dxy

    舌头如何感觉酸味？ 

    －受体被酸性物质所激发，从而发现酸味。 

                                                                          ——Lucy Heady 

    研究人员已经发现了哺乳动物的舌头怎样尝到酸味：它们通过一个独立的专门的感受器。 

    哺乳动物的味觉可以分为甜味、咸味、苦味、酸味和鲜味（谷氨酸钠的味道，中国菜中经常使用）。直到现在，只发现了甜味、苦味和鲜味的味觉感受器，研究人员还不确定其他两种味觉是否有特殊的感受器。 

    三种已知的味觉感受器能够被大分子（如蔗糖）所触发，舌头上特殊的细胞能够抓住并识别它们。但是我们能够尝到的咸味和酸味仅仅是很小的离子：酸味是氢离子（H^＋）而咸味主要是钠离子（Na^＋）。有些研究人员推测舌头上的许多细胞通过大脑复杂的神经信号的信息来识别这些信息。“这个模型非常杂乱。”圣地亚哥the Howard Hughes Medical Institute 的Charles Zuker说道。 

    因此，Zuker的研究小组沿着已知的三种味觉感受器的方法寻找酸味感受器。 

    Angela Huang，Zuker研究小组的一位研究生，首先通过研究老鼠的基因蛋白组筛选出存在于细胞膜的所有蛋白质：蛋白质可以识别外界的信号，并能把它传给神经细胞。这种方法导致产生了10000个候选蛋白质。 

    他们通过假设味觉感受器应该只在很少种类的组织（特别是舌味觉细胞）中被发现，把名单缩减到900个。然后他们研究已知的其他味觉感受器的基因序列，发现了第一个候选者——名叫PKD2L1的单一蛋白质。 

    为了验证PKD2L1的功能，研究小组发明了一种基因改造的小鼠，它可以在表达有PKD2L1的细胞内产生一种毒素来杀灭这些细胞。插入小鼠的探针能够显示当它们吃酸的食物时没有神经信号被刺激，他们已经把这个研究结果发表在《自然》杂志上。而且它们的行为变得与理论相符合：它们不停的吃酸的食物，而正常小鼠是不会吃酸的食物的（只有人类吃酸的食物，其他动物都不吃）。 

    试验是成功的 

    Zuker的研究小组偶然发现关于酸味感受器的一个令人吃惊的事实：它似乎可以在脊髓神经元中表达。“这首次表明味觉感受器能够表达身体的其他部位的刺激。也许这个味觉感受器能够在神经系统中帮助身体监控酸度。”他说道。 

    另一个研究小组也同样对味觉感受器进行关注，同样的找到了PKD2L1作为候选者。来自北卡罗来纳州达辣谟Duke University Medical的Hiroaki Matsunami和他的同事们发现PKD2L1和 PKD1L3能够被试验盘中老鼠细胞中的酸性物质所活化。他们还发现在接触食物时这些蛋白质在舌头上的分布很有规律，但是还不能确定哪一个是酸味的感受器。Zuker的研究解决了这个问题。 

    这两个研究共同指明了我们了解酸味的方法，来自宾夕法尼亚州费城the Monell Center的Gary Beauchamp说道。 

    品尝试验 

    但是这不意味着所有问题都得到解决。令人奇怪的是，品味是个瞬时发生的感觉，而Matsunami的研究表明在传导酸和细胞感受酸信号之间有一个延迟。这暗示着在哺乳动物嘴里也许有其他感觉味道的东西。 

    Beauchamp也认为酸味感受器怎样或者为什么工作也未得到解释。存在其他味觉感受器有着明确的进化动机：苦味可以检测出毒物，而甜味可以检测出糖份，一种必须的能量来源。“现在仍旧不确定为什么我们需要酸味感受器，”Beauchamp说道，“诸如酸味感受器可以检测未成熟的果实的说法是完全牵强的。” 

    对于Zucker的研究小组来说，下一步的研究将会是咸味感受器。“我们要对剩下的蛋白进行研究，直到所有的味觉感受器研究完成。”Zuker说道。 

    文章的英文原文收录在 http://www.nature.com/news/2006/060821/full/060821-9.html。

    参考文献： 

    Huang A. L., et al. Nature, 442. 934 - 938 (2006). | Article | 

    Ishimaru Y., et al. PNAS, 103. 12569 - 12574 (2006).

    How the tongue tastes sour

    -Receptor found that is triggered by acidic foods. 

    Lucy Heady

    Researchers have worked out how a mammal's tongue detects sour tastes: it's all down to a single, specialized receptor, they say. 

    Taste in mammals is classified into sweet, salty, bitter, sour and umami (the taste of monosodium glutamate, commonly found in Chinese takeaways). Until now, only the sweet, bitter and umami taste receptors had been identified, and researchers were unsure whether the other two tastes had specialized receptors for them at all. 

    The three tastes with known receptors are triggered by large molecules, such as sucrose, that latch on to and are recognized by specialized cells on the tongue. But salty and sour are different in that they are the tastes of very simple ions: hydrogen ions (H+) for acidity and, mainly, sodium ions (Na+) for salt. Some researchers have speculated that many cells in the tongue might be able to pick up these signals, relaying the information in a complex pattern of nerve signals to the brain. "This kind of model is very messy," says Charles Zuker of the Howard Hughes Medical Institute in San Diego.

    So Zuker's team — the same lab that pinned down the previous three taste receptors — set out to hunt for a sour taste receptor. 

    Angela Huang, a graduate student in Zuker's lab, first trawled through the mouse genome to pick out any proteins that exist in cell membranes: proteins that can pick up signals from the outside world and transmit them to nerves. That left about 10,000 candidates. 

    They screened these by assuming that a taste receptor would only be found in a small number of tissue types (specifically tongue taste cells). That whittled the list down to 900. They then looked for gene patterns known to exist in other taste receptors, leaving a single protein called PKD2L1 as a prime candidate.

    To check on the action of PKD2L1, the team created genetically engineered mice that produced a toxin in cells expressing PKD2L1, killing these cells. Probes placed inside the mouse brains then showed that no neural activity was prompted by sour-tasting foods in these mice, they report in Nature1. And their behaviour changed to match: they kept licking sour foods, whereas normal mice would run away from acidic snacks (only humans have a taste for sour foods; other animals avoid them).

    Sweet success

    Zuker's team also hit upon a surprising fact about the sour receptor: it seems to show up in neurons of the spinal cord. "This is the first time that a taste receptor has been shown to respond to stimuli in another part of the body," says Zuker. This 'taste' sensor might help the body to monitor acidity in the nervous system, he says.

    Another group of scientists who were similarly on the trail of a sour-taste receptor also hit upon PKD2L1 as a candidate. Hiroaki Matsunami from Duke University Medical Center in Durham, North Carolina, and colleagues showed PKD2L1 and PKD1L3 being activated by acid in mouse cells in the lab dish2. They also found that these proteins were well positioned on the tongue for contact with food, but were unable to confirm that there was just one dedicated receptor for sour taste. Zuker's work fills that gap.

    The two studies together certainly seem to point the way to understanding sour taste, says Gary Beauchamp of the Monell Center in Philadelphia, Pennsylvania. 

    Taste test

    But it seems that not everything is understood. Strangely, while taste is an instantaneous perception, Matsunami's work showed a delay between the introduction of acid and the cells firing off a 'sour' signal, says Zuker. This indicates that something else might be going on inside the mouth to help mammals identify the taste.

    Beauchamp adds that it is also unclear how or why a sour receptor would come to be. There is a clear evolutionary motivation for the existence of some other taste receptors: bitterness detects poison and sweetness detects sugar, an essential source of energy. "It is still not entirely convincing why we need a sour taste receptor," says Beauchamp. "None of the suggestions for sour taste, such as being able to detect unripe fruits, are entirely compelling."

    For Zucker's team, what comes next is a search for the salt receptor. "It's just a case of going through those proteins that are left behind when all the other taste receptors are gone," says Zuker.


    References

    Huang A. L., et al. Nature, 442. 934 - 938 (2006). | Article |

    Ishimaru Y., et al. PNAS, 103. 12569 - 12574 (2006).


编辑：7100