If you are tempted to blame (or credit) Mom for making you the person you are today, your case may have just gotten stronger.
In a scientific tour de force spanning at least two decades, McGill University neurobiologist Michael Meaney has exquisitely explained how mothering styles regulate an offspring’s lifelong stress responsiveness by directly altering gene expression. The work, which Meaney presented in a captivating special lecture, adds a new chapter to the question of how early-life environment influences health throughout life.
Meaney has addressed this question on its most fundamental level, spelling out the precise cascade of molecular steps by which early environmental factors “program” gene expression and regulate the stress response. In the process, he has uncoiled what looks to be a critical pathway by which “social factors” are imprinted onto the genome.
The research is “a beautiful piece of work, in terms of tracing out the steps from behavior to lifelong changes in neurobiology in unprecedented detail,” says Stanford neurobiologist Robert Sapolsky, an expert on the neural effects of stress.
The belief that parenting styles influence what kind of adult a child becomes permeates society, Sapolsky says. A wealth of data document that negative social factors such as abuse, family strife, emotional neglect, harsh discipline, or poverty increase risk for an array of mental and physical diseases, from depression to diabetes. There is equally strong evidence that early development represents a critical period during which exposure to these sorts of stressors can be particularly damaging. But the question of how has remained open.
To find out, Meaney turned to mother rats—250 of them. Like their human counterparts, rat moms have different nurturing styles: some lick and groom their pups abundantly and make it easy for them to nurse, while others are less inclined toward these behaviors. Meaney took advantage of this natural variability in mothering style, dividing his rats into frequent and infrequent lickers. He tested the offspring of the two groups to determine any behavioral differences and found that pups of frequent lickers were more exploratory in new surroundings, less fearful, and less reactive to stressors as adults.
And the effect was reversible: a pup initially raised by an infrequent licker could “recover” if subsequently adopted by a frequent licker. This finding proved that the effect was a result not of the pup’s DNA but of the mother’s nurturing style. Importantly, it also suggested that properly timed “interventions” could overcome earlier influences.
Revealing the Scaffolding
Meaney then looked inside the brain, at the hippocampus. This area is critical to reining in the flood of stress hormones, particularly glucocorticoids, released in the “fight or flight” response but harmful to the brain if left unchecked. Meaney found that pups reared by frequent lickers had higher concentrations of “glucocorticoid receptors,” the molecular locks that trigger a compensatory braking action by the hippocampus. The more of these receptors the rat pups had in the hippocampus, the more efficient was their regulation of the stress response.
Ultimately, Meaney took the investigation right down to DNA—or, more specifically, to chromatin, the form DNA takes when it is tightly wound to its protein backbone. Chromatin acts as a gatekeeper to “transcription factors” in the cell’s nucleus that wish to send a message to the DNA, to turn a gene on, for example. To deliver the message, the densely wrapped coil of chromatin must be “opened up” at the right places to allow transcription factors to reach gene promoters and boost the gene’s expression. One way nature does this is by changing the chemical milieu that surrounds the chromatin ball. It is now clear that “nurture” does this too: for the glucocorticoid receptor gene, a mother rat’s lick does the trick.
“Frequent licking and grooming by the mother permanently alters the DNA not by changing its sequence, but by changing the chemical environment in which DNA operates,” Meaney said in his lecture. The molecular processes involved are common across all of biology, from yeast to humans, he says, and they “serve as the logical interface between a dynamic environment and a fixed genome.”
So, can we blame (or credit) our parents for how we handle stress? “Not quite,” Sapolsky says, because it is not possible to “concretely translate ratlike differences in mothering style to human mothering style.” Rather, he says, the work provides “a very tangible lesson in how much early experience matters, and how you can begin to imagine how early experience that is relevant to humans gets translated, on a molecular level, into differences in adulthood.”