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Influence of Toxoplasma gondii on host cell function
Names of investigators
Genevieve Syn, Sarra E. Jamieson, Jenefer M. Blackwell
Toxoplasma gondii is a ubiquitous pathogen capable of infecting most animals, including humans and is a significant health burden in developed and developing countries. Infection in healthy individuals is usually asymptomatic, although chronic infection with T. gondii has been reported to be a risk factor for neurodegenerative and psychiatric disorders, including Parkinson’s Disease, Alzheimer’s Disease, and schizophrenia. Vertical transmission of T. gondii in mothers infected for the first time during pregnancy can lead to congenital toxoplasmosis, with affected babies born with characteristic brain and eye lesions including retinochorioditis, hydrocephalus and intracranial calcifications. Pathogenic pathways that may link congenital and latent toxoplasmosis have been largely unexplored. In this study, we used the Illumina 450K Human Methylation Beadchip and the Illumina HT-12 v4 Expression Beadchip to identify global changes in the host methylome and transcriptome following T. gondii infection in the clinically relevant WERI retinal cell line. Only genes in concordance, i.e. hypermethylated and decreased expression or hypomethylated and increased expression, were selected for downstream pathway analyses. Seven pathways were significantly enriched post-infection, including two neurologically-associated pathways: dopamine-DARPP32 feedback in cAMP signalling (p-value = 8.3 x 10-5; adjusted p-value = 0.02); and amyloid processing (p-value = 1.0 x 10-3; adjusted p-value = 0.043). These pathways are of interest for their roles in utero in eye and brain development (i.e. synapse formation, neuronal migration) and in adulthood where the dopamine pathway is involved in Parkinson’s disease and schizophrenia and amyloid processing contributes to Alzheimer’s disease. Thus, epigenetic dysregulation of these pathways by T. gondii may contribute to the known pathologies of T. gondii at the congenital level and its later life effect on the brain.
To understand more broadly the influence of the parasite on host cellular processes, we undertook further gene expression profiling using human foreskin fibroblast cells infected with T. gondii. We observed enrichment of genes involved in oxidative phosphorylation (OXPHOS) and mitochondrial dysfunction. We identified eleven hub genes (HIF-1α, CASP8, FN1, POU5F1, CD44, ISG15, HNRNPA1, MDM2, RPL35, VHL and NUPR1) and ten predicted upstream regulators, including 4 endogenous regulators RICTOR, KDM5A, RB1 and D-glucose. We characterized a number of mitochondrial parameters in T. gondii infected human foreskin fibroblast cells. In addition to the usual rapid recruitment and apparent enlargement of mitochondria around the parasitophorous vacuole we observed fragmented host mitochondria in infected cells, not linked to cellular apoptosis, from 24 hours post-infection. An increase in mitochondrial superoxide levels in T. gondii infected cells was observed that required active parasite invasion. Measurement of OXPHOS proteins showed decreased expression of Complex IV in infected cells, followed by decreased expression of Complexes I and II. No change occurred in Complex V. No difference in host mitochondrial membrane potential between infected and mock-infected cells was observed at any time. Our results show perturbation of host mitochondrial function following T. gondii infection that likely impacts on pathogenesis of disease.
Plain language summary
The toxoplasma parasite is very common throughout the world. In Australia between 30-40% of people will be infected with this parasite, usually through eating contaminated, undercooked meat. Most infected people will be unaware they have the parasite as it causes no symptoms. However, the parasite can cause serious problems if mothers catch the parasite for the first time whilst they are pregnant as the parasite can pass across the placenta to the developing baby. If the baby becomes infected they can be born with eye and brain disorders. Latent acquired infection has also been associated with neurodegenerative and psychiatric disorders in adults. To understand how acute fetal and acquired latent infection with the parasite results in the eye and brain disorders we are looking at how the parasite influences host cell function. We believe that changes we reported previously in host cell mitochondrial function might relate directly to clinical signs being focused to the eye and the brain of babies infected in utero. If this is the case, we might be able to provide evidence to support the repurposing of drugs currently used to treat genetic eye and brain diseases that are associated with mitochondrial dysfunction for the treatment of babies born with congenital toxoplasmosis. We also have some exciting new data on how the parasite affects dopamine and amyloid processing pathways which might provide the link to its association with neurodegenerative and psychiatric disorders.
Funders of the project
PhD studentship support
Wesfarmers Centre Seed Funding
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