When diffusion works backwards

Diffusion of dye in water.

Diffusion is an often confusing concept, but one thing that most people know for sure about diffusion: it spreads things out. Just like when you put a drop of red food coloring into a glass of water, over time, it spreads to make the whole glass of water pink.

But sometimes, diffusion seems to work backwards: it is possible for diffusion, with the help of some (chemical) reactions, to accumulate a type of molecule in one spot.

In our recent work, we found that this is precisely what is happening to the Dorsal gradient. Dorsal is a protein and transcription factor that acts as a morphogen in the early embryo. This just means that it has a concentration gradient in space and tells cells what to do in a concentration-dependent fashion. For a longer reminder of what Dorsal does, see here and here. But for now, what is important is that Dorsal is usually bound to its inhibitor Cactus, and signaling through the Toll receptor on the ventral side of the embryo causes Cactus to be degraded, so that Dorsal is free to go into the nuclei on that side of the embryo.

There are four processes that we know are happening. Together, these four result in accumulation of Dorsal on the ventral side. (1) Cactus (Cact) binds Dorsal (Dl); (2) there is a diffusive flux of Dl/Cact complex to the ventral side (leftward on figure). This diffusive flux is made possible by (3): Toll signaling causes the destruction of Cact on the ventral side. Finally, (4) free Dl is captured by the nuclei, so that it does not diffuse back.

We have known all of this for a long time, but it wasn't until recently that we realized what this meant: that Cactus "shuttles" Dorsal to the ventral side of the embryo. This is because Dorsal/Cactus complex is lost on the ventral side of the embryo (through Toll signaling), which creates an imbalance. Because there is more Dorsal/Cactus complex on the dorsal side of the embryo, it diffuses to the ventral side. On the ventral side, the Dorsal/Cactus complexes that arrive there by diffusion also get broken up by Toll signaling (and the freed Cactus is degraded), and Dorsal goes into the nuclei. Free Dorsal protein does not diffuse back to the dorsal side because it gets stuck in the nuclei. Over time, this means more and more Dorsal gets deposited on the ventral side of the embryo.

If you've read some of my other blog posts, you'd notice that I've remarked on this before: that clearly there is more Dorsal on the ventral side of the embryo than on the dorsal side. This is why. Unfortunately, as this "shuttling" mechanism is [an ever-so-slight] shift in the standard viewpoint of what Dorsal and Cactus are doing in the early embryo, not everyone agrees this is happening. The problem is, we know the four processes in the figure above are happening, and together, those guarantee that shuttling is happening. The mechanism also explains a lot of previously confounding data.

One final note: shuttling has been discovered before in other contexts. Most notably, in BMP signaling, which is occurring in the fly embryo at the same time as Dorsal signaling, as well as in BMP signaling in vertebrates. Additionally, shuttling has been hypothesized to be occurring upstream of Toll signaling.