Delivering Proteins Directly to the Brain

Diseases of the brain, whether congenital or degenerative, that lead to the gradual death of brain cells over the years, are a major focus for many in the fields of medicine and research, with efforts aimed at finding cures or finding solutions for improving patients' well-being. However, a key challenge remains: treating the brain with medication is notoriously difficult. The primary obstacle is the blood-brain barrier, a specialized characteristic of the blood vessels surrounding the brain. This barrier, formed by specialized molecular mechanisms, such as tightly packed cells, restricts the passage of substances from the bloodstream to the brain cells. Consequently, it blocks foreign substances, such as large proteins and most bacteria, from entering the brain's sensitive environment.  While this protective function is crucial for protecting the brain against harmful invaders, it also hinders the delivery of medications to the brain through the bloodstream, complicating the treatment of diseases affecting the brain.

Over the years, various strategies have been employed to temporarily bypass the blood-brain barrier, with varying degrees of success.  Yet, the quest for an effective method to deliver drugs directly to the brain is ongoing. A new study conducted in Israel presents a surprising approach to this problem: using a naturally occurring parasite known to penetrate the brain, which has been genetically engineered to deliver specific proteins. This method could potentially revolutionize the treatment of brain diseases in the future.

The blood-brain barrier restricts substances from exiting blood vessels and reaching brain cells.  The blood-brain barrier | Gunilla Elam / Science Photo Library

Mouse, Be Brave!

The study used a single-celled parasite called Toxoplasma gondii. This parasite is extremely small—only slightly larger than a bacterium—and can infect many mammals, though it undergoes sexual reproduction exclusively in the bodies of felines. Toxoplasma gondii is particularly known for its unique effect on mice: past studies have shown that it alters their behavior, making them less likely to flee from cats. This behavioral change allows the parasite to easily reach the cats and continue its reproductive cycle.

Toxoplasma gondii is surprisingly common in humans, with about one in three individuals carrying the parasite in their brain cells. The parasite is usually dormant, and most carriers are unaware of its presence. The parasite usually remains dormant, so most carriers are unaware of its presence. However, if the immune system becomes compromised, such as in individuals with HIV/AIDS, the parasite can reactivate and pose serious health risks. Fetuses are particularly vulnerable due to their underdeveloped immune systems, with the risk increasing if the mother becomes infected earlier during pregnancy. Newborns infected in the womb may suffer from severe congenital disease. Testing for the parasite is generally performed in high-risk cases or if there is suspicion of infection or symptoms during pregnancy.

bout one in three people carry it in their brain cells. Toxoplasma gondii | Kateryna Kon, Shutterstock

The Parasite in the Room

Toxoplasma gondii has coexisted with mammals for millions of years, evolving complex mechanisms to efficiently infect their cells. This long-standing relationship is why researchers in Oded Rechavi's lab at Tel Aviv University's Faculty of Life Sciences chose to harness the parasite rather than trying to eliminate it."We like the idea of solving problems using developments that already exist in nature," said  Shahar Bracha, the study's lead researcher, in an interview with the Davidson Institute website. "In many fields, such as medicine and research, we can find suitable situations in nature where relevant mechanisms have evolved, and draw inspiration from them rather than engineering a new solution entirely from scratch."

In an interview with Haaretz, Rechavi explained the parasite's mechanism of action: "It knows how to hitch a ride on the immune system and reach the brain through it. Once in the brain, it penetrates neurons and can stay there for life. Once inside, the parasite forms a kind of cyst in which it continues to secrete proteins permanently." With these traits in mind, the researchers aimed to harness the parasite's ability to reach brain cells and secrete proteins. While these proteins typically help the parasite take over the cell, molecular tools that allow genetic editing in various organisms can be employed to modify the parasite to secrete other proteins with potential medical benefits.

A Ride to the Brain

As a first step, the researchers chose to focus on Rett syndrome, a severe and rare disorder that primarily affects girls, leading to a decline in their motor functions and cognitive abilities. Rett syndrome is caused by a defect in a protein called MeCP2, which is crucial for brain cell function. Because it involves a single protein, treating this disorder might be more straightforward than treating conditions involving multiple proteins. If doctors in the future could introduce the healthy protein into the brains of girls with Rett syndrome, their condition might improve.

When Toxoplasma gondii infects brain cells, it secretes proteins it produces directly into them. The researchers used genetic engineering to attach the MeCP2 protein to these secreted proteins, allowing the parasite to deliver it into brain cells along with the other proteins it normally produces and secretes. This method proved highly effective; in an experiment on human cells, the genetically engineered parasites successfully introduced large amounts of MeCP2 into the cells, reaching the cell nucleus where it functions.

To verify that MeCP2 was active in the cells it had been delivered to, the researchers used organoids—three-dimensional structures of cells that mimic an organ, in this case, the brain. The infected organoids efficiently received the protein and produced additional proteins indicative of normal MeCP2 presence. Finally, the researchers tested the protein delivery's effectiveness in a living organism. They infected mice with the parasite and monitored its spread. The genetically engineered parasite was as effective as the regular parasite, primarily targeting the mouse's brain cells. The successful secretion of MeCP2 into the mouse brain demonstrated the method's effectiveness in crossing the blood-brain barrier in a living organism, not just in cell cultures. In the brains of the mice, the parasite functioned as expected,  efficiently secreting MeCP2.

 Rett syndrome is caused by a defect in the MeCP2 protein, which is essential for brain cells. MeCP2 protein complex with DNA | Source: Ramon Andrade 3dciencia / Science Photo Library

Just the Beginning

While this research demonstrates the feasibility of the new method, there is still a long journey ahead before parasites can be used in routine medical treatment, and this is not expected in the near future. Although Toxoplasma gondii does not typically pose a threat to most carriers, further studies are needed to ensure it can be harnessed for medical purposes without causing harm. In the future, it may even be possible to genetically engineer the parasite to minimize any associated risks even further.

Moreover, significant development and improvement are required to use this new method effectively. The study did not attempt to control where the protein was released or the exact amount of protein the parasite secretes into brain cells. In the future, it may be possible to engineer parasites to target specific areas of the brain and regulate both the timing and levels of protein secretion. For now, this research serves as a source of inspiration, offering a glimpse into the potential for future treatments for diseases currently deemed incurable and showcasing humanity's capacity to leverage existing natural mechanisms for medical advancement.