Providing clues into why the severity of a common parasitic infection can vary greatly from person to person, a new Johns Hopkins study shows that each one of three strains of the cat-borne parasite Toxoplasma gondii sets off a unique reaction in the nerve cells it invades.
Past research suggests that the parasite, estimated to infect 25 percent of people worldwide, can trigger or exacerbate psychotic symptoms and schizophrenia in genetically predisposed people.
The findings of the new study, published in the March issue of the journal Infection and Immunity, help explain why the infection causes serious disease in some but not in others and clarify its role in psychiatric disorders, the researchers say.
"We already know that toxoplasmosis can play a role in some psychiatric disorders, but up until now we didn't know why. Working with human nerve cells, our study shows the exact alterations triggered by each strain that can eventually manifest themselves as symptoms," says senior investigator Robert Yolken, M.D., a neurovirologist at the Johns Hopkins Children's Center.
The researchers injected human nerve cells with the three most common toxoplasma strains, each of which caused a different pattern of gene expression. One of the most basic functions in all living organisms, gene expression occurs when a gene is switched on to release a substance that tells cells what to do or not do, determining the cells' biologic behavior. Gene expression can be turned on and off, stepped up or down by various factors, including viral and bacterial invasions.
Cells infected with toxoplasma type I -- the most virulent strain in mice -- had the greatest impact on gene expression, altering more than 1,000 genes, 28 of them linked to brain development and central nervous system function and 31 others to nerve impulse and signaling.
Cells injected with the less virulent types II and III had low and moderate levels
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Johns Hopkins Medical Institutions