They are recovering, but poorly
Brain stem cells are updated only in a limited way
Phys Tech, Naked Science
A group of scientists from MIPT, Stony Brook University and the Cold Spring Harbor Laboratory observed how neural stem cells are divided and consumed in the hippocampus in mice - an area of the brain critical for learning and memory. Optimistic predictions about the presence of symmetrical division – when two stem cells are obtained from one stem cell – have not been confirmed. If such a division occurs, then no more than ten percent of the time. This means that the replenishment of stem cells that could give rise to new neurons is a rare or non–occurring process. In addition, the researchers determined the spatial features of the disappearance of stem cells in the aging brain. The article was published in Scientific Reports (Mineyeva et al., Spatial geometry of stem cell proliferation in the adult hippocampus).
Neurogenic niche of the hippocampus. Stem cells (Nestin-GFP) are shown in green, astrocytic protein (GFAP) is shown in red. Three-dimensional reconstruction of a series of confocal images. Provided by the authors of the study.
Olga Mineeva, an employee of the MIPT Brain stem Cell laboratory, comments: "Our results primarily indicate that in a normal mature brain, the ability of hippocampal stem cells to renew by symmetrical divisions is limited. And the increase in the total number of stem cells does not occur at all or does not affect significantly. These results require a critical revision of a number of studies that have shown the presence of symmetrical divisions of hippocampal neural stem cells."
Although there is still debate about the possibility of the appearance of new neurons in the adult brain in humans, their generation in the brains of other adult mammals is recognized as an irrefutable fact. The process of formation of new neurons from stem cells is called neurogenesis. After the growth and development of the young brain is completed, in most adult mammals, only two areas remain in the brain that preserve stem cells responsible for neurogenesis: a thin layer in the wall of the lateral ventricles of the brain (or subventricular zone) and a thin layer in the dentate gyrus of the hippocampus (or subgranular zone). The second zone is of particular interest to researchers, as it is located in the hippocampus, whose work is critically important for the implementation of important cognitive functions – for example, learning, memory and emotional behavior. At the same time, modern data show that neurogenesis itself in the hippocampus may be important for the implementation of these functions. But it is also known that the number of neural stem cells in the hippocampus decreases with age. Therefore, researchers face a number of questions: how is the pool of these stem cells preserved during ontogenesis, what are the mechanisms of its maintenance and renewal, and, most importantly, is it possible to influence these processes and thereby prolong intensive neurogenesis into old age, and therefore prolong the youth of the brain.
The key to answering these questions may be figuring out how stem cells divide in the adult brain. Theoretically, the division of a neural stem cell can occur in two ways (Fig. 1). The first option is when two of the same stem cells are obtained from one stem cell. This kind of division, when two daughters like her are formed from the mother cell, is called symmetrical. The second option is when two daughters are obtained from one stem cell, one of which is the same as the mother's, and the second becomes the precursor of a neuron. These divisions are commonly called asymmetric. Stem cells can divide mainly in one way or use both types of divisions to generate neurons. As can be seen, the method of stem cell division determines both the total number of stem cells, and, consequently, whether the reserve of stem cells is consumed, at what rate, whether it can be restored or even increased.
Figure 1: Types of cell division. With symmetrical division (on the left), two of the same stem cells are formed from one stem cell (green), and with asymmetric division (on the right), one stem cell and a neuron precursor (red cell) or an astrocyte (purple cell) are formed / MIPT Press Service.
So, if hippocampal stem cells often divide symmetrically, then their pool can self-sustain and recover. Potentially, pharmacological induction of symmetrical divisions may even increase the stem reserve. If symmetrical divisions are an extremely rare event, and mostly asymmetric ones occur, then activation of stem cell division after a period of rest will lead to the inevitable disappearance of the stem cells themselves. The data on the decrease in the number of stem cells with age indirectly indicate in favor of the second, "pessimistic" scenario and the possible absence of symmetrical divisions. But how to investigate this issue directly? An unusual solution was proposed by a group of scientists from MIPT, Stony Brook University and Cold Spring Harbor Laboratory.
The authors identified dividing cells in a classical way for this kind of research, namely, they injected experimental animals with a synthetic analog of the thymidine nucleotide – bromodeoxyuridine (BrdU), which is embedded in the doubling DNA chains of cells during their division (we already talked about this method when we wrote about another work by the same group of researchers). The fact that it was hippocampal stem cells that shared was judged by another marker – green fluorescent protein (GFP), the synthesis of which was controlled by the regulatory sequence of the nestin protein gene, a marker of stem cells. Stem cells are distinguished from other precursors of neurons by their special recognizable shape: they have a large body and one long apical process, strongly branching from above. Thus, if symmetrical divisions of stem cells occur in the dentate gyrus of the hippocampus, such divisions on slices can be seen as a pair of closely spaced green cells with a large apical process and with BrdU in the nuclei. However, labeled cells may be nearby not only because of symmetrical division. They can be descendants not of one cell, but of two different neighboring cells, each of which is divided asymmetrically (Fig.2). The question arises how to distinguish between these two situations.
Figure 2: On a slice of the brain, there are stem cells labeled with BrdU (fragment in the middle) among a multitude of non-dividing or resting stem cells (light green). Such labeled pairs can occur either due to the activation of an asymmetric type of division of two unrelated stem cells located side by side (option on the left), or due to the symmetrical division of one stem cell (on the right). / According to the drawing provided by Olga Mineeva, the author of the article. MIPT Press Service.
Scientists have come up with a statistical method by which it is possible to assess whether the labeled cells happened to be nearby or not by chance. Namely, due to their origin from two different maternal cells divided asymmetrically (Fig. 1, left part) or from a common maternal cell divided symmetrically (Fig. 1, right part). The essence of the method was to measure the actually observed probability of detecting pairs of labeled stem cells and compare it with a completely random probability that can be artificially modeled in the general population of stem cells. As a result: if the actually observed number of pairs is higher than the simulated random one, it means that not all pairs can be explained by chance and some of them were formed as a result of symmetric divisions.
In order to measure the real and randomly modeled number of cell pairs, the location of all stem cells containing and not containing the division marker of BrdU was found on microscopic images of slices. Thus, its position in space was determined for each cell. Then, using the obtained coordinates, the distances between pairs of actually observed labeled cells and the number of pairs of cells at specific distances were determined (Fig. 3A). Then the random distribution of labeled cells was artificially modeled for comparison. To do this, we used all the positions in which labeled cells could appear, that is, the coordinates of all dividing and non-dividing stem cells. Using a random number generator, the same number of cells were selected from the list of coordinates that were actually observed on the slice, and they were conditionally taken as labeled. The same slice was obtained, but now with pseudo-labeled cells scattered randomly on it (Fig.3B). Let's say if we saw 11 labeled cells on the slice (as in Fig.3A), then 11 pseudo-labeled cells will be scattered among all possible positions (Fig.3B) and the distance distribution is calculated between them. This operation was repeated many times to get statistics from all slices.
Comparing the real distribution with the simulated random distribution, the authors saw that they did not differ from each other. However, the lack of differences between the real and randomly generated picture could be due to the low sensitivity of the method. To test this, the researchers tried to add a given number of pairs of converging dividing cells to the model and estimate how many symmetrical divisions are needed for the new artificially generated distribution to begin to differ from the random one. It turned out that you need to add more than 10% of symmetric division events. From this, scientists concluded that symmetrical divisions of nerve stem cells do not occur in the real adult hippocampus, or they occur, but extremely rarely and their share is no more than 10%.
Figure 3: Diagram of the location of real stem cells on the slice and in the model. Gray dots are non–dividing stem cells. Black dots are actually detected labeled cells. Blue dots are randomly scattered pseudo–labeled cells / MIPT Press Service.
The authors used the same method to describe the disappearance of stem cells with age. The older the mouse, the fewer stem cells remain in the neurogenic zone of the hippocampus. For example, a seven-month-old mouse has nine times fewer of them than a two-week-old mouse, and voids are found in the neurogenic reserve in the space where the stem cells were at first.
The authors simulated the aging of the neurogenic zone of the mouse hippocampus by performing the same random search operation as in the first experiment. However, now, among all the available cells in young mice, they randomly selected and left the amount that was actually observed in the hippocampus of a seven-month-old mouse. It turned out that the remaining stem cells in the real hippocampus of seven-month-old mice were distributed more evenly than randomly selected ones. Based on this, the researchers conclude that the disappearance of hippocampal stem cells depends on their position in space: cells located close to each other have a greater chance of disappearing soon, which leads to voids in the stem cell layer. Moreover, experts have found out that in different parts of the hippocampus, stem cells are consumed with varying degrees of unevenness. Interestingly, different areas of the hippocampus are responsible for different cognitive functions. However, scientists are in no hurry to associate such a disappearance of stem cells with functional features – perhaps this is due to simpler factors, such as the distribution of blood vessels.
The latest work of these authors in a new way confirmed the previously put forward assumption that stem cells cannot be renewed indefinitely. Earlier, on the basis of other data, the authors of this study put forward the concept that each stem cell, after emerging from a state of rest, undergoes a limited number of cell divisions, giving rise to new neurons and astrocytes, but not new stem cells (Encinas et al., 2011). An important consequence of this concept of the irreplaceability of the hippocampal resting stem cell pool is the possible negative effect of substances activating stem cell division on neurogenesis in the hippocampus in the long term. Thus, by increasing neurogenesis through enhanced recruitment of stem cells, these factors can cause premature depletion of the neurogenic niche and, as a possible consequence, lead to cognitive impairment due to the subsequent shortage of new neurons.
Figure 4: A) Remaining stem cells in a seven-month-old mouse. C) Stem cells of a two-week-old mouse, among which the same number of cells that remained in a seven-month-old mouse was randomly selected (blue). C, D) The real distribution of distances between the nearest cells (red) differs from the random one (blue) / MIPT Press Service.
Grigory Enikolopov, Head of the MIPT Brain Stem Cell Laboratory: "There may not be a total ban on symmetrical divisions, and an increase in the number of stem cells is possible under certain conditions. The search for such effects that stimulate the division and renewal of neural stem cells, but at the same time do not exhaust their pool prematurely, should continue."
Portal "Eternal youth" http://vechnayamolodost.ru