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Neuronal DNA double strand breaks as novel epigenetic actors: roles in cognition, health and neuro-inflammatory diseases

Periodic Reporting for period 1 - NEURINFDNA (Neuronal DNA double strand breaks as novel epigenetic actors: roles in cognition, health and neuro-inflammatory diseases)

Reporting period: 2016-01-04 to 2018-01-03

Cognitive deficits are manifested years prior detecting neuronal loss in neurodegenerative diseases or during persistent infections of the central nervous system (CNS). Accumulating evidence show that epigenetic alterations contribute to neuronal dysfunction, as they change the chromatin structure and affect gene expression. In this context, DNA double-strand breaks (DSBs) are now emerging as central regulators of neuronal epigenetics.
My recent findings opened innovative perspectives of research, by showing that DSBs are not always associated with neuronal death, but rather constitute novel epigenetic signals that contribute to cognitive processes. To date, the role of DSBs in pathogen persistence and the mechanisms whereby DSBs affect neuronal function are totally unknown. Here, we postulate that perturbations in sensing, production and/or repair of DSBs may underlie the behavioral impairment observed in many CNS infectious diseases.
Persistent neuronal viral infections alter neuronal function, sometimes without overt immune response. The underlying mechanisms may result from persistence of the viral genome in interaction with neuronal DNA, thereby hijacking the chromatin remodeling system of the host cell, or from the secretion of proinflammatory cytokines. Parasites such as Toxoplasma also persist in the CNS and cause behavioral impairment. Their long-lasting impact on neuronal function may involve the modulation of the epigenome.
Here, we proposed to analyze the role of DSBs in cognitive alterations that accompany neurological infectious diseases. In particular, we: 1) characterized the role of pathogens and the associated immune response to neuronal localization of DSBs; 2) analysed which mechanisms of neuronal DSBs, detection and repair contribute to cognitive impairment.
"Our general scientific objective was to test whether CNS infections and the associated neuro-inflammation could alter the DNA double-strand break (DSB) response in neurons, and thereby impair behaviour. The first aim was completed using two pathogens and models of neuro-inflammation. The second aim was partly answered by the discovery published by another team that Topoisomerase 2 generates DSBs upon physiological neuronal activity. The last aim has started recently, once delays in the import and the breeding of a mouse model were lifted.
We implemented models of infections of primary cultures of mouse or rat neurons, and explored the changes in levels of several proteins involved in the DNA DSB response. By immunofluorescence staining, we explored which cell types in culture and in the brain of infected mice were accumulating DSBs, and whether this process was associated with cell death.
Regarding the main deliverables, we showed that infections by Toxoplasma gondii (TG) and Bornavirus (BoDV) cause DNA DSBs in neurons, by mechanisms at least dependent on the direct persistence of the pathogen in the host cell. In the case of BoDV, the mechanisms involved rely on the nucleoprotein of the virus and DSBs may be required for the replication of the virus in the nucleus of the neurons. I also identified that among many neuro-inflammtory molecules tested, one could cause DSBs in neurons in a dose-dependent manner, and seems to act by a mechanism relying on DNA repair factors depletion in neurons. We identified antibodies that were suitable to immunoprecipitate H2A.X to pursue the experiments of chromatin immunoprecipitation followed by sequencing that will help determine the sites of DSBs during these infections.
Funding by MSCA was a major factor contributing to my competitive recruitment in 2016 as a senior researcher (CR1) on a tenure track position at CNRS. I also mentored 2 master students, including one who is starting a Ph.D. training under my supervision to continue this project. Since the start of the MSCA fellowship, I authored 2 articles in top-tier journals as first author, and participated to the revision experiments of a PNAS paper published by my host team. Two papers are in preparation on the work (one on TG and one on BoDV), funded by the present grant and should be submitted by June 2018.
Additionally, my reintegration during the 2015-16 epidemic crisis of Zikavirus, combined with my knowledge in super-resolution microscopy allowed my contribution to a study, which revealed that Zikavirus is present within the spermatozoa of infected patients. As a wish of public engagement, we explained our findings in interviews and on national TV.
I also attended the international EMBO Conference on ""Hijacking host signaling and epigenetic mimicry during infections"" at Pasteur institute, where I was able to actively network with many experts in the field. I was also invited to represent the Centre of Excellence in Neurodegenerative Diseases of Toulouse, the Neurotoul, in China in the context of an international cooperation in Research.
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Our findings indicate that DSBs may play an important role in the pathophysiology of the neuronal infection by the BoDV. It is the first time that infection-dependent alterations of the DSB response are not involved in the elimination of infected cells, nor in the process of tumorization. Since other findings in our lab show that BoDV alters neuronal epigenetics and cognitive function, it is tempting to speculate that DSB response may be the missing mechanistic link between molecular alteration of chromatin and pathological phenotype. Second, increasing evidence suggest the role of Toxoplasma gondii (TG) in neuropsychiatric disorders such as schizophrenia or bipolar disorders. TG causes behavioural alterations in mice notably by reducing their innate fear to predator’s urine. However, neither the neuronal circuits involved in this behaviour nor underlying molecular mechanisms are understood. Our findings indicate that TG causes the accumulation of DSB specifically in neurons of the parietal cortex and the amygdala, two main regions involved in the behaviour altered by the infection. Hence, these findings may be the first clue of molecular mechanisms underlying remote epigenetic alterations that could cause the long-lasting changes in neuronal function in the course of TG infection. Our next expected results will be the identification of the neuronal DNA sequences where DSBs happen in the context of CNS infections. We hypothesize that specific sets of genes may be affected and linked with behaviour and cognitive functions.
Hence, in the field of fundamental research, understanding the mechanisms of neuronal DSBs on cognitive processes will have a vast impact, which extends well beyond CNS infections, and may help us understand common pathological mechanisms between a neurodegenerative disease like Alzheimer disease, and chronic infection of the brain and neuropsychiatric syndromes.
Beyond CNS infections, our findings on neuro-inflammation-induced DSBs open new perspective of research on a process that may be central to several neurological diseases, because the pro-inflammatory factor involved in DSBs accumulation is also a key player in sickness syndrome. By impacting on DSB levels and affecting DNA repair, our findings open new perspective of research that may lead to identification of new targets to reduce the production of DSBs and/or promote repair capacity. This strategy may ultimately lead to efficient therapeutics that, in combination with disease-targeted therapeutics, will efficiently cure several neurological diseases.