Friday, March 27, 2009

Cocoons that glow in the dark

I have found the perfect use for the salad tray of our fridge. I've removed the human food that was in there and replaced it with this...

aromatic twigs of bay, mashed up mulberry leaves and... silkworms (of course!).

I think they look very contented in there. In fact, if they were capable of smiling I am sure they would be. I love the way they pick out the branches and crawl along to the top then pause there, their heads circling in the air in their quest to move ever higher. I suppose they are searching for the perfect spot, with their silk already beginning to spill from them.

The reservoirs for the silk are two glands located either side of the intestine, and by this 'ripe' stage of the fifth instar these two glands are a quarter of the silkworm's mass. The silk is extruded by spinnerettes below the silkworm's mouth and their output is astonishing. In this Nature paper from 2003 Professor Florian Wurm (such a wonderfully appropriate name) from the University of Technology in Lausanne, describes how the one thousand cells in these two glands manage to produce 300mg of protein in four days. That is 80 micrograms per cell per day. He puts this in context: the productivity of mammalian cells, for example, comes nowhere close - the best that they can muster is only 50 picograms of protein a day. That is a million times more from the silkworm (according to my calculations).

The silkworm, then, is a veritable little protein-making factory, and Professor Tomita and co-workers from the Japan Science and Technology Corporation in Hiroshima (report in another Nature paper) have joined in the quest to encourage the silkworm to produce something even more valuable than silk - that is various human proteins. Human albumin, for example, is in short supply. Even in a small country like Switzerland 1.2 tons are needed annually. This is usually obtained from processing 50 000 litres of blood plasma. And then there are other, even more valuable proteins like antibodies, which also require large cultures of human cells for their production. The silkworm is a potential provider of these too.

Apart from the greater productivity of the silkworm cells, there are other benefits of using insects as protein factories: the protein is produced in a pure form (and so does not require expensive and environmentally-draining purification), and there is virtually no risk of contamination by dangerous (to humans) viruses or prions.

Professor Wurm reports that the genetic make-up of silkworms is altered by injecting a 'piggybac vector' into the egg of a silkworm. The piggybac vector is a way of changing the DNA of the embryo (without using a virus) so that the resulting larvae will produce silk that contains human protein (a form of collagen). To keep track of the larvae which carried the altered DNA a couple of markers were included: red-fluorescent eyes when the larvae were young, and green fluorescent silk (as well as the human protein). Glow-in-the-dark eyes and silk - my 'traditional' silkworms are beginning to seem a little boring in comparison.


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