Less protein for maggots means longer lives for fruit flies
Less protein for maggots means longer lives for fruit flies
幼虫期减少蛋白质摄入可延长果蝇寿命
Back in the 1930s, scientists noticed that water fleas and rats fed restricted diets while young went on to live longer. The same effect has since turned up in fruit flies and mice. What we did not know was how a meal eaten in infancy could affect health weeks, months, or even years later. A recent Nature study might have found the answer. A team led by Fumiaki Obata, a biologist at the RIKEN Center for Biosystems Dynamics Research in Kobe, Japan, found a protein that carries a record into adulthood of what fruit flies ate as larvae, influencing how long they live.
早在20世纪30年代，科学家们就注意到，幼年时期饮食受限的水蚤和大鼠寿命更长。此后，在果蝇和小鼠身上也发现了同样的现象。但我们一直不清楚幼年时期的饮食如何影响数周、数月甚至数年后的健康。最近发表在《自然》杂志上的一项研究可能找到了答案。由日本神户理化学研究所生物系统动力学研究中心的生物学家小畑史明（Fumiaki Obata）领导的团队发现了一种蛋白质，它能将果蝇幼虫时期的饮食记录带入成年期，从而影响其寿命。
Hungry maggots
饥饿的幼虫
Lab fruit flies are fed a mix of yeast and sugar, with yeast being their main source of protein. “We decreased only the yeast concentration in the diet, from eight percent to either one or two percent,” Obata explains. This low-protein diet was introduced roughly halfway through the larval period. Once the flies emerged from their pupae as adults, they returned to standard food. In Obata’s experiment, the protein-restricted flies, males and females alike, outlived their well-fed siblings. But it came at a cost. The flies were paler and lighter, weighing sometimes 28 percent less. The females also laid fewer eggs.
实验室果蝇通常以酵母和糖的混合物为食，其中酵母是它们主要的蛋白质来源。“我们只降低了饮食中酵母的浓度，从8%降至1%或2%，”小畑解释道。这种低蛋白饮食是在幼虫期大约一半时开始引入的。一旦果蝇从蛹中羽化为成虫，它们就会恢复正常饮食。在小畑的实验中，无论雌雄，蛋白质摄入受限的果蝇寿命都比饮食充足的同类更长。但这也有代价：这些果蝇颜色更浅、体型更轻，体重有时减轻了28%。雌性果蝇产卵的数量也更少。
“It’s very common, actually. Reproduction and lifespan are always in a trade-off relationship,” Obata says. “This is also the case in this early-life dietary restriction. They have fewer eggs and they are slightly smaller. But they have a lifespan extension.” This typical trade-off wasn’t the whole story, though. When the team added amino acids, the building blocks of proteins, back into the low-yeast larval food, the lifespan boost disappeared. Somehow, the adult flies’ bodies were remembering how much protein they had eaten as larvae. Obata and his colleagues called this a nutritional memory and set out to find where exactly in the body that memory was kept.
“这其实很常见。繁殖和寿命之间总是存在权衡关系，”小畑说。“这种早期饮食限制也是如此。它们产卵更少，体型稍小，但寿命却延长了。”然而，这种典型的权衡并非故事的全部。当研究团队将蛋白质的组成部分——氨基酸——重新添加回低酵母的幼虫食物中时，寿命延长的效果消失了。不知何故，成年果蝇的身体记住了它们在幼虫时期摄入了多少蛋白质。小畑和他的同事将此称为“营养记忆”，并着手寻找这种记忆究竟储存在身体的哪个部位。
Nutritional memory
营养记忆
Comparing gene activity in adult flies raised on low-protein and standard diets turned up around 100 candidate proteins that could store the information about early protein intake. “We basically went one by one, checking which would be important,” Obata says. To narrow the search down, the team tagged the larval food to tell which proteins in an adult fly’s body were built from food it ate as a larva. Larvae were raised on a synthetic diet in which two amino acids, lysine and arginine, were made with rarer, heavier isotopes of carbon and nitrogen atoms. Once the larvae became adult flies, the team switched to food with a second set of amino acids tagged with different, lighter isotopes.
通过比较低蛋白饮食和标准饮食下长大的成年果蝇的基因活性，研究人员发现了约100种可能储存早期蛋白质摄入信息的候选蛋白质。“我们基本上是一个接一个地检查，看哪些是重要的，”小畑说。为了缩小搜索范围，研究团队对幼虫食物进行了标记，以区分成年果蝇体内哪些蛋白质是由幼虫时期的食物构建的。幼虫被喂食一种合成饮食，其中赖氨酸和精氨酸这两种氨基酸由更稀有、更重的碳和氮同位素制成。当幼虫变成成虫后，研究团队换成了含有第二组氨基酸的食物，这些氨基酸被标记了不同的、较轻的同位素。
Then the researchers used mass spectrometry to weigh fragments of proteins from the flies’ heads. The same protein fragment might register as a few units heavier or lighter, depending on whether it was built from larval or adult food. “This is basically only achievable with this stable isotope experiment,” Obata says. Three days into adulthood, nearly 64 percent of the proteins in the flies’ heads were still made from amino acids eaten by larvae. By day six, that only dropped to about 46 percent. Prominent among them were ribosomal proteins, which are part of the complex that makes all other proteins. Flies fed a protein-restricted diet as larvae had fewer of these and made new proteins more slowly during their first week of adulthood.
随后，研究人员利用质谱分析法对果蝇头部的蛋白质片段进行了称重。同一个蛋白质片段可能会显示出几个单位的重量差异，这取决于它是源自幼虫时期的食物还是成年时期的食物。“这基本上只有通过这种稳定同位素实验才能实现，”小畑说。在进入成年期的第三天，果蝇头部近64%的蛋白质仍由幼虫时期摄入的氨基酸构成。到了第六天，这一比例仅降至约46%。其中最突出的是核糖体蛋白，它是制造所有其他蛋白质的复合体的一部分。幼虫期摄入低蛋白饮食的果蝇，其核糖体蛋白较少，在成年后的第一周内合成新蛋白质的速度也较慢。
This immediately raised a question. Flies go through metamorphosis inside a pupal case, and pupae don’t eat. So the larval amino acids that ended up in adult ribosomal proteins had to be stored somewhere during metamorphosis, and whatever held them had to be sensitive to how much protein the larvae ate. The team went looking for a protein that both responded to the larval diet and survived into adulthood. And they found it.
这立即引发了一个问题：果蝇在蛹壳内经历变态发育，而蛹是不进食的。因此，最终进入成年核糖体蛋白的幼虫氨基酸一定在变态发育过程中被储存在了某个地方，而且储存它们的物质必须对幼虫摄入的蛋白质含量敏感。研究团队开始寻找一种既能响应幼虫饮食又能存活到成年期的蛋白质。最终，他们找到了它。
Storage protein
储存蛋白
It was a protein called larval serum protein 2, or Lsp2. Larvae stockpile it as a pantry for metamorphosis when they stop eating. “The flies express this Lsp2 massively in early life,” Obata says. “And we knew that there is some carryover of this Lsp2 protein into the adult stage, and that Lsp2 is responsive to dietary protein in the early stage. That led us to think that this would be the best candidate in the end.” Protein-restricted larvae made less Lsp2, and the levels stayed low well into adulthood, even after the flies went back to eating normally.
这是一种被称为“幼虫血清蛋白2”（Lsp2）的蛋白质。幼虫将其储存起来，作为停止进食后变态发育的“粮仓”。“果蝇在生命早期会大量表达这种Lsp2，”小畑说。“我们知道这种Lsp2蛋白会残留到成年阶段，而且Lsp2对早期的饮食蛋白质有反应。这让我们认为它最终会是最佳候选者。”蛋白质摄入受限的幼虫产生的Lsp2较少，即使在果蝇恢复正常饮食后，其水平在成年期依然保持在较低状态。
Researchers found that low Lsp2 in larvae apparently tells the adult body to keep producing less of it. When the team genetically silenced Lsp2 in larvae, the adult flies ended up with fewer ribosomal proteins, slower protein production, and longer lives—just like the flies raised on the low-protein diet. The team also noticed that Lsp2 is unusually rich in two amino acids, phenylalanine and tyrosine. Removing tyrosine from the larval food, or cutting phenylalanine to a quarter of its usual level, was enough to lower Lsp2 and extend lifespan; restricting isoleucine, an amino acid that doesn’t show up frequently in Lsp2, did nothing. “The low-tyrosine, low-phenylalanine flies cannot make enough [Lsp2],” Obata explains. “It’s the same thing that happens under the low-protein diet condition.”
研究人员发现，幼虫体内较低的Lsp2水平似乎在向成年后的身体发出信号，要求其持续保持较低的产量。当研究团队在幼虫体内通过基因手段沉默Lsp2时，成年果蝇最终表现出核糖体蛋白减少、蛋白质合成速度变慢以及寿命延长的特征——就像那些在低蛋白饮食下长大的果蝇一样。研究团队还注意到，Lsp2中富含苯丙氨酸和酪氨酸这两种氨基酸。从幼虫食物中去除酪氨酸，或将苯丙氨酸降至正常水平的四分之一，就足以降低Lsp2并延长寿命；而限制异亮氨酸（一种在Lsp2中不常出现的氨基酸）则没有效果。“低酪氨酸、低苯丙氨酸的果蝇无法制造足够的[Lsp2]，”小畑解释道。“这与低蛋白饮食条件下发生的情况是一样的。”
The full chain of events
完整的事件链
Obata’s team reconstructed starts with less larval protein, which leads to less Lsp2, which means fewer ribosomes and slower protein production in young adults—which results in a longer life. But not all links in this chain are equally solid.
小畑团队重建的事件链始于：幼虫蛋白质摄入减少，导致Lsp2减少，这意味着年轻成虫体内的核糖体减少、蛋白质合成速度变慢，最终导致寿命延长。但这条链条上的所有环节并非都同样稳固。
Missing links
缺失的环节
What’s most firmly established today is the final step. “If you have less translation and less protein production, that leads to the lifespan extension,” Obata says. This lifespan extension, he argues, relies on improved proteostasis, the cell’s ability to keep its proteins in good working order. The link between Lsp2 and ribosomes is less understood. “What we have to find out is why this Lsp2 is preferably going to the ribosome,”
目前最确凿的是最后一步。“如果你减少了翻译和蛋白质合成，就会导致寿命延长，”小畑说。他认为，这种寿命延长依赖于蛋白质稳态的改善，即细胞保持蛋白质良好运作的能力。而Lsp2与核糖体之间的联系尚不完全清楚。“我们必须弄清楚为什么这种Lsp2会优先进入核糖体。”