By Karen Dewey
Everyone must eat to live; we all know this from day one. We get all our energy from food, but what else does a good diet do for us? Our bodies store energy in fat deposits that have effects on a cellular level, effects so small that we never really notice them. In 2010, neuroscientists James Chell and Andrea Brand discovered a signaling pathway that begins with the food you eat and launches the growth and development of dormant neuronal stem cells.
Neuronal stem cells, or neuroblasts, exist in a state of what Chell and Brand call “quiescence”. Quiescent cells don’t grow or proliferate; a genetic lock keeps them from developing any further. For decades, scientists knew that neuroblasts begin growing in response to a particular signal, called a mitogen, released by nearby fat cells. But how exactly does the signal reach a neuroblast? To find out, Chell and Brand investigated the PI3K/Akt pathway, a cellular pathway driven by insulin that is known to influence cell growth and use of nutritional stores. They chose the fruit fly, Drosophila, as their model organism.
In Drosophila, quiescent neuroblasts begin growing and multiplying when the fly is just a larvae. To track the development of neuroblasts as they mysteriously entered the growth phase, Chell and Brand created a lineage of flies with labeled grh-GAL4A genes, which are expressed in all neuroblasts during reactivation, then watched the neuroblasts develop as the flies matured. They also used the GAL4A marker to label the gene dilp6, which codes for a signal called dILP6, part of the PI3K/Akt pathway, target of their investigation, and found it expressed in glial cells lurking just above the neuroblasts.
The next step was finding evidence that the dilp-6 wasn’t just a false alarm – was the PI3K pathway active in neuroblasts? Looking for the pathway involves a label called PH-GFP, which binds to cell membranes when the PI3K pathway is active. Sure enough, PH-GFP appeared in the neuroblasts, indicating that the PI3K pathway was up and running. Furthermore, PI3K-inhibited mutants had severely restricted neural growth, suggesting that PI3K pathway is essential for neuroblast growth.
Chell and Brand wanted to see if bypassing the natural fat cell trigger and manually activating PI3K could help neuroblasts to grow. They raised larvae on a sucrose-only diet to ensure that no dietary amino acids would be able to activate fat cells, then activated PI3K themselves. Not only did the neuroblasts grow, they grew much more than they normally would. Morphologically the cells were the same, indicating that, with a little help, the PI3K pathway can induce growth on its own without nutritional input.
But PI3K isn’t the whole story either. It’s only the first part of a pathway that also involves Akt. When PI3K is active, Akt levels increase. Chell and Brand investigated whether Akt on its own could induce neuroblast growth. Going back to their sucrose-only diet flies, they expressed Akt in dormant neuroblasts, which entered the growth phase in response. As well as growing, neuroblasts multiply, or proliferate. Whether activated by PI3K or Akt, proliferation occurs at the same rate.
After establishing the role of PI3K and Akt in neuroblast growth, Chell and Brand completed the picture by investigating dILP-inhibited mutants. With no way to activate the PI3K pathway, the neuroblasts were unable to grow and the CNS (including the brain and spinal cord) was underdeveloped.
As things stood, Chell and Brand had pieced together the details of a cellular pathway controlling the growth of neural stem cells. First, glial cells release dILP, which activates the PI3K pathway, releasing Akt, triggering growth in the nearby neuroblasts. But how do fat cells fit into this picture? Why is this pathway considered nutrionally activated? It had already been demonstrated that fat cells send mysterious signals called mitogens to trigger dILP release. Chell and Brand proved it again, using their sucrose-only flies.
The crucial point is that fat cells only release mitogens in response to high levels of dietary amino acids. This is one reason why you have to eat your protein; your body can’t make enough amino acids on its own to support cellular functions. If flies are deprived of protein, dietary amino acids are unavailable, and nothing will tell the fat cells to activate the PI3K pathway. We would expect to see less or no dILP release in the cells of flies on sucrose-only diets, and that is what Chell and Brand found. Since there’s no known way to label dILP6 directly, they investigated the number of genes actively coding for dILP6 instead. In protein-deprived cells, these genes were far less common, to the point that they weren’t even expressed in the early larval stages. In other words, cells don’t even bother to make dILP6 without input from dietary protein.
Unless of course, you decide to trick them into doing it. To check their assumptions once again, Chell and Brand activated dilp6 and forced the sucrose-only flies to produce dILP6. Neuroblasts began to grow and proliferate as expected, although they divided less frequently than they did in other phases of the experiment or in nature.
Another wrinkle in the process: there are actually seven forms of dILPs in fruit flies. Are they all as important as dILP6? Structurally and functionally they seem to have a lot in common. Mutant flies that can’t produce dILP2 don’t survive very long, in fact, and turning on dILP2 expression in these fly embryos can save them. So dILP2 is important as well, but are all seven dILPs? Well, it’s rare to find a mutant that fails to express not one, not two, but seven specific proteins, and it’s even harder to make a mutant strain like that. At the point we are at today, such a fly can’t really be experimented on. To get as close as possible, Chell and Brand created a temperature-sensitive fly that could make dILPs just fine, but could only transport them through the cell in the right climate. At the wrong temperatures, neuroblast development was severely compromised because the cells couldn’t transport any dILPs.
By carefully investigating each step of the PI3K/Akt pathway, Chell and Brand have managed to illustrate an entire cellular pathway. First, your body breaks down protein you’ve eaten into amino acids. Your fat cells react to the presence of amino acids in your bloodstream and release a mitogen. This mitogen travels to glial cells, special nerve cells that provide support and structure to neuroblasts, which release dILPs. These dILPs trigger the PI3K pathway in neuroblasts, telling them to begin growing and multiplying.
But why is it so important for neuroblasts to grow and multiply anyway? If you’ll remember, neuroblast is just another name for a neuronal stem cell. Stem cells are incredibly exciting to scientists, doctors, and anyone waiting on medical research to cure a deadly or debilitating disease. While controversy over the use of embryonic stem cells for research and medical development is an important ethical issue, stem cells are actually produced in the adult body as well. The problem is that they aren’t always necessarily doing anything useful.
Stem cells are found in the muscles, bone marrow, and as we’ve seen in the brain. For diseases such as Alzheimer’s and other mentally degenerative disorders, hippocampal stem cells could promise a new kind of therapy in the hopefully not-to-distant future. Emotional disorders like depression and anxiety disorders can also be improved with neural regeneration. But in order for stem cells to be of any help they must first be active. Understanding the mechanics of activation takes us a tiny step further to one day controlling stem cell activity for therapeutic purposes.
Don’t get too excited just yet, though. The P3IK pathway is just one pathway after all, and the fat cell mitogen release is still poorly understood. Furthermore it can be dangerous to simply activate stem cells; there’s a risk of cancer and other detrimental developments. Still, Chell and Brand’s research is exciting on more than a research level. If nothing else, it reminds us of the many thousands of things our bodies do every day to keep us alive and running in ways we never even think about.