Nanoplastics cause abnormal branch growth in neurons

Tiny pieces of plastic can enter brain cells and alter their physical development, with the smallest particles causing the most noticeable changes. New research reveals that while low levels of microscopic polystyrene plastics do not kill brain cells or stop them from communicating, particles measuring just 50 nanometers wide prompt nerve cell branches to grow abnormally long. These findings were published in the journal NanoImpact, raising new questions about how environmental plastic pollution might affect neurological health over time.

Global plastic production continues to climb every year, generating massive amounts of waste that eventually breaks down into microscopic fragments. These fragments can enter the human body through the water we drink, the food we eat, and the air we breathe. Once inside, these tiny particles travel through the bloodstream and can lodge in various organs, including the lungs, liver, and kidneys.

Recent research has revealed that plastic particles can also cross the blood-brain barrier. This barrier is a highly selective border of cells that usually protects the brain from harmful substances circulating in the blood. Finding plastic within brain tissue has sparked widespread concern about potential neurological risks. This discovery prompted researchers to investigate exactly how these synthetic materials interact with delicate brain cells.

Most prior laboratory tests on plastic toxicity used exceptionally high doses or large plastic particles. Scientists often tested these massive doses on robust, immortalized cancer cell lines rather than normal brain tissue. This approach left a large gap in our understanding of how realistic amounts of small plastics might affect healthy, developing brain networks. To address this blind spot, a team of researchers from the University of Eastern Finland designed an experiment to observe the effects of low doses of microscopic plastics on highly sensitive brain cells.

Veronika Górová, a doctoral researcher at the A.I. Virtanen Institute for Molecular Sciences, led the study. Górová and her colleagues focused their efforts on understanding how the sheer physical size of a plastic fragment changes its biological impact. They hypothesized that tinier particles would be absorbed more easily by cells, leading to more distinct biological changes than their larger counterparts.

The researchers chose to study primary cortical neurons, which are specialized cells taken directly from the outer layer of the brain of fetal mice. Neurons are the primary messengers of the nervous system, using electrical and chemical signals to process information and control the body. By using fresh cells rather than immortalized lab strains, the team created a model that more closely mimics how a living brain might react to foreign materials.

To test their hypothesis, the team exposed these neurons to tiny spheres made of polystyrene, a very common type of plastic used in everything from food packaging to building insulation. They used particles in three extremely small sizes: 50 nanometers, 100 nanometers, and 250 nanometers in diameter. For perspective, a human hair is roughly 80,000 to 100,000 nanometers wide, making even the largest of these tested plastics entirely invisible to the naked eye.

The neurons were submerged in liquid containing these plastic spheres for 24 hours. The researchers intentionally kept the concentration of the plastics low. They wanted to simulate a more realistic environmental exposure and observe subtle changes in the cells, rather than simply poisoning the neurons with an overwhelming amount of foreign material.

After the exposure period, the team used advanced microscopes to look inside the neurons. They successfully observed the 250-nanometer plastic pieces accumulating inside the bodies of the brain cells. The team noted that as the concentration of the plastic increased in the surrounding liquid, the amount of plastic absorbed by the cells also increased.

The microscopes used in the study could not clearly visualize the 50-nanometer pieces due to their incredibly small size. However, the researchers suspected these tiny pieces were also entering the cells. To determine if the plastics were harming the basic survival of the neurons, the researchers performed a test to measure the metabolic health of the cells.

They found that these low doses did not impair the basic survival or metabolic function of the neurons. The cells continued to process energy normally, showing no signs of dying off. It was only when the researchers applied extremely high doses of the plastics, far above their intended test range, that the neurons began to show signs of damage and reduced survival rates.

The team then investigated whether the tiny plastics affected the physical shape of the cells. Neurons grow long, thin extensions called neurites, which eventually become the wiring that connects different parts of the brain together. Proper neurite growth is an essential part of brain development and learning.

Using specialized imaging software, the researchers measured the length of these branches after the plastic exposure. They discovered that neurons exposed to the 50-nanometer plastics grew longer branches than those exposed to clear liquid. The cells exposed to the larger 100-nanometer and 250-nanometer plastics did not show this abnormal branch lengthening.

To understand what was happening at a deeper level, the team examined the neuronal transcriptome. The transcriptome is the complete set of genetic instructions, or RNA molecules, that a cell is actively reading and using at any given time. By looking at these instructions, scientists can see which genes a cell is turning on or off in response to stress.

The genetic analysis revealed subtle alterations in the cells exposed to the 50-nanometer plastics. The researchers found changes in the activity of genes known to control nerve branch growth and cell development. For instance, a specific gene associated with extending nerve branches, which relies on calcium to function, was highly active. This genetic shift matched the physical branch lengthening they had seen under the microscope.

Conversely, the larger 250-nanometer plastics did not cause these same genetic shifts. “It is important to understand that not only the concentration and material, but also the size of the particles matters,” Górová said in a press release. “With decreasing nanoparticle size, we observed more pronounced, although still relatively subtle changes.”

Finally, the scientists checked to see if the plastics disrupted the electrical communication between the neurons. They placed the cells on microscopic sensor plates capable of detecting the tiny electrical sparks neurons use to talk to one another. After monitoring the cells for an entire day following the plastic exposure, the team saw no changes in the firing rate or the strength of the electrical signals.

The results from the electrical tests were not statistically significant, meaning the plastic did not reliably alter the cells’ communication abilities. The brain cells maintained their normal chatter despite the presence of the foreign material. This suggests that while the smallest plastics change the physical structure and genetic reading of the cells, they do not immediately shut down the brain’s basic electrical network.

While this study offers a detailed look at how microscopic plastics interact with individual brain cells, the researchers noted several limitations to their work. The experiment involved growing isolated nerve cells in a dish, which lacks the protective barriers and complex interactions found in a complete, living brain. The human brain contains multiple types of support cells that might help clear away foreign materials or react differently to the plastics.

Additionally, the laboratory exposure only lasted for 24 hours. In the real world, humans and animals are exposed to a continuous, lifelong stream of environmental plastics. The researchers point out that a brief exposure in a lab setting cannot fully replicate the cumulative effects of decades of plastic accumulation in the human body.

The team also focused entirely on polystyrene. While polystyrene is a heavily researched material, it is just one of many different types of plastics polluting the environment. Future studies will need to test other common materials, such as polyethylene, to see if different chemical makeups trigger different reactions in nerve cells.

The researchers plan to continue exploring how these materials influence neurological health over longer periods. “In the future it would be interesting to have a look at the effects with more complex models and prolonged exposures, to get closer to the real-world scenario,” Górová said. By slowly building more realistic models, the scientific community hopes to eventually determine the true risk that everyday plastic pollution poses to the developing human brain.

The study, “Polystyrene nanoplastics modulate neurite length in a size-specific manner,” was authored by Veronika Górová, Thuy Thi Lai, Alexey M. Afonin, Kore Nemeth, Anssi Pelkonen, Tarja Malm, Pasi Jalava, Riikka Lampinen and Katja M. Kanninnen.