Stem cells on the brain

I confess: I have stem cells on the brain.

Yeah, I probably  have a few stem cells left in this old brain of mine, but that’s not what I mean….rather, I mean that I am really curious and excited about how stem cells do their jobs in our brains.

More specifically, I wonder about things such as how stem cells regulate brain growth during our development and also what goes awry for them to cause cancer. I also want to figure out how we can harness the power of stem cells to treat diseases and injuries of the brain and the CNS as a whole including the spinal cord.

Where did my interest in neural stem cells come from?

When I was in high school biology I got extra credit for isolating an intact frog brain during the whole frog dissection section of the class. I was fascinated by its structure. This would be the reason, according to my daughters, why I was so popular in high school.

More recently, about a dozen years ago, I was a relatively new postdoc who had made a conditional knockout mouse model for a gene called N-Myc, named for being like Myc. A conditional knockout is a mouse in which you can in a controllable fashion delete a single gene and see what happens to cell and developmental biology. You can even make your knockout happen in only one cell type.

For example, for my first studies, I deleted N-Myc specifically in neural stem cells.

I was a neurosci newbie so I had a lot to learn.

What did we know back then around the year 2000?

Not a lot.

No one really understood what N-Myc did normally or how it caused cancer, but it was very highly overexpressed in many cancers, especially neural cancers in kids such as neuroblastoma.

In 2002, we published a study using the conditional knockout of N-Myc to take it away specifically from neural stem cells in the brain. This was the first published conditional knockout of any Myc gene in any type of stem cell so it was exciting.

What did we find?

As I was making the N-Myc knockout mice I got to the point where I had mice that were homozygous (meaning both copies of the N-Myc gene were targeted) for the knockout in neural stem cells. Surprisingly the knockout mice, at least to my relatively inexperienced eye, seemed normal and they certainly did not die. Sometimes knockout mice, with the loss of just 1 out of about 30,000 genes, die from not having just one certain gene.

But our mice lacking N-Myc in neural stem cells were living at the normal rate and didn’t seem wildly abnormal in behavior or appearance.

Then I had one of those amazing moments in science.

When I examined the brains of the N-Myc knockout mice I was floored. I could see the N-Myc knockout brain was super tiny.

I mean, really really small. See image above from our paper. “Null” means knockout.

So we took away just one gene, N-Myc, from neural stem cells and those mice couldn’t grow a normal brain.

Interestingly, the N-Myc-less brain was mostly organized normally and proportionate, but just shrunk like a shrinky-dink. One exception was the back part of the brain (outlined with dashed lines), the cerebellum, was even tinier in the knockout than the rest of the brain.

It turns out the cerebellum in both people and mice is responsible for coordination as well as movement and remarkably, our N-Myc knockout mice, upon further careful examination, exhibited tremors and ataxia. People with cerebellar injuries and disorders also have tremors and ataxia too.

In addition too much N-Myc causes a cerebellar tumor called medulloblastoma. So there was compelling evidence that N-Myc was important for brain growth and especially for controlling the development of the cerebellum. A fantastic paper from my colleague Anna Kenney published around the same time as our knockout paper, strongly implicated N-Myc in medulloblastoma and you can see in the image to the right below from her paper how the medulloblastoma tumor has tons of N-Myc (bluish-purple stain) whereas the normal mature cerebellum doesn’t.

Some animals, specifically echo-locaters such as bats and whales, have abnormally large cerebella so a question I still wonder about to this day is whether during their development they have more N-Myc in their cerebella than other animals such as people that have relatively puny cerebella.

Anyhow, back to the N-Myc knockout mice. After isolating the brains, I weighed them and the N-Myc knockout brain was about half the size of the brains of its wildtype siblings.

This was my exciting introduction to neural stem cells and neuroscience.

I’ve been hooked ever since.

A few years after we published our findings on how loss of N-Myc in neural stem cells strongly disrupts brain growth, another exciting moment came. A paper appeared in which researchers reported that in a rare condition called Feingold Syndrome in humans, the N-Myc gene was mutated and one of the most striking things about Feingold Syndrome was that the people had microcephaly–small heads and tiny brains.

At that moment, the power of the mouse as a model system for human disease hit home for me. Here I had created a mouse mutant for N-Myc and gotten a mouse with a tiny brain only to later read a new study that humans with mutated N-Myc had tiny brains. More recently I have posted on why the human brain is huge compared to other animals and I think N-Myc has a lot to do with it in addition to neural stem cells’ love of sugar.

Another level of meaning here is the fact that too much N-Myc causes cancers of the nervous system characterized by too much growth. So when we mutate N-Myc it slows brain growth and in cancers with too much N-Myc the brain and nervous system grows too much in the form of cancer.

Connect the dots. N-Myc tells neural stem cells how big a brain to grow. To some extent the related c-Myc gene also has a role in normal brain growth and definitely is implicated in brain tumors, an area that my colleague and fellow CIRM grantee, Rob Wechsler-Reya just published an extremely important paper on recently. CIRM blogger Amy Adams has a nice post up giving you the big picture on Rob’s paper and his research.

More recently in my own lab, we have also found that N-Myc is extremely important in iPS cells and human and mouse ES cells as well.

Our knockout mice have also been used to study N-Myc function in a host of other stem cell and tissue types, and a general pattern is that stem cells generally love their N-Myc. In fact, without it, they undergo an identity crises and most often lose their “stemness”.

We still are doing research on what it is about N-Myc that makes stem cells grow nervous system tissue and I am very excited about some of the things we are learning that have important relevance for not only cancer, but also regenerative medicine. One clue about how N-Myc does its magic comes from a recent study my lab did with neuroscientists Veronica Martinez-Cerdeno where we found (see green image near the top of the post of a developing cerebellum) that taking away both N-Myc and c-Myc increases levels of an important factor called p27 (shown in green) in stem cells of the cerebellum. Normal cerebella have almost no green staining for p27 at all. Normally p27 makes cells stop growing so if N-Myc keeps p27 levels low as it seems that may be an important way in which N-Myc allows stem cells and the brain to keep on growing. We are learning more about how this all works.