Does neurogenesis continue throughout life? New studies suggest yes – and it may play a key role in memory

The topic of adult neurogenesis has been controversial over the past couple of decades due to a lack of strong evidence that new neurons are generated in humans after adolescence. However, the development of new technologies and experimental techniques – as well as an alarming rise in Alzheimer’s disease and other dementias^1,2 – has brought adult neurogenesis back into the limelight.

There is a prevailing belief that neurogenesis – i.e. the growth and development of new neurons – halts once we reach adulthood. Indeed, most neurons are generated in the embryo, before birth. 

Adult neurogenesis, however, refers to the continued generation of neurons from neural stem cells in the adult brain. These newborn cells eventually become indistinguishable from those born during embryonic development³. 

Determining whether new neurons are continuously generated in the adult brain in humans, and how this process may be altered in both normal, ‘healthy’ aging and in neurodegenerative diseases such as Alzheimer’s, is an important question with potentially game-changing therapeutic potential.

In animal models, reductions in adult neurogenesis have been associated with a number of cognitive deficits including impairments in spatial memory, long-term memory and fear conditioning. Impaired neurogenesis has also been associated with a number of psychiatric disorders including depression, anxiety, addiction and schizophrenia. For example, patients with major depression have shown reduced levels of neurogenesis in the hippocampus, while antidepressants are known to increase neurogenesis in this region – an action which may be important in their therapeutic action^4,5. 

The hippocampus is involved in the consolidation of memories to other brain regions as well as in memory retrieval, and is one of the most affected areas in Alzheimer’s disease⁶. The dentate gyrus – a subsection of the hippocampus – plays a key role in memory retrieval, and is one of only two regions where the addition of new neurons has been observed throughout life in mammals. The other is the subventricular zone of the lateral ventricles, where the differentiation of new neurons plays a role in the sense of smell – although the latter is unlikely have any functional significance in humans^7,8. Although adult neurogenesis in these two regions has been observed rodents and monkeys⁹, neither has been reliably observed in humans. 

A seminal study by Eriksson et al. in 1998¹⁰ provided the first direct evidence of adult neurogenesis in the hippocampus of humans. The researchers were not, however, able to quantify the number of new neurons in the samples, nor did it provide any insights into the dynamics of adult neurogenesis. 

More recently, a study in 2013¹¹ built upon research in the field by devising a method to retrospectively determine the birth date of cells in the hippocampus. They achieved this by measuring the concentration of ¹⁴C in the DNA. The theory behind this approach was that, due to above-ground nuclear bomb testing during the Cold War, atmospheric ¹⁴C levels were elevated during the time period from 1955-1963; since then, levels of ¹⁴C in the atmosphere have steadily declined. Much of that ¹⁴C has reacted with oxygen to form CO₂ which has in turn been taken up by plants through photosynthesis before passing through the food chain into animals and humans. As a result, atmospheric ¹⁴C levels are mirrored in human genomic DNA, as the ¹⁴C is incorporated into chromosomes during cell division at a concentration corresponding to that in the atmosphere at the time – thus providing a method of determining the cells’ date of birth. Effectively, a radioactive timestamp. 

By measuring concentrations of ¹⁴C in human post-mortem hippocampal cells from subjects aged 19-92, the researchers could devise a model to calculate the rate of cell turnover which suggested that neurogenesis may indeed persist throughout adulthood. However, this was only a model – actual evidence of immature or newborn neurons being incorporated into adult brains in humans was still lacking. 

Last year, a study led by Sorrells et al.¹² attracted a lot of attention in the neuroscience community after publishing findings in which the authors failed to show any evidence of new neurons in post-mortem brain samples from adult humans. The research looked at the numbers of neural precursor cells and immature neurons in human port-mortem samples and concluded that neurogenesis in the hippocampus was most significant within the first year of life, before declining rapidly throughout childhood until eventually halting entirely in the brains of adults. 

A paper published in Nature earlier this year, however, found quite the contrary. 

The research¹³, led by Moreno-Jiménez et al. at the Department of Molecular Neuropathology, Universidad Autónoma de Madrid, Spain, aimed to investigate the extent of adult neurogenesis in the human hippocampus using post-mortem brain samples obtained from both healthy individuals and Alzheimer’s disease patients. 

Previous studies have shown both increases¹⁴ and decreases¹⁵ in neurogenesis in post-mortem tissue derived from patients with Alzheimer’s disease. Moreno-Jiménez and her team, however, sought to achieve more definitive results by improving upon the tissue-processing methods used in previous studies; proposing that previous failures to show direct evidence of adult neurogenesis in humans may actually be down to the experimental methods used in those studies. 

Thus, by using state-of-the-art methods to tightly control the conditions by which the brain samples were obtained and prepared, the team achieved a greater sensitivity allowing them to identify thousands of immature neurons in the dentate gyrus of hippocampi from 13 neurologically healthy human subjects and 45 subjects with Alzheimer’s disease. While the numbers of new neurons declined throughout life in both cohorts – suggesting a natural age-related decline in neurogenesis – immature neurons were identified right through to the ninth decade of life. 

The researchers identified newborn neurons using fluorescent antibodies, allowing them to visualise a specific marker of neuroblasts (neuronal precursor cells which will later develop into neurons) called doublecortin (DCX) (Figure 1). Moreno-Jiménez et al. reported a several-fold higher density of these types of immature cells in the dentate gyrus than has been reported in previous studies – most likely owing to the enhanced tissue preparation methods. The team also demonstrated the co-existence of several other neuroblast-associated markers, confirming that the cells identified were in fact immature neurons.

Figure 1: Confocal microscopy images showing (1) the entire hippocampus, (2) the dentate gyrus (DG) showing the abundant presence of DCX+ immature neurons (yellow triangles) and (3) a high-power-magnification image of one of these cells. Moreno-Jiménez et al. (2019)

Figure 2: The density of DCX+ neural precursor cells
is lower in Alzheimer’s disease patients compared with
controls and declines with age, with counts dropping
further as patients progress through the six Braak stages
of Alzheimer’s disease. Moreno-Jiménez et al. (2019)

As you may have guessed, the number of DCX-positive cells was significantly lower in the hippocampi of Alzheimer’s disease patients compared with neurologically healthy controls (Figure 2). Moreover, neurogenesis was even further reduced in the later stages of the disease. 

Even more interesting is that the reductions in adult neurogenesis were also detected in the early stages of the disease – before the presence of neurofibrillary tangles or senile plaques thought to underlie the pathophysiology of Alzheimer’s. 

It is possible therefore that detection of impairments in adult hippocampal neurogenesis using non-invasive methods may serve as an early biomarker for the disease, and that therapeutic targeting of cellular pathways underlying neurogenesis in the hippocampus could potentially mitigate the memory deficits characteristic of Alzheimer’s disease and other dementias. 

A new study published in Cell Stem Cell this week¹⁶ by researchers at the University of Illinois in Chicago reported similar findings in post-mortem samples taken from a cohort of 18 patients with Alzheimer’s disease and other mild cognitive impairments, aged between 79 and 99 years old. 

Similarly, the researchers investigated the levels of DCX-positive neuroblasts in the samples, finding that these neural precursor cells, among other markers of developing neurons, were present in the hippocampus of brain tissue taken from patients well into their 90’s. 

As well as supporting Moreno-Jiménez et al.’s conclusions that a decline in neurogenesis may be an important underlying feature of Alzheimer’s – potentially driving the decline in cognitive ability from the early stages of the disease – the study goes further by showing that higher counts of DCX-positive neuroblasts are associated with higher scores in measures of cognitive ability and an overall better clinical diagnosis. 

It is worth noting, however, that while the results of these studies strongly support the general consensus among neuroscientists that neurogenesis does indeed persist throughout life in the hippocampus of humans, the existence of these neuroblast markers does not necessarily prove that these precursor cells all develop into fully mature neurons – studies have shown that newborn neurons that do not complete cell maturation and integration processes are eliminated during adult neurogenesis in mice¹⁶. Nonetheless, the results are in stark contrast to previous research which has generally found a decline in neuroblast numbers in the adult hippocampus throughout life¹¹. These findings therefore provide crucial new evidence – reigniting the age-old debate about the relevance of adult neurogenesis in humans. 

Since adult-born neurons have been found to contribute to learning and memory in animal models, it is quite feasible that this may also be true in humans. Adult neurogenesis may indeed be a fundamental part of learning and memory, with impairments in the addition, maturation or survival of newborn neurons potentially leading to memory deficits such as those seen in Alzheimer’s disease and other forms of dementia. 

While these findings are the first to present direct evidence suggesting that neurogenesis does indeed continue throughout life in humans, research in the field is still in its infancy. Nevertheless, the results raise a host of questions regarding their potential to pave the way for new treatments for neurodegenerative diseases. What if we could isolate and therapeutically target a particular pathway or pathways involved in the regulation of adult neurogenesis in the hippocampus? Could this lead to improvements in memory in those suffering from Alzheimer’s and other dementias, potentially slowing or preventing the progression of the disease – or perhaps even offer a way of staving off the natural decline in memory due to ageing in healthy individuals? Future research will inevitably seek to answer these questions. 

On a final note – numerous studies have shown that both exercise and sleep can significantly increase neurogenesis in mice, as well as stave off the natural decline due to aging^17–20. Yet another reason to get more of both! 

References:

1. Winblad, B. et al. Defeating Alzheimer’s disease and other dementias: a priority for European science and society. The Lancet Neurology 15, 455–532 (2016).

2. Chan, K. Y. et al. Epidemiology of Alzheimer’s disease and other forms of dementia in China, 1990–2010: a systematic review and analysis. The Lancet 381, 2016–2023 (2013).

3. Laplagne, D. A. et al. Functional Convergence of Neurons Generated in the Developing and Adult Hippocampus. PLOS Biology 4, e409 (2006).

4. Gonçalves, J. T., Schafer, S. T. & Gage, F. H. Adult Neurogenesis in the Hippocampus: From Stem Cells to Behavior. Cell 167, 897–914 (2016).

5. Miller, B. R. & Hen, R. The current state of the neurogenic theory of depression and anxiety. Current Opinion in Neurobiology 30, 51–58 (2015).

6. Braak, H. & Braak, E. [Morphology of Alzheimer disease]. Fortschr Med 108, 621–624 (1990).

7. Bergmann, O. et al. The Age of Olfactory Bulb Neurons in Humans. Neuron 74, 634–639 (2012).

8. Wang, C. et al. Identification and characterization of neuroblasts in the subventricular zone and rostral migratory stream of the adult human brain. Cell Research 21, 1534–1550 (2011).

9. Kornack, D. R. & Rakic, P. Continuation of neurogenesis in the hippocampus of the adult macaque monkey. PNAS 96, 5768–5773 (1999).

10. Eriksson, P. S. et al. Neurogenesis in the adult human hippocampus. Nature Medicine 4, 1313–1317 (1998).

11. Spalding, K. L. et al. Dynamics of Hippocampal Neurogenesis in Adult Humans. Cell 153, 1219–1227 (2013).

12. Sorrells, S. F. et al. Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults. Nature 555, 377–381 (2018).

13. Moreno-Jiménez, E. P. et al. Adult hippocampal neurogenesis is abundant in neurologically healthy subjects and drops sharply in patients with Alzheimer’s disease. Nature Medicine 25, 554 (2019).

14. Jin, K. et al. Increased hippocampal neurogenesis in Alzheimer’s disease. PNAS 101, 343–347 (2004).

15. Li, B. et al. Failure of Neuronal Maturation in Alzheimer Disease Dentate Gyrus. J Neuropathol Exp Neurol 67, 78–84 (2008).

16. Tobin, M. K. et al. Human Hippocampal Neurogenesis Persists in Aged Adults and Alzheimer’s Disease Patients. Cell Stem Cell (2019). doi:10.1016/j.stem.2019.05.003

17. Sierra, A. et al. Microglia Shape Adult Hippocampal Neurogenesis through Apoptosis-Coupled Phagocytosis. Cell Stem Cell 7, 483–495 (2010).

18. Kreutzmann, J. C., Havekes, R., Abel, T. & Meerlo, P. Sleep deprivation and hippocampal vulnerability: changes in neuronal plasticity, neurogenesis and cognitive function. Neuroscience 309, 173–190 (2015).

19. Lucassen, P. J. et al. Regulation of adult neurogenesis by stress, sleep disruption, exercise and inflammation: Implications for depression and antidepressant action. European Neuropsychopharmacology 20, 1–17 (2010).

20. Vukovic, J., Colditz, M. J., Blackmore, D. G., Ruitenberg, M. J. & Bartlett, P. F. Microglia Modulate Hippocampal Neural Precursor Activity in Response to Exercise and Aging. J. Neurosci. 32, 6435–6443 (2012).

21. Clemenson, G. D. et al. Enrichment rescues contextual discrimination deficit associated with immediate shock. Hippocampus 25, 385–392 (2015).

Twitter image by Jason Snyder. 

Groundbreaking research uncovers a new function of serotonin – the epigenetic regulation of gene expression

“Our findings represent a dramatic divergence from the current dogma, which works primarily on the premise that neurotransmitters like serotonin and dopamine act solely through the activation of their membrane receptors in the brain to regulate brain cell activity” 

– Dr Ian Maze, a senior author of the research.

I have previously written about the increasingly evident role of epigenetic modifications in the regulation of gene expression, and how these post-translational modifications can lead to a number of genetic changes involved in a host of psychiatric disorders; from major depression to PTSD to schizophrenia. If you haven’t already read that article, I suggest reading it first as it will help you to understand the basis of the exciting research which follows.

Structure of euchromatin
From Sun et al. (2013)

Serotonin, also known as 5-HT, has previously been shown to form covalent bonds with proteins in the cystoplasm of neurons via interactions with an enzyme called transglutaminase 2 (TGM2), leading to a change in the activity of those proteins. However, this is the first time that serotonin has been shown to bind to proteins in the nucleus – specifically the histone proteins which wrap around DNA-containing euchromatin (the active parts of the genome from which genes are transcribed) to form nucleosomes. Simply put, if the DNA encoding a gene is wound tightly within the nucleosome, it is less likely to be expressed; however if the gene is wound less tightly, it is more likely to be expressed. There are a multitude of modifications to these histone proteins which can alter the expression of the genes contained, such as methylation, acetylation or phosphorylation; but this is the first time that serotonin – a monoamine neurotransmitter – has been shown to directly interact with the genome through a histone modification dubbed “serotonylation”.

TGM2 enables serotonylation
of H3 by 5-HT (serotonin).
From Farrelly et al. (2019).

In the current study published earlier this week in Nature, Farrelly et al. show that serotonin is able to “serotonylate” particular histones via an interaction with the enzyme TGM2, ultimately leading to changes in the expression of the particular gene contained within the structure. TGM2 was shown to be both necessary and sufficient for serotonylation at H3, although the interaction was only found to occur at H3 histones. 

Mass spectrometry following in vitro TGM2 binding assays revealed Gln5 as the specific amino acid on H3 which binds serotonin via a TGM2-dependent interaction; this was confirmed by the observation that when Gln5 was mutated to a different amino acid, this “transamidation” interaction with serotonin could not occur. That is, the glutamine found at position 5 of the H3 histone of H3K4me3 nucleosomes is the particular site at which serotonin binds via TGM2. 

This histone serotonylation process was found to be significantly apparent – and perhaps even necessary – in pathways involved in neuronal development and differentiation. Interestingly, however, the modification was even shown to occur in non-serotonergic neurons, and yet more surprisingly, in non-neuronal cells. 

This suggests that serotonin – further to its role in neurotransmission – may play a role in activating the expression of genes throughout the brain and body by directly interacting with chromatin. While specific interactions with a common transcription factor complex called TFIID were demonstrated in neuronal growth pathways, precisely how histone serotonylation activates gene expression throughout the genome has yet to be elucidated. Nonetheless, this finding dramatically alters our understanding of the role of serotonin – and perhaps other neurotransmitters such as dopamine – in the body. 

Future research will investigate whether pathological changes in serotonin transmission – such as in mood disorders or as a result of the pharmacological action of drugs such as SSRIs – could interfere with this newly discovered role of serotonin. Dr. Maze’s laboratory is also investigating the role of other monoamine neurotransmitters, such as dopamine, in such histone modifications.

Source:

Farrelly, L.A., Thompson, R.E., Zhao, S., Lepack, A.E., Lyu, Y., Bhanu, N.V., Zhang, B., Loh, Y.-H.E., Ramakrishnan, A., Vadodaria, K.C., et al. (2019). Histone serotonylation is a permissive modification that enhances TFIID binding to H3K4me3. Nature. Available at: https://doi.org/10.1038/s41586-019-1024-7.

Window installed into a live brain

For the first time, we can peer through a glass window into a live brain and see the individual neurons up close.

What if we had a glass window into the brain that lets us look inside? For the first time ever, a team of physicists, chemists and biologists has done just that. Led by a microscopy pioneer, they peered into a living mouse's brain using powerful technology.

"You can look into the brain and see a true neuron in action," said physicist Stefan Hell, who leads the Max Planck Institute of Biophysical Chemistry's Department of NanoBiophotonics. His team's achievement is described in the latest issue of the journal Science.

Hell is well-known in the field for inventing a super-resolution "stimulated emission depletion" or STED microscope in the 1990s that can distinguish among features in living samples on a scale so small that general wisdom said it would be impossible.

With that microscope, Hell and his colleagues at Max Planck can discern features down to 70 nanometers in the living brain -- four times beyond what had been the physical limit.

An electron microscope can show powerful levels of detail, but only on dead cells mounted and prepared just so. Recently Hell's team took a live mouse that had been genetically modified so its neurons produce a fluorescent agent. They placed the mouse under anesthesia, opened its skull, and replaced part of the bone with a glass window.

Then, the STED microscope lens was attached to the window so light could be focused on an upper layer of the live mouse's brain. Operating almost like an ultra-precise spotlight, the microscope only illuminated individual neurons carrying the fluorescent marker. All the other cells were dark, letting the neurons shine. (The mouse survived the procedure.)

The resulting images from the live brain have an unprecedented level of clarity, Hell said. Little protrusions with thin necks and a cup-like shape at the end can be seen on the neurons. These "dendritic spines" are the input, the place where a neuron receives signals from a synapse.

Since their technique requires a transgenic animal whose neurons fluoresce, Hell said the plan isn't to make such a window for any human brains. However, there is potential to use this approach for research into treating or even preventing certain neurological diseases.

"I think we can learn a lot about what's going wrong, for example, at a synapse in certain cases," Hell said. "That door is now slammed wide open because one can access a level of functionality, a level of detail that is really critical."

Next, Hell said he and his colleagues want to help neuroscientists use their method to learn about brain functions that are still poorly understood. They would also like scientists to advance research into potential treatments by studying malfunctioning synapses in live animal brains. As physicists, he added, his team will work on producing sharper images.

From Discovery News

New Fluorescent Dyes Highlight Neuronal Activity

Researchers at the University of California, San Diego School of Medicine have created a new generation of fast-acting fluorescent dyes that optically highlight electrical activity in neuronal membranes. The work is published in this week’s online Early Edition of the Proceedings of the National Academy of Sciences.

The ability to visualize these small, fast-changing voltage differences between the interior and exterior of neurons – known as transmembrane potential – is considered a powerful method for deciphering how brain cells function and interact.

However, current monitoring methods fall short, said the study’s first author Evan W. Miller, a post-doctoral researcher in the lab of Roger Tsien, PhD, Howard Hughes Medical Institute investigator, UC San Diego professor of pharmacology, chemistry and biochemistry and 2008 Nobel Prize co-winner in chemistry for his work on green fluorescent protein.

“The most common method right now monitors the movement of calcium ions into the cell,” said Miller. “It provides some broad indication, but it’s an indirect measurement that misses activity we see when directly measuring voltage changes.”

 

The new method employs dyes that penetrate only the membrane of neurons, either in in vitro cells cultured with the dye or, for this study, taken up by neurons in a living leech model. When the dyed cells are exposed to light, neuronal firing causes the dye momentarily to glow more brightly, a flash that can be captured with a high-speed camera.

“One of the tradeoffs with using voltage-sensing dyes in the past is that when they were reasonably sensitive to voltage changes, they were slow compared to the actual physiological events,” said Miller. “The new dye gives big signals but is much faster and doesn’t perturb the neurons. We essentially see no lag time between the optical signal and electrodes (used to double-check neuronal activity).”

The new method provides a wider view of neuronal activity, said Miller. More importantly, it makes it possible for neuroscientists to do accurate, single trial experiments. “Right now, you have to repeat experiments with cells, and then average the results, which is physiologically less relevant and meaningful.”

For Tsien, the new dyes address a career-long challenge.
“These results are the first demonstration of a new mechanism to sense membrane voltage, which is particularly satisfying to me because this was the first problem I started working on as a graduate student in 1972, with little success back then,” said Tsien. “Later, we devised indirect solutions such as calcium imaging or dyes that gave big but slow responses to voltage. These techniques have been very useful in other areas of biology or in drug screening, but didn’t properly solve the original problem. I think we are finally on the right track, four decades later.”