Human Herpes Simplex Virus‐1 Encoded ICP27 Promotes the Alternative Splicing of MAPT Resulting in increased 4R Ratios within Cells of Neuronal Origin

Abstract

AbstractBackgroundFamilial linked Alzheimer’s Disease (AD) accounts for a small percentage of current cases, with the remaining caused by unknown etiologies. Independent of causation, AD is defined by three common characteristics: inflammation, extracellular b‐amyloid aggregation and intracellular Tau accumulation. Human herpes simplex virus 1 (HSV‐1), a neurotropic virus found in ∼50% of the population, was recently linked to AD progression in clinical samples and animal models. HSV‐1 infections induce inflammation and APP cleavage producing Ab42, but a role for viral‐mediated Tau accumulation has not been defined. In healthy brains, alternative splicing of MAPT, the gene encoding Tau, results in equimolar ratios of 3R and 4R isoforms of Tau protein. However, in AD tissues, unequal ratios of either 3R or 4R results in Tau microtubule disassociation, hyperphosphorylation and aggregation. Host factors that regulate MAPT alternative splicing include SRPK1. Interestingly, HSV‐1 encodes a protein, ICP27, that impedes the SRPK1 functions. Thus, we hypothesized that HSV‐1 infections alter MAPT neuronal splicing resulting in altered 3R/4R Tau ratios in an ICP27 dependent fashion .MethodWe used infection with WT and mutant HSV‐1 and ectopic ICP27 expression to monitor MAPT splicing in iPSC‐derived 3D cortical organoids and 2D neuronal models.ResultsWe observed that 3D cortical organoids infected with HSV‐1 accumulate phospho‐Tau and increase MAPT 4R mRNA splicing. HSV‐1 infection of neuronal SHSY‐5Y cells, results in a significant increase in 4R isoform relative to 3R, which we did not observe when we infected these cells a virus that lacks ICP27 expression, suggesting that this viral protein is necessary for the altered splicing. Further, using lentiviruses that express either wild‐type ICP27 or a mutant ICP27 that lacks the SRPK1 interaction domain, we observed increased 4R splicing only with the wild‐type protein, revealing that ICP27 is sufficient for modulating Tau isoforms and that the SRPK1 domain is required. Finally, we found that levels of 4R splicing was increased within infected astrocytes, thus underscoring the impact HSV‐1 imparts on cells of both neuronal and non‐neuronal origin.ConclusionIn sum, our findings reveal the remaining critical third link in how neurotropic human herpesviruses can impact and influence AD progression.