Recently I’ve noticed our posts have strayed from talking about diseases to being about the causative agents themselves. We’ve looked at bacteria and viruses and then shown the diseases they cause. Instead of doing the other way around as was originally intended. Based on this we should re-name the blog ‘Pathogen/Microbe/Thing we want talk about of the Week!’, but it doesn’t have a very good ring to it…
To put things right, and because ‘Mad Cow Disease’ is very cool in its own right, we will return to a disease for our topic this week, rather than another bacteria or virus.
And the Boring, Sensible Name for Mad Cow Is <Drum roll>
Mad Cow, or as it is known as in the science world Bovine Spongiform Encephalopathy (BSE) is a disease that has disastrous effects on the brain of infected animals. It is not just restricted to cows either; the disease is called Scrapie in sheep and Creutzfeldt–Jakob Disease (CJD) in humans. This is the bit where I tell you what causes it but instead we have to make a quick detour through molecular biology 101. Looks like I got some splainin’ to do.
People who haven’t heard of DNA (deoxyribonucleic acid – for those playing at home) probably intersect with people who haven’t heard of the internet in a Venn Diagram so it’s probably safe to assume if your reading this you’ve at least heard of it.
But just to recap quickly. DNA is the instructions for life. The cell uses specialised tools, called enzymes, to ‘interpret DNA’ into another molecule called RNA (ribonucleic acid – hello ladies). Another set of enzymes ‘interprets’ the RNA molecule into a string of amino acids. The amino acids then self-assemble by folding themselves up into what we call a protein. If DNA is the instructions for life then proteins are the tools which get the job done.
The ability of proteins to self-assemble into shapes that have a function is an extremely important ‘skill’. These shapes are determined by the properties of the amino acids and their order in the string. Amino acids vary in size and charge and so the string will bend and twist in order to resolve the effects of size and charge amongst the amino acids. Another important point is that normally there is only one or a small number of ‘stable’ ways the protein can have its amino acids shapes and charges resolved. These results in the same piece of DNA being able to make the same protein with the same shape, structure and function over and over again reproducibly.
For those of you who know this stuff inside out please don’t tear it to shreds. It serves as a simple enough explanation for the purpose of this post. If this site hangs around for long enough maybe I can expand on this explanation in the future. Maybe.
Despite having only a small number of possible shapes or confirmations available after being made some proteins are able to change their shape based on a stimulus. Examples of these proteins are ‘receptors’. Some proteins have are able to detect specific signals in their environment and upon interaction with the signal the receptor will alter its shape. This can allow it to send a signal of its own or perform a new function. Often the new shape is less stable and so the protein will shift its shape back again after a period of time.
Enzymes, which I mentioned above, are a very important type of protein. Enzymes are catalysts which means they speed something up. The definition is that an enzyme for a specific chemical reaction is able to enhance the reaction without being consumed in the process.
I mentioned ‘receptors’ and enzymes specifically to illustrate two important functions of proteins important for the story here. If you want the TL;DR version – proteins are the shit. If it happens then a protein was involved at some point.
Ummm, what about Mad Cow?
Okay, back to the crazy cattle. So I mentioned at the start that this disease wasn’t caused by a bacteria or virus. It is, in fact, caused by a prion. Prions are miss-folded proteins that *can* sometimes take on a new role or stop performing the role they would normally perform if folded properly. If the protein that becomes miss-folded was integral to the function of the cell, or can interrupt normal cellular activity, then the cell may die.
In the case of BSE the protein responsible for disease, when folded properly, is probably involved in transporting products into or out of the cell (it seems to vary depending on the type of BSE but the point is that it does have a normal function) but when it folds incorrectly it results in the eventual death of the organism. What’s important to remember here is that we are talking about the same string of amino acids. When folded ‘as intended’ we call it a protein, when it miss-folds (in a specific way) we call it a prion.
Prion diseases are very unusual because the body is very good a destroying proteins that don’t behave. In the case of prions, the most stable shape it can form is the one responsible for disease so the body has a lot of trouble getting rid of the mistake. In the case of BSE it’s a little worse than usual. Normally miss-folded proteins are also dysfunctional but this prion acquires a new ‘ability’ when formed, the ability to catalyse the formation of more prions. Essentially the prion can act as an enzyme that catalyses the transition from the correctly folded form of the protein to the aberrantly folded version. To put it another way, once the BSE prion is formed it converts all the correctly folded forms into more prions.
So what? So one protein now isn’t doing its job, we have tonnes of proteins. One broken protein can’t really have that big of an impact can it? Duh of course it can, that’s why there is a disease.
More like Bovine Sponge-form Encephalopathy. Who’s with me?
The correctly folded BSE protein is water-soluble and probably loosely associated with the cell membrane. The prion is not soluble and instead of floating happily within the cell, it forms protein aggregates (clumps of protein) which interrupt cellular activity. The protein is normally made all over the body but in particularly large amounts within the central nervous system where the disease is most obvious (hence encephalopathy) and the clumps look a lot like holes under a microscope (hence spongiform).
Once you’re the brain starts to accumulate these holes it ceases to function normally. When your brain’s stops acting normal then so do you and common symptoms include (remember we are talking cattle here) changes in temperament, such as nervousness or aggression; abnormal posture; lack of coordination and difficulty in rising; decreased milk production; or loss of body weight despite continued appetite.
‘How about a human angle?’ I hear you say
Good on you for recognising that I hadn’t got to that yet. You might have heard a little while ago, particularly during the 1980’s and 1990’s, the UK had a bit of a problem with their bovine. Although there a number of theories as to exactly what happened and how BSE became a huge problem they all seem to follow a common thread.
Cattle are herbivores, they like-a the grass-a, but in industry that results in slow growing cows. In order to make cattle grow larger faster their diet is supplemented with a number of things including anti-biotics, hormones (not as common anymore) and protein. The standard protein additive is soya bean meal as soya is easy to grow and contains a relatively large amount of protein compared to alternatives. Soya, however, does not grow well in the UK. This led to the practise of substituting other protein sources in for the expensive (for the UK farmer to try and grow or import) soya, primarily meat and bone meal was used. These products were made from the left-over bits of cow and sheep following the slaughter process. As the cows were eating other cows it was only a matter or time till the BSE prion formed spontaneously, spread throughout that animal, that animal was slaughtered and its left-overs distributed in cattle feed to new cows. As the prion is very stable, even at higher temperatures, the sterilisation procedures had no effect and the prion spread throughout the cattle population.
Humans that ate the disease cattle could pick up the prion and develop symptoms and eventually die aswell. Since 1986 when the UK government acknowledged the problem 165 people have contracted BSE (which is called TSE when it is transmitted – Transmissible Spongiform Encephalopathy) and died as there is no cure.
Lets end on a happier note
Australia has never had a single reported case of BSE as Australian cattle are often ‘pasture-fed’ and so don’t often receive high protein diets. The availability of soya in Australia is also a factor so we needn’t ever use meat and bone meal for feed. Also worldwide regulation and surveillance has developed to prevent a major epidemic taking hold again. In fact regulation sparked controversy recently when Japan stopped buying beef from the US after a breach in regulation was detected prompting a worldwide decline in US beef sales. Following a detected but isolated case of BSE in 2003 and a breach in regulations in 2006 (a few cow carcases were shipped with brain and spine intact, against the rules) many countries stopped buying US beef dropping the export 40% over three years from 2003 to 2007. The US beef industry is only beginning to recover.
As a result of this a US biotech company, Hematech, has genetically engineered cattle that lack the gene that makes the protein responsible for BSE. Theoretically this means we have cattle that are completely immune to BSE!
This article is making me hungry. I’m off to get me a crazy free steak, as soon as my vegan fiancée isn’t looking…