Toxoplasma Injected Neurons Are Depolarized While Surrounding Neurons Remain Unaltered

Abstract

Toxoplasma gondii is a neurotropic intracellular parasite that causes a life-long latent infection by encysting in the brain. Toxoplasma’s ability to asymptomatically persist within the central nervous system (CNS) is quite unusual, leading some investigators to investigate how this chronic infection might change baseline physiology. These investigations have been limited to understanding global changes, such as global neurotransmitter release or electrophysiology via electroencephalography as no mechanism existed to interrogate in vivo how Toxoplasma might change infected neurons. At the single cell level only one in vitro study demonstrated hyper and hypoexcitable states of neuron firing, but even this study failed to look at the individual neuron. We have overcome this barrier by developing a novel mouse model that allows us to permanently mark and track CNS host cells that have been injected with parasite effector proteins, parasite proteins that are secreted into host cells and manipulate host cells processes. Using this novel system, we have determined that: i) Toxoplasma persists in neurons because parasites primarily interact with neurons; ii) the vast majority of these Toxoplasma-injected neurons (TINs) (~95%) do not harbor parasites; and iii) TINs are not homogenously distributed throughout the brain. Based upon these findings and prior work, I hypothesize, that neurons injected with T. gondii effector proteins will be significantly changed such that they will show altered physiology. Given how widely neuron physiology can vary by region and neuron subtype, I used a stepwise approach to address this hypothesis. First, I developed a MATLAB-based mapping program to neuroanatomically map TINs. Second, using this program, I determine that the isocortex and striatum are enriched for TINs. Third, within these regions, I used immunofluorescence assays to identify specific neuron subtypes commonly injected by Toxoplasma, including medium spiny neurons (MSNs) in the striatum. Finally, I leveraged the well-described electrophysiology of MSNs to compare single cell recordings of striatal TINs and nearby non-injected MSNs. Based upon these studies, I determined that the electrophysiology of striatal TINs substantially differs from surrounding striatal non-injected neurons, suggesting that interacting with Toxoplasma is sufficient to alter the electrophysiology of neurons possibly through a TINs-directed immune response or direct manipulation of the TIN from injected Toxoplasma proteins or active infection.