"People think that the placenta only serves to promote blood flow between a mom and her baby, but that's really not all it's doing," says Tracy L. Bale. (Credit: Daniel Lobo/Flickr)

Placenta can send stress signals to baby’s brain

An enzyme in the placenta appears to play a role in how stress during pregnancy can boost a baby’s risk of developing autism or schizophrenia.

Published in the Proceedings of the National Academy of Sciences, the new study shows that in mice the enzyme OGT (O-linked-N-acetylglucosamine transferase) translates maternal stress into a reprogramming signal for the brain before birth.

“OGT seems to be serving a role as the ‘canary in the coal mine,’ offering a readout of mom’s stress to change the baby’s developing brain,” says Tracy L. Bale, senior author on the paper and a professor in the animal biology department at University of Pennsylvania School of Veterinary Medicine.

OGT is known to play a role in gene expression through chromatin remodeling, a process that makes some genes more or less available to be converted into proteins.

Placenta tampering

In a study published last year in PNAS, Bale’s lab found that placentas from male mice pups had lower levels of OGT than those from female pups, and placentas from mothers that had been exposed to stress early in gestation had lower overall levels of OGT than placentas from the mothers’ unstressed counterparts.

“People think that the placenta only serves to promote blood flow between a mom and her baby, but that’s really not all it’s doing,” Bale says. “It’s a very dynamic endocrine tissue and it’s sex-specific, and we’ve shown that tampering with it can dramatically affect a baby’s developing brain.”

To elucidate how reduced levels of OGT might be transmitting signals through the placenta to a fetus, Bale and Christopher L. Howerton, a postdoctoral researcher and coauthor of the study, bred mice that partially or fully lacked OGT in the placenta.

They then compared these transgenic mice to animals that had been subjected to mild stressors during early gestation, such as predator odor, unfamiliar objects, or unusual noises, during the first week of their pregnancies.

Disrupted testosterone

The researchers performed a genome-wide search for genes that were affected by the altered levels of OGT and were also affected by exposure to early prenatal stress using a specific activational histone mark and found a broad swath of common gene expression patterns.

They chose to focus on one particular differentially regulated gene called Hsd17b3, which encodes an enzyme that converts androstenedione, a steroid hormone, to testosterone. The gene is particularly interesting in part because neurodevelopmental disorders such as autism and schizophrenia have strong gender biases, where they either predominantly affect males or present earlier in males.

Placentas associated with male mice pups born to stressed mothers had reduced levels of the enzyme Hsd17b3, and, as a result, had higher levels of androstenedione and lower levels of testosterone than normal mice.

“This could mean that, with early prenatal stress, males have less masculinization,” Bale says. “This is important because autism tends to be thought of as the brain in a hypermasculinized state, and schizophrenia is thought of as a hypomasculinized state. It makes sense that there is something about this process of testosterone synthesis that is being disrupted.”

Slower growth at weaning

Furthermore, the mice born to mothers with disrupted OGT looked like the offspring of stressed mothers in other ways. Although they were born at a normal weight, their growth slowed at weaning. Their body weight as adults was 10 to 20 percent lower than control mice.

Because of the key role that that the hypothalamus plays in controlling growth and many other critical survival functions, researchers then screened the mouse genome for genes with differential expression in the hypothalamus, comparing normal mice, mice with reduced OGT and mice born to stressed mothers.

They identified several gene sets related to the structure and function of mitochrondria, the powerhouses of cells that are responsible for producing energy. And indeed, when compared by an enzymatic assay that examines mitochondria biogenesis, both the mice born to stressed mothers and mice born to mothers with reduced OGT had dramatically reduced mitochondrial function in their hypothalamus compared to normal mice.

Such reduced function could explain why the growth patterns of mice appeared similar until weaning, at which point energy demands go up.

Can’t keep up

“If you have a really bad furnace you might be okay if temperatures are mild,” Bale says. “But, if it’s very cold, it can’t meet demand. It could be the same for these mice. If you’re in a litter close to your siblings and mom, you don’t need to produce a lot of heat, but once you wean you have an extra demand for producing heat. They’re just not keeping up.”

Mitochondrial dysfunction in the brain has been reported in both schizophrenia and autism patients.

In future work, Bale hopes to identify a suite of maternal plasma stress biomarkers that could signal an increased risk of neurodevelopmental disease for the baby.

“With that kind of a signature, we’d have a way to detect at-risk pregnancies and think about ways to intervene much earlier than waiting to look at the term placenta.”

Narayan Avadhani collaborated on the study, which received support from the National Institute of Mental Health.

Source: University of Pennsylvania

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