。
科学家注意到当这些睡眠时间短的果蝇睡醒时腿会颤抖，这个现象引导科学家研究使其腿发生颤抖的基因。他们还发现这种颤抖的主要作用是唤醒处于睡眠状态的细胞。人类也有此特点。
经过对此类果蝇基因的仔细分析，科学家们发现其中有一个胺酸基因发生了突变。由于此突变，使得此类果蝇细胞膜间的化学物质传输过程被打乱，碳酸氢钾不能像往常那样自由的在细胞间传输。
当科学家对基因发生突变的果蝇进行研究时发现，只有当果蝇特定的基因发生突变时才会出现睡眠时间缩短的现象。这一结果或许能够说明，作一个正常睡眠时间短的人也许并不是件好事。因为他们的基因很可能和正常人不同，但是科学家目前还不能确定此研究结果。
早期的研究中科学家已经证实果蝇确实和哺乳动物在睡眠过程中有相似的特点。像人类一样，果蝇在睡眠时很安静，通常保持每天睡眠6至12小时。睡眠过程中果蝇丧失了大部分对外界刺激作出反应的能力。当它们正常的睡眠被打乱时，他们下一次睡眠所用的时间也会相应延长。
科学家还注意到咖啡因对人类和果蝇同样具有减少睡眠时间的作用。果蝇在睡眠和苏醒时细胞内物质发生的化学变化也和人类十分相似。
睡眠是一个非常复杂的生理活动，它包含许多基因的相互作用，只是其中一些基因对睡眠的控制力比其它基因大。此次科学家发现的基因属于控制睡眠的主要基因，因为它能够通过改变神经元的兴奋性影响生物体的睡眠。
