Tiny vesicles in breast milk carry stress signals to the infant brain

When a parent experiences an infection while breastfeeding, their body can pass biological stress signals to the newborn through tiny particles hidden in the milk. A recent study reveals that modifying the parent’s environment to be more engaging and supportive can block these stress signals, protecting the infant’s developing brain and future behavior. The research was published in the journal Molecular Psychiatry.

Breast milk provides more than simple calories and basic nutrition. It contains a massive variety of hormones, immune cells, and microscopic bubbles called extracellular vesicles. These tiny, membrane-bound packages operate like biological mail carriers. They travel safely through the harsh environment of the infant gut, enter the bloodstream, and drop off genetic material in distant tissues.

Inside these vesicles are regulatory molecules known as microRNAs. To understand microRNAs, it helps to look at how cells normally function. Your DNA acts as a master instruction manual, filled with blueprints for keeping the body alive. When a cell needs to function, it creates messenger RNA to carry those instructions to the cellular factories that build proteins.

MicroRNAs are small pieces of genetic code that act like a dimmer switch, attaching to the messenger RNA and stopping it from building proteins. Because they pause the construction phase, this process is known as post-transcriptional regulation. By delivering these microRNAs to an infant, milk vesicles can turn specific cellular functions on or off in the developing body.

Scientists understand that maternal illness during pregnancy can alter fetal brain development. Less is known about how infections during the postnatal nursing period might change the genetic messages passed through milk. Julia Martz of the Massachusetts College of Pharmacy and Health Sciences, Baila Hammer of Touro University, and a team of colleagues set out to investigate this dynamic.

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The researchers wanted to see if an immune challenge in a lactating parent alters milk composition enough to change the trajectory of an infant’s brain. They also wanted to know if a better living environment could buffer the mother and infant against these biological changes. To test this, the researchers divided lactating rats into two different living conditions. Half the animals lived in standard, plain laboratory cages, while the other half lived in enriched environments featuring extra space, climbing structures, and toys.

On the tenth day of nursing their pups, half the mothers in each housing group received an injection of a bacterial component known as lipopolysaccharide. This component is derived from Escherichia coli bacteria. It does not cause a real infection, but it tricks the body into sensing an invader, prompting temporary inflammation and mild sickness behaviors like lethargy. Researchers often use this substance because it creates a predictable, uniform immune response without the chaotic variables of a live, replicating pathogen.

The remaining mothers received a harmless saline placebo injection. Two hours later, researchers collected milk from all the experimental groups. They used a high-speed centrifuge to isolate the extracellular vesicles from the milk and sequence the genetic cargo hidden inside.

The researchers found that simulated infections heavily altered the composition of the milk. Not only did the fat content of the milk drop, but the levels of a stress hormone called corticosterone increased. Beyond basic nutrition, the immune challenge also changed the profile of the microRNAs packaged into the milk vesicles.

This effect was mostly seen in the mothers housed in standard cages. For these mothers, the immune challenge resulted in dozens of altered microRNAs compared to the healthy control group. The enriched environment provided a strong protective effect against this outcome. Sick mothers living in the complex environments saw a much less drastic shift in their milk’s genetic cargo, and their milk’s fat content remained normal.

The researchers then looked at the brains of the nursing pups, focusing specifically on the hippocampus. The hippocampus is a brain structure located deep within the temporal lobe, and it heavily regulates learning, memory, and emotional processing. Like the milk vesicles, the pups nursing from sick, standard-housed mothers showed altered microRNA profiles in their hippocampus.

In many cases, the specific microRNAs altered in the infant brain matched the ones found in the mothers’ milk. This overlap suggests that the milk vesicles may reach the brain to deposit their instructions, or at least trigger a chain reaction that shifts the infant’s brain chemistry. Just like the milk samples, pups nursing from enriched-housed mothers largely avoided these widespread genetic changes.

This genetic shift had lasting behavioral consequences for the developing animals. When the pups grew into adulthood, they underwent behavioral testing. One test involved placing the rats in an open, brightly lit arena to measure anxiety. Animals that feel anxious tend to hug the walls, while more relaxed animals will freely explore the exposed center.

A second test evaluated how much the animals preferred interacting with a new, unfamiliar rat versus an inanimate object. This kind of social preference test helps researchers measure sociability and developmental milestones in rodents. The adult offspring of sick, standard-housed mothers showed higher levels of anxiety-like behaviors in the open arena, and they also exhibited a reduced preference for socializing with other rats.

The enriched living spaces entirely prevented these adult behavioral changes. Even though the mothers in the enriched cages experienced the exact same immune challenge, their adult offspring behaved just like the healthy control groups. The researchers also noted that maternal stress hormones could not explain this behavioral rescue. Milk corticosterone levels remained high even in the enriched mothers, meaning the protected vesicle cargo was a more likely source of the behavioral buffer.

The researchers noted that this was a small study, utilizing seven or eight litters per testing condition, for a total of about thirty litters. This sample size carries a few inherent limitations. Milk composition changes constantly across the natural nursing timeline of any mammal, adapting to the growing infant’s daily needs. Because this experiment only sampled milk over a narrow two-day window in the middle of the lactation phase, it remains unknown how early or late stages of nursing might respond to immune stress.

Another limitation involves the exact physical pathway of the genetic cargo. While the researchers found matching microRNAs in the milk and the infant brains, they did not visually trace the physical journey of the vesicles. It is entirely possible the milk vesicles act indirectly in the body. For instance, the vesicles might alter the infant’s gut bacteria, which in turn send signals that influence the hippocampus.

Future studies will need to use fluorescent markers to track exactly where these maternal vesicles travel inside the offspring. Other components of the milk, such as structural fats and immune proteins, might also work alongside the microRNAs to shape infant brain development. Isolating these microscopic variables takes immense resources and time.

Despite these open questions, the findings suggest a very physical link between a caregiver’s environment and the biological quality of their care. Supporting nursing parents with better environmental conditions and reduced daily stress might do more than just improve their mood. It could directly shape the genetic instructions passed on to the next generation, building a more resilient infant brain.

The study, “Investigating milk-derived extracellular vesicles as mediators of maternal stress and environmental intervention,” was authored by Julia Martz, Baila Hammer, Tristen J. Langen, Benjamin N. Berkowitz, Benzion Berkowitz, Jasmyne A. Storm, Jueqin Lu, Deepali Lehri, Sanoji Wijenayake, Jordan Marrocco, and Amanda C. Kentner.