Engineered Toxoplasma gondii Could Deliver Therapeutic Proteins Across the Blood-Brain Barrier

A parasite capable of breaching the blood-brain barrier and infecting its hosts’ brains may eventually provide a way to transport vital therapeutics.

Toxoplasma gondii – a parasite found in almost every type of warm-blooded creature on Earth – may be modified to carry therapeutic proteins into brain cells, opening potential treatment routes for conditions that are otherwise hard to reach.

Experiments in laboratory-grown human brain tissue and live mice produced minimal side effects, indicating that the approach is feasible. With additional research and refinement, it could also have wider uses, including in brain research.

"In this study, we show that T. gondii can be used to address many of the challenges associated with protein delivery for research and therapeutic applications," writes a team led by neuroscientist Shahar Bracha of the Massachusetts Institute of Technology.

"We demonstrate the use of T. gondii as a versatile delivery system in cultured fibroblasts, in vitro-differentiated neurons, primary neurons, human brain organoids and in vivo in mice, and characterize factors that affect delivery patterns under different conditions."

Why the blood-brain barrier blocks therapeutic proteins

As its name indicates, the blood-brain barrier is a membrane separating blood vessels from brain tissue and the central nervous system. It stops potentially harmful material in the bloodstream from reaching the latter.

While this protection is generally beneficial, its resistance to hydrophilic and large molecules prevents the passage of virtually all proteins. That creates a major obstacle for delivering therapeutic proteins that could treat disorders affecting the brain.

T. gondii has evolved ways to pass through the blood-brain barrier. By itself, this is far from beneficial for people or other animals.

This protozoan causes toxoplasmosis, a condition associated with unpleasant symptoms and potentially serious complications. However, Bracha and her colleagues asked whether T. gondii’s capacity to cross the blood-brain barrier could instead be put to beneficial use.

Engineering Toxoplasma gondii for brain protein delivery

In the central nervous system, T. gondii mainly survives within and interacts with neurons, using three distinct organelles to release substances. The researchers focused on two of these organelles, modifying them to secrete proteins known to treat neurological conditions in humans.

To assess the method, the team trialled engineered T. gondii across several systems. They exposed laboratory-grown clusters of human brain tissue, known as organoids, to T. gondii designed to deliver MeCP2. This protein is used to treat Rett syndrome, a rare genetic disorder that affects brain development and is almost always caused by mutations in the MECP2 gene.

In the lab-grown tissue, the delivered protein bound to the organoid’s DNA and changed gene expression compared with controls exposed to unmodified T. gondii. The researchers interpreted this as successful delivery of a functional protein.

Mouse tests and future research uses

To investigate how the method could operate in a living organism, the researchers infected mice with edited T. gondii microbes. Control groups received either unedited microbes or saline injections alone.

The edited T. gondii infected hosts as efficiently as the unedited parasites, while also delivering MeCP2 and causing minimal inflammation compared with the saline control group.

An estimated 25 to 30 percent of people worldwide carry T. gondii, with most experiencing no symptoms, meaning the infection is generally benign. If it is already something humans live with, adapting it to work for us could offer substantial benefits.

The team considers the findings a possible new path not only for treating neurological conditions, but also as a powerful research method for examining protein activity in neurons.

"Neurons are particularly difficult to target with existing methods," they write. "T. gondii's ability to robustly deliver intracellular proteins to neurons emphasizes its potential as a research tool."

The research was published in Nature Microbiology.