Editor: B Davies
It’s a commonly held belief that you can’t grow new brain cells as adult; you’re born with one hundred billion neurons and that’s as many as you’re getting. However, this isn’t quite the case. While new neurons don’t form in most parts of the human central nervous system (the brain and spinal cord), there are two special areas where new neurons do indeed arise after birth. These areas are found in specific parts of the brain with rather complicated names: the subgranular zone of the dentate gyrus and the subventricular zone of the lateral ventricle. These two areas (which we call the ‘SGZ’ and ‘SVZ’ for short) contain what we call ‘neural stem cells’ (NSCs), which are able to produce new neurons throughout adult life. This production of new neurons from stem cells is called ‘neurogenesis’.
As patients survive longer with neurological diseases and as we expect longer stays in space, it becomes more and more important to understand any links between immobility and neurogenesis for two reasons. Firstly, changes to neurogenesis could affect brain health – it may be that changes to neural stem cells following reduced mobility feed back into disease like MS or DCM and actually become part of the cause. Adult neurogenesis is greatly decreased in Huntington’s disease patients when compared to healthy people, suggesting that there could be a link between reduced neurogenesis may play a role in the disease3. Secondly, exploring the link may help us understand the effects of exercise on the brain. Reduced movement has been shown to impair memory function and learning4 and to change the chemical environment of the brain5. We may also be able to better understand the link between exercise and prevention of neurodegenerative conditions like Alzheimer’s disease, which is associated with degeneration in neurogenic areas6.
For all the above reasons, a team from Italy lead by Rafaella Adami recently set out to explore whether reduced movement lead to changes in neural stem cells7.
The study was done in mice. While mice do have some notable differences to humans in terms of the neural stem cells (see below), these experiments require the dissection of large amounts of brain tissue and immediately after death and so are practically impossible to do in humans.
The researchers wanted to recreate the conditions seen in situations (e.g. neurological diseases) where people can’t move very much. In these situations limbs are ‘unloaded’ – people aren’t using their arms or legs to move their weight around. in something called the ‘hindlimb unloading model’8 (HU) mouse model. Mice are suspended by their tales from the ceiling of a cage, taking the load off their hind legs, but leaving them free to walk on their front legs. Thus the hind legs don’t bear the mouse’s weight and are ‘unloaded’ (see figure 2). Adami et al put a group of mice in this position for 14 days, over which time their back leg muscles shrank significantly, as they would if they were unable to move them due to neurological disease (or if they were in space and carrying no weight!). After 14 days the mice were killed and their brains where dissected to examine the neural stem cells in the SVZ. Brains from mice which had been allowed to run around their cages freely where used for comparison (control).
What were the results of the study?
Firstly the researchers looked at the number of proliferating (dviding/reproducing) cells found in the SVZ. In this case, proliferating cells were the stem cells that were dividing to make neurons, so more proliferation suggests more neurogenesis. Adami et al found that there were 70% fewer proliferating cells in the HU mice compared to controls – so neurogenesis was reduced.
The team then wondered if this reduced proliferation meant that the stem cells themselves had changed in some way. To explore this possibility, they then took NSCs out of the HU and control mouse brains and grew them in a dish, to form a ball of stem cells and neurons. They saw that stem cells from HU mice divded more slowly than in controls, taking 7 days to double in number (the controls only took 2 days). They also checked that this slower rate of growth wasn’t due to cells dying.
Overall, these findings led the team to their first key result: reducing movement reduces the proliferative capacity of neural stem cells.
Adami et al then wondered what caused this reduced proliferation. They discovered that it was because the more of the HU mouse stem cells appeared to have become stuck in the ‘resting state’ when compared to the control mouse stem cells. 69% of HU stem cells were found to be in a resting state, compared to 57% of controls. Far more of the control cells were in a very active, dividing state (21% vs 13% of HU mice).
The researchers then looked at whether the neural stem cells were able to form mature neurons. They found that 6.8% of control stem cells could form mature neurons, whereas only 0.5% of HU stem cells could.
This lead the team to their second key result: reducing movement reduces the maturation capabilities of neural stem cells.
Next, Adami et al explored whether the metabolism (energy production) of neural stem cells in HU mice had changed. Most neural stem cells produce energy by a process called glycolysis, which by produces a byproduct known as lactate. HU stem cells produced significantly less lactate than controls cells, suggesting that reduced movement gives neural stem cells an abnormal metabolism.
Finally, to try and understand what could be underlying these changes, the researchers looked at gene expression in the neural stem cells. They found that expression of 2 genes were significantly different between HU and control samples. A gene known as CDKrap1 was 3.5x lower in HU stem cells than in controls, while a gene known as cdk6 was 2.3x high in HU stem cells. Overall, it appears that reduced movement changes the genes expressed in neural stem cells. Adami et al haven’t commented on what these different levels of cdkrap5 might mean, but they think that the higher levels of cdk6, which helps keep cells in the resting state rather than dividing, could explain the reduced neurogenesis seen in HU mice.
What do these results mean for DCM?
Right now, not a great deal. This work is still very much ‘blue sky research’ intended to see if the neural stem cells are worth further study for neurological disease (or space travel!). While its fascinating to see that that restricting movement leads to change in neural stem, we have to be cautious in how far we extrapolate the results to humans. Firstly, while mice and humans may be similar, they aren’t the same (newly born neurons rom the SVZ actually end up in a totally different places in mice and people). Secondly, while DCM can involve reduction in movement if nerve damage progresses to an extreme stage or pain becomes debilitating, it’s not quite as clear cut as in this mouse model. Therefore it’s hard to say if neural stem cells would undergo the same changes in DCM patients as they do here. Thirdly, it’s difficult to understand the implications of the results when we don’t fully understand how/if reduced neurogenesis contributes to neurological diseases. Furthermore, the consequences of reduced neurogenesis are likely to vary across conditions – changes to neurogenesis might be completely in DCM than they are for something like Huntington’s.
The next step will be to explore the nature of neural stem cells in other mouse models of reduced movement, such as multiple sclerosis, spinal cord injury and DCM to see if neural stem cells undergo similar reductions in neurogenesis. Then we’ll need to determine how/if reduced neurogenesis might contribute to the problems we see in these conditions. If such a contribution was confirmed, this could be a breakthrough in our understanding of how DCM develops. We might even then be able to developing new treatments which target the neural stem cells themselves. However, there are many steps we must take before we reach that stage – for now we’ll have to move slowly. Watch this space for more!
2. van Praag, H., Shubert, T., Zhao, C. & Gage, F. H. Exercise Enhances Learning and Hippocampal Neurogenesis in Aged Mice. J. Neurosci. 25, 8680–8685 (2005).
3. Ernst, A. & Frisén, J. Adult Neurogenesis in Humans- Common and Unique Traits in Mammals. PLOS Biol. 13, e1002045 (2015).
4. Wang, T. et al. iTRAQ-based proteomics analysis of hippocampus in spatial memory deficiency rats induced by simulated microgravity. J. Proteomics 160, 64–73 (2017).
5. Dupont, E., Canu, M.-H., Stevens, L. & Falempin, M. Effects of a 14-day period of hindpaw sensory restriction on mRNA and protein levels of NGF and BDNF in the hindpaw primary somatosensory cortex. Brain Res. Mol. Brain Res. 133, 78–86 (2005).
6. Guure, C. B., Ibrahim, N. A., Adam, M. B. & Said, S. M. Impact of Physical Activity on Cognitive Decline, Dementia, and Its Subtypes: Meta-Analysis of Prospective Studies. Biomed Res. Int. 2017, 1–13 (2017).
7. Adami, R. et al. Reduction of Movement in Neurological Diseases: Effects on Neural Stem Cells Characteristics. Front. Neurosci. 12, 336 (2018).
8. Barbosa, A. A. et al. Bone mineral density of rat femurs after hindlimb unloading and different physical rehabilitation programs. Rev. Ceres 58, 407–412 (2011).