The Epigenetics Revolution (36 page)
Read The Epigenetics Revolution Online
Authors: Nessa Carey
Tags: #Science/Life Sciences/Genetics and Genomics
The later group used a mouse model of early-life stress. In this model, baby mice were taken away from their mothers for three hours a day, for the first ten days of their lives. Just like the baby rats that hadn’t been licked or groomed much, these babies developed into ‘high-stress’ adults. Cortisol levels were increased in these mice, especially in response to mild stress, just like the relatively neglected rats.
Figure 12.2
Strong nurturing of baby rats sets up a cascade of molecular events that result in increased expression of the cortisol receptor in the brain. This increased expression makes the brain very effective at responding to cortisol and down-regulating stress responses via the negative feedback loop described in
Figure 12.1
.
Strong nurturing of baby rats sets up a cascade of molecular events that result in increased expression of the cortisol receptor in the brain. This increased expression makes the brain very effective at responding to cortisol and down-regulating stress responses via the negative feedback loop described in
Figure 12.1
.
The researchers working on the mice studied the arginine vasopressin gene. Arginine vasopressin is secreted by the hypothalamus, and stimulates secretion from the pituitary. It is shown in
Figure 12.1
. The stressed-out mice, those that had suffered separation from their mothers in early life, had decreased DNA methylation of the arginine vasopressin gene. This resulted in increased production of arginine vasopressin, which stimulated the stress response
7
.
Figure 12.1
. The stressed-out mice, those that had suffered separation from their mothers in early life, had decreased DNA methylation of the arginine vasopressin gene. This resulted in increased production of arginine vasopressin, which stimulated the stress response
7
.
The rat and mouse experimental studies show us two important things. The first is that when early life events lead to adult stress, there is probably more than one gene involved. Both the cortisol receptor gene and the arginine vasopressin gene can contribute to this phenotype in rodents.
Secondly, the studies also show us that a particular class of epigenetic modification is not in itself good or bad. It’s where the modification happens that matters. In the rat model, the decreased DNA methylation of the cortisol receptor gene is a ‘good’ thing. It leads to increased production of this receptor, and a general dampening down of the stress response. In the mouse model, the decreased DNA methylation of the arginine vasopressin gene is a ‘bad’ thing. It leads to increased expression of this hormone and a stimulation of the stress response.
The decreased DNA methylation of the arginine vasopressin gene in the mouse model occurred through a different route to the one used in the rat hippocampus to activate the cortisol receptor gene.
In the mouse studies, separation from the mother triggered activity of the neurons in the hypothalamus. This set off a signalling cascade that affected the MeCP2 protein. MeCP2 is the protein we met in
Chapter 4
, which binds to methylated DNA and helps repress gene expression. It’s also the gene which is mutated in Rett syndrome, the devastating neurological disorder. Adrian Bird has shown that the MeCP2 protein is incredibly highly expressed in neurons
8
.
Chapter 4
, which binds to methylated DNA and helps repress gene expression. It’s also the gene which is mutated in Rett syndrome, the devastating neurological disorder. Adrian Bird has shown that the MeCP2 protein is incredibly highly expressed in neurons
8
.
Normally, MeCP2 protein binds to the methylated DNA at the arginine vasopressin gene. But in the stressed baby mice, the signalling cascade mentioned in the previous paragraph adds a small chemical group called a phosphate to the MeCP2 protein and because of this MeCP2 falls off the arginine vasopressin gene. One of the important roles of MeCP2 is attracting other epigenetic proteins to where it is bound on a gene. These are proteins that all cooperate to add more and more repressive marks to that region of the genome. When the phosphorylated MeCP2 falls off the arginine vasopressin gene, it can no longer recruit these different epigenetic proteins. Because of this, the chromatin loses it repressive marks. Activating modifications get put on instead, such as high levels of histone acetylation. Ultimately, even the DNA methylation is permanently lost.
Amazingly this all happens in the mice in the first ten days after birth. After that, the neurons essentially lose their plasticity. The DNA methylation pattern that’s in place at the end of this stage becomes the stable pattern at this location. If the DNA methylation levels are low, this will normally be associated with abnormally high expression of the arginine vasopressin gene. In this way, the early life events trigger epigenetic changes which get effectively ‘stuck’. Because of this, the animal continues to be highly stressed, with abnormal hormone production, long after the initial stress has vanished. Indeed, the response continues long after the animal would even normally ‘care’ about whether or not it has its mother’s company. After all, mice are not renowned for hanging about to look after their ageing parents.
In the depths
Researchers are gradually gathering data that suggest some of the changes seen in the rodent models of early stress may be relevant in humans. As mentioned earlier, there are logistical, but more importantly ethical, issues which make it impossible to perform the same kinds of studies in people. Even so, some intriguing correlations are emerging.
The original work in the rat model was carried out by Professor Michael Meaney at McGill University in Montreal. His group subsequently performed some interesting studies on human brain samples from individuals who had, sadly, committed suicide. The group analysed the levels of DNA methylation at the cortisol receptor gene in the hippocampus from these cases. Their data showed that the DNA methylation tended to be higher in the samples from people who had had a history of early childhood abuse or neglect. By contrast, the DNA methylation levels at this gene were relatively low in the suicide victims who had not had traumatic childhoods
9
. The high DNA methylation levels in the abuse victims would drive down expression of the cortisol receptor gene. This would make the negative feedback loop less efficient and raise the circulating levels of cortisol. This was consistent with the findings from the rat work, where the stressed-out animals from the less nurturing mothers had high levels of DNA methylation at the cortisol receptor gene in the hippocampus.
9
. The high DNA methylation levels in the abuse victims would drive down expression of the cortisol receptor gene. This would make the negative feedback loop less efficient and raise the circulating levels of cortisol. This was consistent with the findings from the rat work, where the stressed-out animals from the less nurturing mothers had high levels of DNA methylation at the cortisol receptor gene in the hippocampus.
Of course, it isn’t just people who have had abusive childhoods who develop mental illnesses. The global figures for depression are startling. The World Health Organisation estimates that over 120 million people worldwide are affected by depression. Depression-related suicides have reached 850,000 per annum and depression is predicted to become the second greatest contributor to the global disease burden by 2020
10
.
10
.
Effective treatment for depression took a big step forwards in the early 1990s with the licensing by the US Food and Drug Administration of a class of drugs called SSRIs – selective serotonin re-uptake inhibitors. Serotonin is a neurotransmitter molecule – it conveys signals between neurons. Serotonin is released in the brain in response to pleasurable stimuli; it’s the feel-good molecule that we met in our happy rat babies. The levels of serotonin are low in the brains of people suffering from depression. SSRI drugs raise the levels of serotonin in the brain.
It makes sense that drugs that cause an increase in serotonin levels would be useful in treating depression. But there’s something odd about their action. The serotonin levels in the brain rise quite quickly when patients are treated with the SSRI drugs. But it usually takes at least four to six weeks before the terrible symptoms of severe depression begin to lift.
This suggests that there is more to depression than simply a drop in the levels of a single chemical in the brain, which perhaps isn’t that surprising. It’s very unusual for depression to happen overnight – it’s not like coming down with the flu. There’s now a reasonable amount of data showing that there are much longer-term changes in the brain as depression develops. These include alterations in the numbers of contacts that neurons make with each other. This in turn is critically dependent on the levels of chemicals called neurotrophic factors
11
. These chemicals support healthy survival and function of brain cells.
11
. These chemicals support healthy survival and function of brain cells.
Researchers in the depression field have moved away from a simple model based on levels of neurotransmitters and into a more complex network system. This involves sophisticated interactions between neuronal activity and a whole range of other factors. These include stress, production of neurotransmitters, effects on gene expression and longer-term consequences for neurons and how they interact with each other. While this system is in balance, the brain functions healthily. If the system moves out of balance, this complicated network begins to unravel. This moves the brain’s biochemistry and function further away from health and closer to dysfunction and disease.
Scientists are beginning to focus their attention in this field on epigenetics, because of its potential to create and sustain long-lasting patterns of gene expression. Rodents are the most common model system for these investigations. Because a mouse or a rat can’t tell you how it’s feeling, researchers have created certain behavioural tests that are used to model different aspects of human depression.
We all recognise that different people seem to respond to stress in different ways. Some people seem fairly robust. Others can react really badly to the same stressful situation, even developing depression. Mice from different inbred strains are like this as well. Researchers exposed two different strains to mildly stressful stimuli. After the stressful situation, the researchers assessed the behaviour of the mice in some of the tests which mimic certain aspects of human depression. One strain was relatively non-anxious, whereas the other was relatively anxious. These strains were called B6 and BALB, but we’ll called them ‘chilled’ and ‘jumpy’, respectively, for convenience.
The researchers focused their studies on a region of the brain called the nucleus accumbens. This region plays a role in various emotionally important brain functions. These include aggression, fear, pleasure and reward. The researchers analysed the expression of various neurotrophic factors in the nucleus accumbens. The one that gave the most interesting results was a gene called
Gdnf
(
g
lial cell-
d
erived
n
eurotrophic
f
actor).
Gdnf
(
g
lial cell-
d
erived
n
eurotrophic
f
actor).
Stress caused an increase in expression of the
Gdnf
gene in the chilled mice. In the jumpy strain it caused a decrease in expression of the same gene. Now, different inbred strains of mice can have different DNA codes so the researchers analysed the promoter region, which controls the expression of
Gdnf
. The DNA sequence of the
Gdnf
promoter was identical in the chilled and the jumpy strains. But when the scientists examined the epigenetic modifications in this promoter, they found a difference. The histones of the jumpy mice had fewer acetyl groups than the histones of the chilled mice. As we’ve seen, low levels of histone acetylation are associated with low levels of gene expression, so this tied up well with the decreased
Gdnf
expression in the jumpy mice.
Gdnf
gene in the chilled mice. In the jumpy strain it caused a decrease in expression of the same gene. Now, different inbred strains of mice can have different DNA codes so the researchers analysed the promoter region, which controls the expression of
Gdnf
. The DNA sequence of the
Gdnf
promoter was identical in the chilled and the jumpy strains. But when the scientists examined the epigenetic modifications in this promoter, they found a difference. The histones of the jumpy mice had fewer acetyl groups than the histones of the chilled mice. As we’ve seen, low levels of histone acetylation are associated with low levels of gene expression, so this tied up well with the decreased
Gdnf
expression in the jumpy mice.
This led the scientists to wonder what had happened in the neurons of the nucleus accumbens. Why had the levels of histone acetylation dropped at the
Gdnf
gene in the jumpy mice? The scientists examined the levels of the enzymes that add or remove acetyl groups from histones. They found only one difference between the two strains of mice. A specific histone deacetylase (member of the class of proteins which removes acetyl groups) called Hdac2 was much more highly expressed in the neurons of the jumpy mice
12
, compared with the chilled out mice.
Gdnf
gene in the jumpy mice? The scientists examined the levels of the enzymes that add or remove acetyl groups from histones. They found only one difference between the two strains of mice. A specific histone deacetylase (member of the class of proteins which removes acetyl groups) called Hdac2 was much more highly expressed in the neurons of the jumpy mice
12
, compared with the chilled out mice.
Other researchers tested mice in a different model of depression, called social defeat. In these experiments, mice are basically humiliated. They’re put in an environment where they can’t get away from a bigger, scarier mouse, although they are removed before they come to any physical harm. Some mice find this really stressful; others seem to brush it off.
In the experiments adult mice underwent ten days of social defeat. At the end of this they were classified as either susceptible or resistant, depending on how well they bounced back from the experience. Two weeks later the mice were examined. The resistant mice had normal levels of corticotrophin-releasing hormone. This is the chemical released by the hypothalamus. It’s the one which ultimately stimulates the production of cortisol, the stress hormone. The susceptible mice had high levels of corticotrophin-releasing hormone and low levels of DNA methylation at the promoter of this gene. This was consistent with the high levels of expression from this gene. They also had low levels of Hdac2, and high levels of histone acetylation, which again fits with over-expression of the corticotrophin-releasing hormone
13
.
13
.
It might seem odd that in one model system Hdac2 levels went up in the susceptible mice, whereas in another they went down. But it’s important with all these epigenetic events to remember that context is everything. There isn’t just one way in which Hdac2 levels (or those of any other epigenetic gene, for that matter) are controlled. The control will depend on the region of the brain and the precise signalling pathways that are activated in response to a stimulus.
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