Toxoplasma gondii Actively Inhibits Neuronal Function in Chronically Infected Mice

Abstract

Upon infection with the obligate intracellular parasite Toxoplasma gondii, fast replicating tachyzoites infect a broad spectrum of host cells including neurons. Under the pressure of the immune response, tachyzoites convert into slow-replicating bradyzoites, which persist as cysts in neurons. Currently, it is unclear whether T. gondii alters the functional activity of neurons, which may contribute to altered behaviour of T. gondii–infected mice and men. In the present study we demonstrate that upon oral infection with T. gondii cysts, chronically infected BALB/c mice lost over time their natural fear against cat urine which was paralleled by the persistence of the parasite in brain regions affecting behaviour and odor perception. Detailed immunohistochemistry showed that in infected neurons not only parasitic cysts but also the host cell cytoplasm and some axons stained positive for Toxoplasma antigen suggesting that parasitic proteins might directly interfere with neuronal function. In fact, in vitro live cell calcium (Ca2+) imaging studies revealed that tachyzoites actively manipulated Ca2+ signalling upon glutamate stimulation leading either to hyper- or hypo-responsive neurons. Experiments with the endoplasmatic reticulum Ca2+ uptake inhibitor thapsigargin indicate that tachyzoites deplete Ca2+ stores in the endoplasmatic reticulum. Furthermore in vivo studies revealed that the activity-dependent uptake of the potassium analogue thallium was reduced in cyst harbouring neurons indicating their functional impairment. The percentage of non-functional neurons increased over time In conclusion, both bradyzoites and tachyzoites functionally silence infected neurons, which may significantly contribute to the altered behaviour of the host.

Transcriptome Analysis of Mouse Brain Infected with Toxoplasma gondii

“…Indeed, recent studies that have analyzed the frequency of neurological symptoms in mice used strains such as C57BL/6 (22, 23), CD1 (24), and SW (3) at 6 to 18 weeks postinfection. Some researchers have used BALB/c mice to analyze behavior changes in the mice at 2 and 6 months after T. gondii infection (5,25). Therefore, the mouse model of T. gondii infection in BALB/c mice used in the present study could be used for evaluating toxoplasmosis at an earlier stage after infection than in the studies described above.…”

“…This effect was related to T. gondii orchestration of a significant increase in dopamine metabolism via tyrosine hydroxylase production, which is the rate-limiting enzyme for dopamine synthesis in neural cells (35). Furthermore, in vitro experiments demonstrated that tachyzoites actively prevented the normal Ca 2ϩ responses of glutamate-stimulated neurons; hence, it was suggested that T. gondii infects neurons and may directly modulate neuronal function (5). In this study, genes that were identified as being negatively correlated with parasite numbers were involved in small-GTPase-mediated signal transduction and GTP or guanyl nucleotide binding.…”

“…Recently, it has been suggested that chronic infection with T. gondii can alter exploratory behaviors, risk assessment, and unconditioned fear responses and that these changes are related to the presence and the location of cysts within the animal (5). In addition, human studies suggest that chronic infection with T. gondii can cause cryptogenic epilepsy (6) and that individuals with schizophrenia have a higher prevalence of antibodies to T. gondii than healthy controls (7).…”

Reassessment of the Role of Aromatic Amino Acid Hydroxylases and the Effect of Infection by Toxoplasma gondii on Host Dopamine

“…For example, infection causes lowlevel inflammation throughout the brain (45), affects the activity level of infected neurons (46), alters microRNA expression in cells (35), and causes changes in host gene expression in the frontal cortex of mice (12). Since chronic infection leads to activated microglia around many, but not all, tissue cysts (47), a potential cause of signaling and behavioral abnormalities would be inflammation.…”

Spinal cord pathology in chronic experimentalToxoplasma gondiiinfection

“…This leads to a control of the infection but also creates a hospitable environment assuring the parasite's long-term survival. The chronic phase of infection is characterized by numerous intact T. gondii cysts scattered throughout the whole brain being able, thus, to alter directly or indirectly neuronal function and presumably the behavior [49,50].…”