Cells Like Turbulence. Well, Some Cells.

Let’s talk cell culture – specifically, how weird it is. There are an awful lot of ways to grow cells, naturally – different media of course, different scales, differences in how crowded you let them get, how often you split them, add nutrients, wash them, all those things. All of those make sense to me, but there’s another big variable that has always struck me as a bit freaky: the shape and size of the vessel you grow them in.

Experienced protein-production types will find nothing odd about that at all. They’re used to switching between roller bottles, shaker bags, T-flasks, all sorts of sizes, shapes, and agitation methods to see if these have an effect on protein production. They don’t always, but sometimes it can be dramatic. I recall a project where the yield of isolated protein went up from (qualitatively speaking) “useless” to “pretty darn good” just by switching the cells into shaker bags. Here’s a brief guide from Corning that goes into some of the options, which does not neglect their own product line, naturally. A big difference comes into play right at the start: do your cells want to adhere to some surface, or would they rather float around freely in solution? (Here’s ThermoFisher on that subject; they are also happy to sell you labware).

Agitation, in all its various forms, is particularly important for the suspension cultures, especially as the volume goes up. You run into exactly the same volume/surface area considerations and mixing problems as you do in scale-up chemistry. Heating and cooling, if they’re being done from the walls of the container, become increasingly distant from the bulk of the solution. And in the case of cell culture, small volumes can take in all the oxygen they need from the surface, but that gets harder and harder as well as the darn volume goes on increasing as the cubic function.

All that makes sense. But even once you’ve narrowed down to (say) suspension cell culture in a defined medium, mechanical agitation can still be a big factor. Here’s a particularly dramatic example: platelet production. It would be very useful to produce blood platelets in vitro from megakaryocytes instead of isolating them from donor samples, but that’s been difficult to realize on scale. The authors (a large team from Japan who clearly have put in a great deal of effort) have discovered that a key feature of natural in vivo platelet production involves turbulent flow.

The idea that blood-flow shear stress was important in platelet production had already been proposed, but adapting this to cell culture had (so far) not produced the desired results. A closer look showed that the megakaryocytes that were exposed to laminar flow did not release platelets, while the ones that experienced turbulent flow did. Enter the Reynolds number and the Navier-Stokes equations, once again. Those things are seriously important, and are a serious headache for physicists, engineers, and mathematicians, since turbulence itself is so poorly understood. The Clay Institute has a million dollars waiting for you if you can tell them some of the most fundamental things about the Navier-Stokes equations, for example, and it just goes on from there.

At any rate, megakaryocytes, it turns out, apparently care very much about turbulence themselves. This new paper goes on to describe the design and testing of a new cell culture reaction that deliberately exposes the cells to turbulence as well as to shear, and it does the job: production significantly increases of what seem (in animal models) to be functional platelets. The cells start releasing much larger amounts of crucial thrombopoietic mediators (such as IGFBP-2, MIF, and others) under these conditions, which seems to be a key part of the whole process. How exactly they’re sensing the turbulent flow and what the coupling mechanisms are for the enhanced protein production are questions that still seem to be open. You’d have to assume some sort of cell-surface mechanoreceptor system, I’d think, but we’ll see. This opens up some new frontiers for even more detailed cell culture conditions, though, and you can be sure that a lot of biotech folks are taking notice. . .