According to Joseph LeDoux, “One of the most important contributions of modern neuroscience has been to show that the nature/nurture debate operates around a false dichotomy: the assumption that biology, on one hand, and lived experience, on the other, affect us in fundamentally different ways” (ref). Indeed. While I know not where the current debate stands, I’d like to point to a fantastic example of just how inextricably linked the genome is to the environment. In their recent paper, “A Biological Function for the Neuronal Activity-Dependent Component of Bdnf Transcription in the Development of Cortical Inhibition” [doi:10.1016/j.neuron.2008.09.024] Hong et al., ask what happens when you take away the ability of a given gene to respond to the environment. This is not a traditional “knockout” experiment – where the gene is inactivated from the moment of conception onwards – but rather a much more subtle type of experimental manipulation. What happens when you prevent nurture from exerting an effect on gene expression?
The team focused on the BDNF gene whose transcription can be initiated from any one of eight promoter sites (I-XIII). These sites vary in activity depending on the phase of development and/or the tissue or type of cell – all of which make for a complex set of instructions able to turn the BDNF gene on and off in precise developmental and/or tissue-specific ways. In the case of promoter IV, it appears to be triggered in the cortex in response to Ca++ release that occurs when neurons are firing – a phenomena called, “neuronal activity dependent transcription” – a top example of how the environment can influence gene expression. Seeing as how BDNF promoter IV is important for this type of environment-induced gene expression, the team asked what happens when you remove this particular promoter?
To do this, the team constructed – keep in mind that these are – mice that contain mutations in several of the Calcium (Ca++) response elements in the promoter IV region. They introduced point mutations so that the Ca++ sensitive protein CREB could not bind to the promoter and activate gene expression. OK, so what happens?
Firstly, the team reports that the mutant mice are more or less indistinguishable from controls in appearance, gait, growth rate, brain size and can also reproduce and transmit the mutations. WOW! Is that one strike AGAINST nurture? The team then shows that BDNF levels are more than 50% reduced in cultured neurons, but that levels of other immediate early genes are NOT affected (as expected). In living animals, the effects were similar when they asked how much gene expression occurs in the sensory cortex when animals are exposed to light (after an extended period of darkness). OK, so there are few effects, so far, other than lower levels of nurture-induced BDNF expression – hmmm. Looking more closely however, the team found that the mutant mice generated lower levels of inhibitory neuron activity – as measured by the firing of miniature inhibitory postsynaptic currents. Follow-on results showed that the total number of inhibitory neurons (parvalbumin and NPY + GABAergic cells) was no different than controls and so it would seem that the activity dependence of BDNF is important for the maintenance of inhibitory synapses.
Hence, the team has found that what “nurture” does (via the BDNF promoter IV in this case) is to exert an effect on the connectivity of inhibitory neurons. Wow, thanks mother nurture! Although it may seem like an obscure role for something as important as THE environment, the team points out that the relative balance of excitation-to-inhibition (yin-yang as covered here for Rett syndrome) is crucial for proper cognitive development.
To explore the notion of inhibory/excitation balance further, check out this (TED link) video lecture, where Michael Merzenich describes this imbalance as a “signal-to-noise” problem wherein some children’s brains are rather noisy (due to any number of genetic/environmental reasons – such as, perhaps, poorly maintained inhibitory connections). This can make it harder to develop and function in life. Perhaps someday, the genetic/environment research like that of Hong and colleagues will inform the rehabilitative strategies developed by Merzenich.