Zinc – can’t live without it, can’t get rid of it. That about sums up the situation with trying to figure out the metal’s many important roles in biology. A long, long list of proteins have zinc-binding functions (with the metalloproteases and the DNA-binding zinc-finger domains being two important ones that immediately come to mind). It’s been recognized as an essential human nutrient for over fifty years, and human zinc deficiency is a real problem in some parts of the world. That’s often caused by a diet high in phytic acid, which complexes zinc into a much less bioavailable form, although there are some rare inherited zinc deficiencies due to defects in zinc transporters themselves.
But while whole-organism depletion of zinc is known, taking this down to the cellular level is not so easy. That’s because it’s been hard to deplete culture media of zinc down to the levels where you really put the squeeze on the cells. They’re very efficient at importing any zinc available, and some of the methods you might use to get rid of it will alter the growth medium in other ways that will confound your results (such as depleting other metal ions which you then have to add back in).
This new paper, though, suggests a way to get the job done. The authors (a team at MIT’s chemistry department) had an ingenious idea. It’s known that there are proteins whose job is to sequester zinc in response to infection (to keep invading pathogens from using it) – so why not use these to clear it out of the culture medium? The S100A12 protein does the trick – immobilizing it onto resin gives you a way to filter your media and remove zinc from them almost completely. This worked on a number of different media, and even on human serum, etc. Surprisingly, you can actually regenerate the resin – treatment of the immobilized protein with 1M acetic acid liberates the zinc while leaving the protein itself undestroyed and ready for another round.
So what happens to cells grown in such de-zinc-ified media? After 36 hours, HEK293 cells are still alive (and indeed show no particular metabolic phenotype). But a detailed look shows that despite this placid exterior, the cells are furiously paddling under the surface. About 75% of all their gene transcripts were significantly altered by the depletion treatment. The pool of “labile zinc” in the cells, interestingly, wasn’t changed very much at all, which makes one think that they must work pretty hard to keep that in place.
Now that there’s an easy way to deplete zinc for such studies, we can look forward to more advances in “metallomics”. Stressing cells in this manner should reveal some new biology, as the massive response of the HEK293 cells in this work indicates. What happens when you do the same thing to cells from other tissue lineages? To cancer cell lines? To bacteria? We can now find out.