We are all familiar with the notion that genes are NOT destiny and that the development of an individual’s mind and body occur in a manner that is sensitive to the environment (e.g. children who eat lots of healthy food grow bigger and stronger than those who have little or no access to food). In the case of the brain, one of the ways in which the environment gets factored into development – is via so-called “sensitive periods” where certain parts of the brain transiently rely on sensory experience in order to develop. Children born with cataracts, for example, will have much better vision if the cataracts are removed in the first few weeks of life rather than later on. This is because the human visual system has a “sensitive period” early in development where it is extra-sensitive to visual input and, after which, the function and connectivity of various parts of the system is – somewhat permanently – established for the rest of the person’s life. Hence, if there is little visual input (cataracts) during the sensitive period, then the visual system is somewhat permanently unable to process visual information – even if the cataracts are subsequently removed. (To learn more about this topic, visit Pawan Sinha’s lab at M.I.T and his Project Prakash intervention study on childhood blindness.)
What the heck is an “in”sensitive period then? Well, whereas visual input is clearly a “good thing” for the sensitive period of visual development, perhaps some inputs are “bad” and it may be useful to shield or protect the brain from exposure. Maybe some environmental inputs are “bad” and one would not want the developing brain to be exposed to them and say, “OK, this (bad stuff) is normal“. As a parent, I am constantly telling my children that the traffic-filled street is a “bad place” and, like all parents, I would not want my children to think that it was OK to wander into the street. Clearly, I want my child to recognize the car-filled street as a “bad thing”.
In the developing brain, it turns out that there are some “bad things” that one would NOT like (the brain) to get accustomed to. Long-term exposure to glucocorticoids is one example – well-known to cause a type of neuronal remodelling in the hippocampus, that is associated with poor cognitive performance (visit Bruce McEwen’s lab at Rockefeller University to learn more about this). Perhaps an “in”sensitive period – where the brain is insensitive to glucocorticoids – is one way to teach the brain that glucocorticoids are “bad” and DO NOT get too familiar with them (such a period does actually occur during early post-natal mammalian development). Of course, we do need our brains to mount an acute stress response, if and when, we are being threatened, but it is also very important that the brain learn to TURN-OFF the acute stress response when the threat has passed – an extensive literature on the deleterious effects of chronic exposure to stress bears this out. Hence, the brain needs to learn to recognize the flow of glucocorticoids as something that needs to be shut down.
OK, so our developing brain needs to learn what/who is “good vs. bad”. Perhaps sensitive and insensitive periods help to reinforce this learning – and also – to cement learning into the system in a sort of permanent way (I’m really not sure if this is the consensus view, but I’ll try and podcast interview some of the experts here asap). In any case, in the case of the visual system, it is clear that the lack of visual input during the sensitive period has long lasting consequences. In the case of the stress response, it is also clear that if there is untoward stress early in development, one can be (somewhat) destined to endure a lifetime of emotional difficulty. Previous posts here, here, here cover research on behavioral/genomic correlates of early life stress.
Genes meet environment in the epigenome during sensitive and insensitive periods?
As stated at the outset – genes are not destiny. The DNA cannot encode a system that knows who/what is good vs. bad, but rather can only encode a system of molecular parts that can assemble to learn these contingencies on the fly. During sensitive periods in the visual system, cells in the visual system are more active and fire more profusely during the sensitive period. This extra firing leads to changes in gene expression in ways that (somewhat) permanently set the connectivity, strength and sensitivity of visual synapses. The expression of neuroligins, neurexins, integrins and all manner of extracellular proteins that stabilize synaptic connections are well-known tagets of activity-induced gene expression. Hence the environment “interacts” with the genome via neuronal firing which induces gene expression which – in turn – feeds back and modulates neuronal firing. Environment –> neuronal firing –> gene expression –> modified neuronal firing. OK.
Similarly, in the stress response system, the environment induces changes in the firing of cells in the hypothalamus which leads (through a series of intermediates) to the release of glucocorticoids. Genes induced during the firing of hypothalamic cells and by the release of glucocorticoid can modify the organism’s subsequent response to stressful events. Environment –> neuronal firing –> gene expression –> modified neuronal firing. OK.
Digging deeper into the mechanism by which neuronal firing induces gene expression, we find an interesting twist. Certainly there is a well-studied mechanism wherein neuronal firing causes Ca++ release which activates gene expression of neuroligins, neurexins, integrins and all manner of extracellular proteins that stabilize synaptic connections – for many decades. There is another mechanism that can permanently mark certain genes and alter their levels of expression – in a long-lasting manner. These are so-called epigenetic mechanisms such as DNA methylation and acetylation. As covered here and here, for instance, Michael Meaney’s lab has shown that DNA CpG methylation of various genes can vary in response to early-life stress and/or maternal care. In some cases, females who were poorly cared for, may, in turn, be rather lousy mothers themselves as a consequence of these epigenetic markings.
A new research article, “Dynamic DNA methylation programs persistent adverse effects of early-life stress” by Chris Murgatroyd and colleagues [doi:10.1038/nn.2436] explores these mechanisms in great detail. The team explored the expression of the arginine vasopressin (AVP) peptide – a gene which is important for healthy social interaction and social-stress responsivity. Among many other interesting results, the team reports that early life stress (using a mouse model) leads to lower levels of methylation in the 3rd CpG island which is located downstream in a distal gene-expression-enhancer region. In short, more early-life stress was correlated with less methylation, more AVP expression which is known to potentiate the release of glucocorticoids (a bad thing). The team reports that the methyl binding MeCP2 protein, encoded by the gene that underlies Rett syndrome, acts as a repressor of AVP expression – which would normally be a good thing since it would keep AVP levels (and hence glucocorticoid levels) down. But unfortunately, early-life stress removes the very methyl groups to which MeCP2 binds and also the team reports that parvocelluar neuronal depolarization leads to phosphorylation (on serine residue #438) of MeCP2 – a form of MeCP2 that is less accessible to its targets. So, in a manner similar to other examples, early life stress can have long-lasting effects on gene expression via an epigenetic mechanism – and disables an otherwise protective mechanism that would shield the organism from the effects of stress. Much like in the case of Rett syndrome (as covered here) it seems that when MeCP2 is bound – then it silences gene expression – which would seem to be a good thing when it comes to the case of AVP.
So who puts these epigenetic marks on chromosomes and why?
I’ll try and explore this further in the weeks ahead. One intriguing idea about why methylation has been co-opted among mammals, has to do with the idea of parent-offspring conflict. According to David Haig, one of the experts on this topic, males have various incentives to cause their offspring to be large and fast growing, while females have incentive to combat the genomic tricks that males use, and to keep their offspring smaller and more manageable in size. The literature clearly show that genes that are marked or methylated by fathers (paternally imprinted genes) tend to be growth promoting genes and that maternally imprinted genes tend to be growth inhibitors. One might imagine that maternally methylated genes might have an impact on maternal care as well.
Lastly, the growth promoting/inhibiting effects of paternal/maternal genes and gene markings is now starting to be discussed somewhat in the context of autism/schizophrenia which have have been associated with synaptic under-/over-growth, respectively.
Building a brain is already tough enough – but to have to do it amidst an eons-old battle between maternal and paternal genomes. Sheesh! More on this to come.