The Non-Homologous End Joining Protein PAXX Acts to Restrict HSV-1 Infection.

Herpes simplex virus 1 (HSV-1) has extensive interactions with the host DNA damage response (DDR) machinery that can be either detrimental or beneficial to the virus. Proteins in the homologous recombination pathway are known to be required for efficient replication of the viral genome, while different members of the classical non-homologous end-joining (c-NHEJ) pathway have opposing effects on HSV-1 infection. Here, we have investigated the role of the recently-discovered c-NHEJ component, PAXX (Paralogue of XRCC4 and XLF), which we found to be excluded from the nucleus during HSV-1 infection. We have established that cells lacking PAXX have an intact innate immune response to HSV-1 but show a defect in viral genome replication efficiency. Counterintuitively, PAXX-/- cells were able to produce greater numbers of infectious virions, indicating that PAXX acts to restrict HSV-1 infection in a manner that is different from other c-NHEJ factors.

Functions of DNA damage machinery in the innate immune response to DNA virus infection.

DNA is potently immunostimulatory, and self-DNA is packaged in the nucleus or mitochondria allowing it to remain silent to cell-intrinsic sensors. However, damaged or mislocalised self-DNA is sensed by our innate immune systems, resulting in the production of type I interferons (IFNI), chemokines and inflammatory cytokines. During DNA virus infection the detection of viral DNA genomes by pattern recognition receptors (PRRs) is essential for the initiation of IFNI responses and host defence against these pathogens. It is intriguing that a number of molecular mechanisms have been found to be common to both of these DNA-induced stress responses and this has potentially important consequences for both sides of the host/pathogen arms race.

The air-water interface determines the outcome of seeding during amyloidogenesis.

Amyloid formation is a hallmark of protein misfolding diseases (e.g. Type II diabetes mellitus). The energetically unfavourable nucleation step of amyloidogenesis can be accelerated by seeding, during which pre-formed aggregates act as templates for monomer recruitment. Hydrophobic-hydrophilic interfaces [e.g. AWI (air-water interface)] can also catalyse amyloidogenesis due to the surfactant properties of amyloidogenic polypeptides. Using thioflavin T fluorescence and electron microscopy, we demonstrate that the outcome of seeding on human islet amyloid polypeptide amyloidogenesis is dependent upon whether the AWI is present or absent and is dictated by seed type. Seeding significantly inhibits (with AWI) or promotes (without AWI) plateau height compared with seedless controls; with short fibrils being more efficient seeds than their longer counterparts. Moreover, promotion of nucleation by increasing monomer concentrations can only be observed in the absence of an AWI. Using biophysical modelling, we suggest that a possible explanation for our results may reside in lateral interactions between seeds and monomers determining the fibril mass formed in seeded reactions at steady-state. Our results suggest that in vivo hydrophobic-hydrophilic interfaces (e.g. the presence of membranes and their turnover rate) may dictate the outcome of seeding during amyloidogenesis and that factors affecting the size of the pre-aggregate may be important.