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. 

Forgotten Memories Brought Back in Mice? Hold Up...

A study published yesterday in Cell[1] has purportedly found that memories formed as infants may be able to be retrieved as adults using optogenetic techniques, and various media outlets have enthusiastically inferred that this suggests we may be able to remember the events from our infancy, some even suggesting we may be able to remember our own birth. 

The study, led by psychologist Paul Frankland, was based on previous research by the group which found that the “forgetting” of memories during infancy may be a result of high levels of hippocampal neurogenesis at this age – i.e. the neurons representing the memories are replaced by new neurons, thus erasing the memories. 

The aim of the current study was to determine if the memories formed during infancy are permanently lost due to a failure in encoding during infancy, or become progressively inaccessible over time due to a progressive loss in the ability to retrieve them as the mice age. 

The researchers placed young mice into a box and gave them a foot-shock, so that when they are placed back into the box they freeze in anticipation of the shock - a classic fear-based training paradigm. The mice had been engineered to contain a specific set of light-sensitive neurons in a particular region of the hippocampus involved in the formation of memories - the dentate gyrus - which allowed the researchers to activate these neurons by firing a laser at them. They then placed the same mice back into the box as adults and activated this set of neurons, thus reinstating the memory and causing the mice to freeze in anticipation of the shock. 

The authors seem to suggest that this means that some hidden traces of the memories created during infancy were retained and were able to be recalled by the researchers by optogenetically activating the specific set of hippocampal neurons which were observed to be activated during the contextual fear encoding (the “dentate gyrus encoding ensemble”), positing that the reactivation of these ensembles was sufficient for “memory recovery” in adulthood. 

They then quantified the activity of an activity-regulated gene, c-Fos, in cortical and subcortical brain regions following the fear learning event. The purpose of this was to determine whether fluorescently-tagged neurons activated during the memory encoding event were preferentially reactivated during the “memory recall” in adulthood – which of course they were, implying successful recall of the memory. 

However, while they may have been able to reactivate a neural pathway they have essentially programmed into the mice’s brains during infancy, I’m not so sure it was the actual memories themselves which were “recovered”. 

It’s worth nothing that these are memories which the researchers have created in the mice by giving them foot-shocks within a particular environmental context; they are not naturally formed memories. 

“When the infant mice were placed in the box and the laser was turned on, the animals’ memories of the electric shock returned and they froze in place.” 

The fact that the mice freeze when they are placed into the same environment in which they were given a foot-shock during infancy does not necessarily mean that they remember receiving the foot-shock. It simply means that a particular behaviour (freezing) has been programmed into their brains and artificially re-instated by firing lasers at the neurons underlying this behaviour. It means that the same neural pathways resulting in a fear-based freezing response were activated – but these may be totally separate from the pathways containing the actual memory of the event (if they even exist). Besides, we are talking about a simple, conditioned response here; an instinctual behaviour in response to pain – much like you learn to quickly move your hand away from a hot plate – not an actual subjective, detailed memory of an event. It’s possible that the mice would freeze if placed in an entirely different context and the same light-sensitive neurons artificially reactivated.

“To first induce memory formation in the animals, the scientists placed the mice in a box and gave them a mild foot shock. While young adult mice retained this memory and froze when put in the box a second time, infant mice forgot this fear-related memory after a day and behaved normally when they encountered the box again” 

Percent freezing levels declined with retention delay in P17, but
not P60, mice. From Guskjolen et al., 2018, Figure 1(B).

So the infant mice were not forming the memories? This appears to contradict the conclusions of the study – i.e. that those memories are simply hard-to-retrieve; hidden deep within the brain, unable to be recovered by natural cues, and that direct stimulation of the engram (in combination with re-exposure to the training context) may reinstate the connections, leading to memory recovery. How can we say that the memories are simply difficult to retrieve when we are unsure if they were ever formed in the first place? 

“However, we found that opto-stimulation of neural ensembles that were engaged during training was sufficient to induce conditioned freezing at the same retention delays. These results suggest that the underlying engram corresponding to the fear conditioning event is not completely overwritten. Rather, this engram presumably exists in an otherwise inaccessible, dormant state, in which “natural” reminders (such as exposure to the training context) most often do not induce successful reactivation(...) This pattern of results is reminiscent of other amnestic states, including mouse models of retrograde amnesia and Alzheimer’s disease, in which opto-stimulation of tagged encoding ensembles (but not presentation of natural cues alone) permits memory recovery." 

Again, the conclusions drawn assume that the artificially activated ensembles encode the actual memory of the event itself – which is not only not confirmed, but hard to believe considering when the mice were put back in the box the second-time as infants, they had not remembered the fear-related memory supposedly created the day before. How, then, do we know that the memory was encoded at all? How do we know it is the memory that is recalled, and not simply a programmed, artificially instated fear-response? This is too simplistic of a model to draw such far-fetched conclusions, and we certainly can’t say that this type of “forgetting” in infancy is akin to other types of amnesia such as that in Alzheimer’s disease. They are completely different processes, at completely different ages. 

Furthermore, less cortical “re-engagement” was observed following optogenetic stimulation of the dentate gyrus engrams in mice trained as infants compared to those trained as adults, further highlighting the possibility that the memories which were purportedly retrieved may not have been formed at all in the infants. 

“Indeed, whereas adult contextual fear memories are successfully consolidated over the course of weeks, equivalent infant memories are being actively forgotten during this period and therefore perhaps not successfully consolidated in the cortex (…) opto-stimulation of tagged dentate gyrus ensembles leads to recovery of an engram that is qualitatively different (and likely impoverished) compared to the equivalent representation in adult animals.” 

The authors even concede that the “memory recovery” did not persist into the light OFF periods – i.e. when the trained mice were placed into the box as adults, they did not freeze unless the hippocampal engrams engineered to be light-sensitive were activated by the researchers – a pattern which has been observed in similar studies involving reactivation of tagged engram cells in the dentate gyrus [2–7]. 

While the study further adds weight to the idea that infantile forgetting is likely due to a failure of memory encoding in the infant brain (something which we knew anyway), the methods used are simply insufficient to be able to draw some of the conclusions the authors propose, and the study certainly does not suggest that we may be able to recover our infantile memories anytime soon.

 References: 

[1] A. Guskjolen, J.W. Kenney, J. de la Parra, B.A. Yeung, S.A. Josselyn, P.W. Frankland, Recovery of “Lost” Infant Memories in Mice, Current Biology. 0 (2018). doi:10.1016/j.cub.2018.05.059.

[2] X. Liu, S. Ramirez, P.T. Pang, C.B. Puryear, A. Govindarajan, K. Deisseroth, S. Tonegawa, Optogenetic stimulation of a hippocampal engram activates fear memory recall, Nature. 484 (2012) 381–385. doi:10.1038/nature11028.

[3] T. Kitamura, S.K. Ogawa, D.S. Roy, T. Okuyama, M.D. Morrissey, L.M. Smith, R.L. Redondo, S. Tonegawa, Engrams and circuits crucial for systems consolidation of a memory, Science. 356 (2017) 73–78. doi:10.1126/science.aam6808.

[4] D.S. Roy, S. Muralidhar, L.M. Smith, S. Tonegawa, Silent memory engrams as the basis for retrograde amnesia, Proc. Natl. Acad. Sci. U.S.A. 114 (2017) E9972–E9979. doi:10.1073/pnas.1714248114.

[5] T.J. Ryan, D.S. Roy, M. Pignatelli, A. Arons, S. Tonegawa, Memory. Engram cells retain memory under retrograde amnesia, Science. 348 (2015) 1007–1013. doi:10.1126/science.aaa5542.

[6] D.S. Roy, A. Arons, T.I. Mitchell, M. Pignatelli, T.J. Ryan, S. Tonegawa, Memory retrieval by activating engram cells in mouse models of early Alzheimer’s disease, Nature. 531 (2016) 508–512. doi:10.1038/nature17172.

[7] S. Ramirez, X. Liu, P.-A. Lin, J. Suh, M. Pignatelli, R.L. Redondo, T.J. Ryan, S. Tonegawa, Creating a false memory in the hippocampus, Science. 341 (2013) 387–391. doi:10.1126/science.1239073.

Doubts regarding research suggesting that “A Very Sugary Diet Makes You Stupid”

Read the article(s):
http://www.medicalnewstoday.com/articles/245531.php
http://newsroom.ucla.edu/portal/ucla/this-is-your-brain-on-sugar-ucla-233992.aspx

I have some doubts about the conclusions reached in this research.

“As a control, the animals were fed on standard rat feed for five days before the fructose diet started. They were also trained on a maze twice per day and tested to see how well they performed. They also placed visual markers in the maze to help the rats remember their way around.”

 Gomez-Pinilla recounts his experience of testing the rats after six weeks on the sugary diet:

    “The second group of rats navigated the maze much faster than the rats that did not receive omega-3 fatty acids … The DHA-deprived animals were slower, and their brains showed a decline in synaptic activity. Their brain cells had trouble signaling each other, disrupting the rats’ ability to think clearly and recall the route they’d learned six weeks earlier.”

Maybe, rather than omega-3 fatty acids negating a negative effect of fructose on synaptic activity, omega-3 combined with fructose may have enhanced activity and protected from damage to the synapses, leading to the rats’ increased performance in the maze tests.

“Our findings suggest that consuming DHA regularly protects the brain against fructose’s harmful effects …”

The researchers appear to have arrived at the conclusion that fructose (in abundance?) may have negative effects on cognitive activity and memory. I don’t believe that the results of this experiment necessarily point to this conclusion.

Both groups of rats were fed fructose, with the second group also being fed omega-3 fatty acids in the form of flaxseed oil and docosahexaenoic acid (DHA)
There should have been a further control group which was not fed fructose at all, to compare the other two groups against. This would determine whether fructose had any effect on the rat’s brain and performance in the maze tests, prior to investigating any effect that omega-3 fatty acids may have in “negating” this effect. Instead, the researchers gave fructose solutions to both groups of rats.

The UCLA article also suggests that the first group of rats, who did not receive omega-3 fatty acids, may have developed a resistance to insulin:  

"The DHA-deprived rats also developed signs of resistance to insulin, a hormone that controls blood sugar and regulates synaptic function in the brain. A closer look at the rats’ brain tissue suggested that insulin had lost much of its power to influence the brain cells."

"He suspects that fructose is the culprit behind the DHA-deficient rats’ brain dysfunction. Eating too much fructose could block insulin’s ability to regulate how cells use and store sugar for the energy required for processing thoughts and emotions."

I believe that this is the more appropriate route for the experiment to proceed. However, it is unclear whether it is fructose itself that is responsible for the DHA-deprived rat’s lower performance, or an interaction between insulin and fructose in the absence of omega-3 fatty acids.

More research should be done to determine an effect of fructose on the rats’ brain and performance, compared against a baseline, control group of rats who are not fed fructose solutions.

Nonetheless, it is known that omega-3 fatty acids protect the brain and enhance cognitive function and memory. However, it is not correct to conclude from this article that fructose has any negative effect on the brain.