Toxoplasma gondii Could Deliver Therapeutic Proteins to the Brain

A parasite capable of passing through the blood-brain barrier and infecting its hosts’ brains could eventually offer a route for delivering important treatments.

Toxoplasma gondii – a parasite found in almost every type of warm-blooded animal on Earth – may be engineered to carry therapeutic proteins into brain cells, opening up possible treatments for conditions that are otherwise hard to reach.

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

“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.”

How Toxoplasma gondii crosses the blood-brain barrier

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 these areas.

That is generally beneficial. However, because the barrier does not readily allow hydrophilic or large molecules to pass through, it excludes virtually all proteins. This creates a major obstacle for therapeutic proteins that could help to treat brain disorders.

T. gondii has evolved ways to traverse the blood-brain barrier. By itself, this is not beneficial for people or other animals.

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

Delivering proteins with engineered Toxoplasma gondii

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

The researchers assessed the method in several systems. They exposed laboratory-grown clusters of human brain tissue, known as organoids, to T. gondii engineered to deliver MeCP2. This protein is used to treat Rett syndrome, a rare genetic disorder affecting brain development that is almost always caused by mutations in the MECP2 gene.

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

Mouse tests and potential neurological research uses

To establish how the technique might perform in a living organism, the researchers infected mice with the modified T. gondii. Control groups received either unmodified microbes or injections containing only saline.

The altered T. gondii infected their hosts as effectively as the unmodified parasites, while also delivering MeCP2 and causing minimal inflammation compared with the saline control group.

An estimated 25 to 30 per cent of people globally carry T. gondii, with most experiencing no symptoms, so the infection is usually benign. If it is something people already live with, there could be substantial benefits in making it work in their favour.

The researchers consider the findings a new avenue not only for treating neurological disorders, but also as a potent 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.