Question

So ever since the beginning of my scientific career, I have been interested in how the brain works and I have taken and studied multiple aspects of this very complicated question. But since twenty five years, I have concentrated on the question how in the evolution of mammals and, in particular, of primates – so us: How in this evolutionary process does the brain get bigger or specifically the cerebral cortex? This is an important question because the cerebral cortex is the seat of the higher cognitive functions. Now I should stress in the very beginning that brain size and cognitive functions is not linearly correlated. It is not that brain size is the only underlying basis of our higher cognitive functions that charecterize humans but it is equally clear that without brain size increase, our cognitive functions would not be possible. Let me give you an example. There are, unfortunately, human patients, human disorders where a newborn baby may have the size of a brain as small as that of a chimpanzee. So half a liter as opposed to, in the adult stage, one and almost one and a half liters. Now, there are genes that are mutated in these unfortunate people. And when you look at the mental capacity of these people, how fit are these people mentally, they are clearly mentally retarded, but although their brain is the size of a chimpanzee, they are still much smarter than the chimpanzee. In other words size – brain size – is one important parameter for cognitive function but clearly not the only one. Now, when in evolution a brain gets larger or the cerebral cortex gets larger, this is a process that happens during the development of the brain. And specifically what happens is that the so-called stem cells that make all subsequent cells that exist in the adult brain produce more neurons in species A than in species B. So in our development, our stem cells in the cerebral cortex make more neurons than in the chimpanzee. And so the question is: What are the molecular factors that govern this process? And specifically our question is: Can we identify those changes in the genome that underlie the higher number of cell divisions and thus the larger number of neurons that are produced during our development as opposed to that of apes?

Method

If one wants to know the genomic changes that underlie the greater number of stem cell divisions in our brain as opposed to apes, our approach was to isolate these stem cells and to study which genes are particularly highly expressed and very active in these stem cells. And so a very gifted PhD student in my group, Marta Florio, devised a method to isolate the cells and this method is based on where we come from as scientists. We are cell biologists and for many years we have studied the cell biology of the cell. It was really our lab that introduced this approach to the field of developmental neuroscience. And so what we made use of, was the fact that these stem cells have processes, cell extensions. There are two principal classes of these stem cells called apical radial glia (ARG) as a specific term, or basal radial glia. And the apical radial glia cells touch both a fluid filled cavity called the ventricle and the other side of the developing brain called the pia mater where the meninges form. Whereas the basal radial glia touch only the meningial side not the ventricle. So we exploited that knowledge to purify these two cell populations and to determine which genes are specifically expressed in these cells. And obviously the goal was to identify those genes that are very highly expressed in the stem cells but not in the neurons derived there from because neurons are post mitotic (they do not divide). Having the so-called transcriptome of these apical and basal radial glia, we then categorize genes: whether or not they were in the apical radial glia, the basal radial glia. In humans, we isolated these cells from human fetal tissue and from mice. And we eliminated, we lost interest in all those genes that were also expressed in the developing mouse stem cells to hope to go for genes that might be human specific. And so we found about three hundred such genes; some of these genes exist in the mouse genome but are not expressed in the mouse developing stem cell; and others don't even exist in the mouse genome. There were fifty-six such genes. So we were most interested in these fifty-six genes which were highly expressed in the relevant stem cells in the human developing brain and not the mouse developing brain. And we asked which ones are the most highest, most highly, most specifically expressed gene in the in the basal stem cell. And the gene that showed the highest degree of specificity, of expression is the one we're talking about today.

Findings

This one gene that showed the highest specificity of expression in the human cortical stem cells but not the mouse and not in human neurons, is a gene that is called ARHGAP11B. Now that this gene is human specific was known before; that is not our finding. However, what we showed, is two things: First, that it is expressed in the stem cell, that's how we found it, and then we worked out what it actually does during brain development and there are several key findings about this gene. The first one is that the gene is indeed human specific; so it does not even exist in chimpanzees. It arose in evolution as was shown by others before we started actually. It was shown that it arose in evolution about five million years ago which is a little later after the lineage that leads to the chimpanzee separated from the lineage that leads to modern humans. That made the gene very interesting already. But the gene exists not only in modern humans but also in Neanderthals and in Denisovans which are our nearest no-longer-living relatives. And as you know, for the Neanderthals, we know from the skull, from the size of the skull, that the brain of the Neanderthal was as big as our brain. So the exciting thing was that here we have a gene: it's expressed in the right stem cell, in development at the right time and it's only found in species that have a 1.3 to 1.4 liter brain. Since our work and that of others had implicated an increase in stem cell division as being the cause for a brain or cerebral cortex size increase, we then asked the question: Would that particular gene when expressed by force in the developing mouse neocortex which doesn't have that gene, would it drive increased cell division of the relevant stem cells? And that is indeed what happened. So a), this gene, when forcibly expressed in the mouse, drove the apical radial glia to make more basal stem cells. And then it induced the basal stem cells to make not just one round of cell division to give rise to two neurons and it's over but to do multiple rounds of cell division. So the gene did exactly what one would have postulated for a gene that would increase brain size. That gene did it. And then, even more exciting, in about half of the mouse embryos into which we introduced the gene, the brain started to fold in development. As you know, our brain is highly folded, that of the mouse is completely smooth. It's called lissencephalic. And when we introduced that human specific gene into a developing mouse brain, in half of the cases the mouse brain started to fold as if it were human. I have told you that ARHGAP11B arose five million years ago by a partial duplication of a ubiquitous gene called ARHGAP11A. It contains the first part of that old gene. The old, the ubiquitous gene when expressed in mice does nothing to increase basal stem cells, whereas ARHGAP11B does. Why does it do that? There is a protein sequence at the end of the ARHGAP11B protein that is absolutely specific to modern humans. It only exists in humans, it does not exist in any other species. So not only is the gene human specific but it has a protein sequence that is human specific. How does that come about? It comes about by a so-called reading frame shift which is due to the fact that fifty-five nucleotides in the nucleic acid are missing. Why are they missing? We thought they were missing when the gene duplicated that they just got lost five million years ago. And we were surprised to find that the DNA of the ARHGAP11B gene contains these fifty-five nucleotides. They are lost only when the messenger RNA, the blueprint that then gives rise to the protein, is made from the gene. That's when they are missing. Why are they missing? Because a singlebase pair, a C is changed to a G. That creates a signal to remove these fifty-five nucleotides. So in other words it's a point mutation, a single base pair change that underlies the loss of fifty-five nucleotides. The reading frame shift that results from that, that gives rise to the new protein sequence and that gives that protein its capacity to drive, to amplify the basal stem cells. So to prove its importance of that single nucleotide change, we made an ancestral version that is identical in essence to the one we carry in us, the 11B gene we carry in us, but it has a C instead of G. When that gene is introduced into the developing mouse brain, it does nothing on the basal stem cells. In other words, if I wanted to overstate the case, I would say a single nucleotide sequence underlies the increase in brain size that contributes to what we are as modern humans.

Relevance

So the potential relevance of this finding is that, if that gene should indeed cause brain size to increase not just in mice but in higher species, then it might provide a basis, it might contribute to higher cognitive functions. In this context, I should say that the G, the C to G change which endows the protein encoded by the gene by its ability to amplify basal stem cells, that G is found in every human individual sequenced to date. Suggesting that it was indeed an advantage in the evolution of modern humans to have the G over the C. I think there are two big open questions.

Outlook

One is to show that, indeed, that gene increases brain size in a stable manner. We are testing that in mice. The work we have done so far was all with mouse embryos. Now we're making stable so-called transgenic mouse lines which express the gene in the appropriate way and we are going to see if the size of the mouse brain then increases. Perhaps it folds regularly. And once we have those mice, of course, we will test them for hopefully increased cognitive function. We are also considering other species such as the ferret to see what the gene does there or maybe even higher species than the ferret. That's one direction of future research. The other direction is this new protein sequence that is human specific that comes by this reading frame shift. What does it actually do? How does it achieve the amplification of the basal stem cells? And so, we are working very hard on the downstream machinery, as we say, that is driven, activated, induced by that new protein sequence. That's the other major question.