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【bio-news】疯牛病、阮蛋白病…………传染性蛋白
In recent years our feeding habits have been the focus of ongoing polemics. Everybody will remember the "mad cow" crisis when the sales of veal plummeted for fear of contagion, thousands of animals were sacrificed and beef imports, especially from the United Kingdom - the focus of the epidemic - were curtailed.
The origin of the crisis was the feeding of cattle with animal feed that was contaminated by a new pathogenic agent - a prion. These cows, after a long incubation period, died of dementia. The prion entered the human food chain without evident symptoms being observed - the pathogen was able to jump the species gap to humans. This created unprecedented alarm amongst the public at large and gave rise to great interest in these diseases - Transmissible Spongiform Encephalopathies (TSEs) and in the great protagonist of the story, the prion.
Prions are proteins - without DNA - capable of causing rare neurodegenerative diseases, currently without a cure, by means of a novel process different from that of the "classic" virus and bacteria. The prion presents two distinct forms: a "healthy" one present in all animals (PrPc) and another, pathogenic one (PrPsc) which is the causative agent of the ailment. When PrPsc enters a living being, through feeding, it looks for the PrPc as a host and transforms it into a new PrPsc. So, the PrPc disappears and the PrPsc accumulates.
Although the PrPc exists in practically all the tissues of the body, the TSEs only appear in the brain. This is why neurones die and spaces or cavities appear which give rise to the typically "spongy" appearance. What route does the prion follow from the mouth to the brain? This is the great mystery. The first obstacle it encounters is the acidity of the gastric juices, then the action of the enzymes that break up the foods we swallow and then it has to pass through the digestive tract wall. The cells containing the PrPc could be the entry gate but ? what cells are they? Are the same in different animal species? Nobody knows.
Answering these questions has been the aim of this thesis and, in order to achieve this objective, we carried out a "sweep" along the digestive tract of rats, primates and of Pyrenean cows - the most affected breed in Navarre. We employed a number of techniques enabling the location of PrPc- using antibodies specifically targeting it. These antibodies, amongst other items, were marked with fluorescent molecules so the positive cells could be subsequently visualised with a microscope.
The results showed that PrPc appears in the endocrine cells that are dispersed throughout the digestive tract. Endocrine cells produce hormones, biochemical messengers that are secreted into the blood and control the correct functioning of the body. This suggests that the conversion of PrPc to PrPsc may occur in these cells and arrive at the brain in the blood. But, in the digestive tract there are dozens of different endocrine cells and not all have PrPc. To identify which ones have and which do not, we detected, simultaneously, the PrPc and another substance, characteristic of each cell type and observed if the marking appeared in the same cells or not. By means of this system we were able to confirm that, curiously, the PrPc appeared in the same cells in all three animal species.
Finally, we also detected PrPc in the nervous system of the wall of the digestive tract. In this case, the disorder may be propagated to the brain through the signals between neurones - the synapses - that link both organs.
http://www.elhuyar.com 使人发疯的蛋白质
最近几年人类的饮食习惯正成为争论的焦点。我们都还记得“疯牛”危机，当时，由于害怕被传染，牛肉的销量直线下降，数以千计的牛被处死，牛肉进口，特别是从疯牛病流行中心—英国的牛肉进口，大幅度减少。
这场危机起源于使用动物饲料喂养牛，这些动物饲料都被一种新的致病体—朊蛋白污染了。这些牛在经历了一段长时间的潜伏期后，死于痴呆。在不引起可见的明显的症状的情况下，朊蛋白进入了人类的食物链，它可以跨过物种差异感染人类。这在公众中引起了空前的恐慌，也使我们对危机的始作恿者—朊蛋白，及其引起的疾病—传染性海绵状脑病（TSEs）产生了极大的兴趣。
朊蛋白是一种蛋白质，不含DNA，通过不同于“经典的”细菌和病毒的奇特致病方式，可引起罕见的神经变性疾病，目前尚无法治愈。朊蛋白以两种不同的形式存在，一种是“有益健康的”，存在于所有的动物，称PrPc，另一种是致病性的，是这类疾病的病原体，称PrPsc。一旦PrPsc通过食物进入生物体，它就会寻找PrPc作为宿主，并将其转变为新的PrPsc。因而PrPc消失而PrPSc蓄积。
尽管PrPc存在于体内所有的组织中，但传染性海绵状脑病仅出现在脑部。因而神经元死亡，形成空洞或空腔样，从而呈现出典型的“海绵状”外观。朊蛋白通过什么途径从嘴巴进入大脑？这是最大的迷团。它遇到的第一道屏障是胃液中的胃酸，然后在酶的作用下吞下的食物分解，接着它还要通过消化道壁。含有PrPc的细胞可以成为其进入的门户，但到底是什么细胞呢？它们在不同的物种中是否都一样呢？这些答案没人知道。
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作者:admin@医学,生命科学 2011-04-07 17:17
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