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.
There’s more evidence supporting a significant role for epigenetics in responses to stress. The naturally jumpy B6 mice were the ones with the increased expression of Hdac2 in the nucleus accumbens, and decreased expression from the Gdnf gene. We can treat these mice with SAHA, the histone deacetylase inhibitor. SAHA treatment leads to increased acetylation of the Gdnf promoter. This is associated with increased expression of the Gdnf gene. The crucial finding is that the treated mice stop being jumpy and become chilled instead[217] – changing the histone acetylation levels of the gene changed the mouse’s behaviour. This supports the idea that histone acetylation is really important in modulating the responses of these mice to stress.
One of the tests used to investigate how depressed the mice become in response to stress is called the sucrose-preference test. Normal happy mice love sugared water, but when they are depressed they aren’t so interested in it. This decreased response to a pleasant stimulus is called anhedonia. It seems to be one of the best surrogate markers in animals for human depression[218]. Most people who have been severely depressed talk about losing interest in all the things that used to make life joyful before they became ill. When the stressed mice were treated with SSRI anti-depressants, their interest in the sugared water gradually increased. But when they were treated with SAHA, the HDAC inhibitor, they regained their interest in their favourite drink much faster[219].
It’s not just in the jumpy or chilled mice that histone deacetylase inhibitors can change animal behaviour. It’s also relevant to the baby rats who don’t get much maternal licking and grooming. These are the ones that normally grow up to be chronically stressed, with over-activation of the cortisol production pathway. If these ‘unloved’ animals are treated with TSA, the first histone deacetylase inhibitor to be identified, they grow up much less stressed. They react much more like the animals who received lots of maternal care. The levels of DNA methylation at the cortisol receptor gene in the hippocampus go down, increasing expression of the receptor and improving the sensitivity of the all-important negative feedback loop. This is presumed to be because of cross-talk between the histone acetylation and DNA methylation pathways[220].
In the social defeat model in mice, the susceptible animals were treated with an SSRI anti-depressant drug. After three weeks of treatment, their behaviour was much more like that of the resilient mice. But treatment with this anti-depressant drug didn’t just result in increased levels of serotonin in the brain. The anti-depressant treatment also led to increased DNA methylation at the promoter of the corticotrophin-releasing hormone.
These studies are all very consistent with a model where there is cross-talk between the immediate signals from the neurotransmitters, and the longer-term effects on cell function mediated by epigenetic enzymes. When depressed patients are treated with SSRI drugs, the serotonin levels in the brain begin to rise, and signal more strongly to the neurons. The animal work described in the last paragraph suggests that it takes a few weeks for these signals to trigger all the pathways that ultimately result in the altered pattern of epigenetic modifications in the cells. This stage is essential for restoring normal brain function.
Epigenetics is also a reasonable hypothesis to explain another interesting but distressing feature of severe depression. If you have suffered from depression once, you are at a significantly higher risk than the general population of suffering from it again at some time in the future. It’s likely that some epigenetic modifications are exceptionally difficult to reverse, and leave the neurons primed to be more vulnerable to another bout.
So far, so good. Everything looks very consistent with our theory about life experiences having sustained and long-lasting effects on behaviour, through epigenetics. And yet, here’s the thing: this whole area, sometimes called neuro-epigenetics, is probably the most scientifically contentious field in the whole of epigenetic research.
To get a sense of just how controversial, consider this. We’ve met Professor Adrian Bird in this book before. He is acknowledged as the father of the DNA methylation field. Another scientist with a very strong reputation in the science behind DNA methylation is Professor Tim Bestor from Columbia University Medical Center in New York. Adrian and Tim are about the same age, of similar physical type, and both are thoughtful and low key in conversation. And they seem to disagree on almost every issue in DNA methylation. Go to any conference where they are both scheduled in the same session and you are guaranteed to witness inspiring and impassioned debate between the two men. Yet the one thing they both seem to agree on publicly is their scepticism about some of the reports in the neuro-epigenetics field[221].
There are three reasons why they, and many of their colleagues, are so sceptical. The first is that many of the epigenetic changes that have been observed are relatively small. The sceptics are unconvinced that such small molecular changes could lead to such pronounced phenotypes. They argue that just because the changes are present, it doesn’t mean they’re necessarily having a functional effect. They worry that the alterations in epigenetic modifications are simply correlative, not causative.
The scientists who have been investigating the behavioural responses in the different rodent systems counter this by arguing that molecular biologists are too used to quite artificial experimental models, where they can study extensive molecular changes with very on-or-off read-outs. The behaviourists suspect that this has left molecular biologists relatively inexperienced at interpreting real-world experiments, where the read-outs tend to be more ‘fuzzy’ and prone to greater experimental variation.
The second reason for scepticism lies in the very localised nature of the epigenetic changes. Infant stress affects specific regions of the brain, such as the nucleus accumbens, and not other areas. Epigenetic marks are only altered at some genes and not others. This seems less of a reason for scepticism. Although we refer to ‘the brain’, there are lots of highly specialised centres and regions within this organ, the product of hundreds of millions of years of evolution. Somehow, all these separate regions are generated and maintained during development and beyond, and thus are clearly able to respond differently to stimuli. This is also the case for all our genes, in all our tissues. It’s true that we don’t really know how epigenetic modifications can be targeted so precisely, or how the signalling from chemicals like neurotransmitters leads to this targeting. But we know that similarly specific events occur during normal development – so why not during abnormal periods of stress or other environmental disturbances? Just because we don’t know the mechanism for something, it doesn’t mean it doesn’t happen. After all, John Gurdon didn’t know how adult nuclei were reprogrammed by the cytoplasm of eggs, but that didn’t mean his experimental findings were invalid.
218
For a useful review of animal models of depression, see Nestler and Hyman (2010),