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Abnormal tau phosphorylation is a key mechanism in neurodegenerative diseases. Evidence implicates infectious agents, such as Herpes Simplex Virus 1 (HSV-1), as co-factors in the onset or the progression of neurodegenerative diseases, including Alzheimer's disease. This has led to divergence in the field regarding the contribution of viruses in the etiology of neurodegenerative diseases. Research indicates that viruses may function as risk factors driving neurodegenerative disease rather than playing a causative role. Investigating HSV-1 in abnormal tau phosphorylation is important for understanding the role of infectious agents in neurodegeneration.We generated cellular models of HSV-1 acute, latent infection, and viral reactivation from latency in cortical brain organoids and investigated the interplay between tau phosphorylation and HSV-1 infection by employing human induced pluripotent stem cell (iPSC)-derived monolayer neuronal cultures and brain organoids. Acute infection with HSV-1 strains 17syn+ and KOS caused nuclear accumulation of phosphorylated tau (p-tau) in neurons and neural precursor cells. Antivirals prevented nuclear accumulation of p-tau. Viral reactivation was accompanied by the nuclear translocation of p-tau. Chromatin immunoprecipitation analysis indicated an interaction of p-tau with the viral chromatin. A reduction in abundance of component of nuclear speckles and their loss of organized morphology in low-denisty host chromatin regions was observed, with strain-specific differences. HSV-1 infection was followed by an increase in the abundance of BRSKs and TAOKs, kinases known to phosphorylate tau.These findings show interaction between p-tau and HSV-1 chromatin and demonstrate the ability of HSV-1 to activate mechanisms that are observed in Alzheimer's disease.

Tau is a microtubule-associated protein that stabilizes the neuronal cytoskeleton. In the family of neurodegenerative diseases known as tauopathies, including Alzheimer’s disease (AD), frontotemporal dementia (FTD), and chronic traumatic encephalopathy (CTE), abnormal tau aggregation destabilizes microtubule structure, contributing to a cascade of cellular processes leading to neuronal cell death. The gut microbiome has increasingly become a target of neurodegenerative disease research since gut microbiome imbalances have been linked to protein aggregation and inflammation through a bidirectional axis linking the gut and brain. Accordingly, the present study examined tau-mediated changes to gut microbiome composition and immune activation in a Drosophila melanogaster model of human mutant tauopathy. Fecal deposit quantification and gastric emptying time courses suggested an abnormal food distribution and reduced gut motility in tau transgenic flies compared to controls. Tau transgenic flies also showed an increase in gut bacteria colony forming units (CFUs) from diluted fly homogenate, indicating an increased bacterial load. Finally, we showed that tau transgenic flies have a trend towards elevated systemic levels of antimicrobial peptides targeting gram-negative bacteria using qPCR, suggesting an enhanced innate immune response to bacterial insult. These data demonstrate qualifiable and quantifiable gut microbial and innate immune responses to tauopathy. Furthermore, these results provide a framework for future studies targeting the gut microbiome as a modifier of neurodegenerative disease.