Neurodegenerative diseases are medical conditions that affect neurons, which are the building blocks of the brain and spinal cord. These diseases not only affect the health of individuals, but also their abilities to work and learn, their families, and society as a whole. Parasites are organisms that live in/on a host and that get their nutrition from that host. How are these two linked, you may wonder? Interestingly enough, research shows that parasites such as Toxoplasma gondii are linked to disorders such as schizophrenia, Alzheimer’s, Parkinson’s and Multiple Sclerosis (Li et al.,2019). T. gondii is a common parasite that infects approximately a billion people worldwide, generally causing cold-like symptoms that resolve quickly, and then becoming silent in the brain for the rest of the persons’ life. In fact, you may have this parasite living inside of you, and not even know it! Because T. gondii lives in the brain, the host’s body must have a strong immune response to make sure it doesn’t reactivate to cause neurological problems. This continuous immune response can lead to swelling in the brain (“neuroinflammation”), which is associated with changes such as cell death, tissue damage, and decreased communication between cells in the brain. It is also known to be linked to neurodegenerative diseases. This led researchers to test, using mice, what changes occur in the brain, in search of a mechanism that explains how these parasites could be linked to brain disorders.

Generally, researchers Li, Severance, Viscidi, Yolken, and Xiao looked at what areas of the brain and what brain cells were affected, and what components of the immune system were activated. First, they used stain to show that areas with a large number of parasites were the ones affected. (Li et al.,2019). They then used another stain to see if the degenerating cells were neurons (communication cells rather than insulating/protective glial cells, also found in the brain), and they were. After this, they used a third stain to determine if parasites occurred in the same areas as the degenerating cells—however, there was no overlap found. This suggests that the damage is caused by the general immune response (inflammation). Next, the researchers looked at what types of brain cells were involved, and found that neurons that secrete the excitatory chemical in the brain (glutamate, involved in learning and memory) and neurons that secrete the inhibitory chemical (GABA, used to control the activity of nerve cells) were both affected. This suggests that the parasite has no preference for one type of brain cell over the other (Li et al., 2019).

Lastly, researchers looked at what components of the immune system were involved in creating the swelling in the brain in the presence of parasites. Results showed increases in the complement system, which is involved in inflammation, and in microglia (cells in the nervous system that work to clear dying cells and debris). They also found elevated levels of proteins called “C1q and C3”, that play a role in the immune system by tagging areas for destruction. Using a stain, they determined that the areas with elevated immune proteins coordinated with areas that were being destroyed. All of these results are related to mechanisms involved in neurodegenerative diseases and provide a potential link to the parasite T. gondii with these diseases.

Why is this research important, you may ask? Researching T. gondii in general could lead to more information on the parasite and the potential for treatment of toxoplasmosis, which affects many people worldwide. In addition, looking at the mechanisms used by these organisms and what changes they cause in the brain could lead to new methods of alleviating or curing symptoms of brain disorders. This research also stimulates many questions including what is the relevance to humans (mice and human brains differ, so can the infection still cause or contribute to degeneration in humans?), how exactly does inflammation contribute to the destruction of brain cells, and why are only certain areas of the brain affected, even if the parasites are distributed throughout? More research is needed to answer these important questions, and could help us learn more about both toxoplasmosis and neurodegenerative disorders.

Reference:

Li, Y., Severance, E.G., Viscidi, R.P., Yolken, R.H., & Xiao, J. (2019). Persistent Toxoplasmainfection of the brain induced neurodegeneration associated with activation of complement and microglia. Infect Immun, 87(8), 1-12. https://doi.org/10.1128/IAI .00139-19.