Abstract
Alzheimer's disease is a progressive neurodegenerative disease characterized neuropathologically by presence of extracellular amyloid plaques composed of fibrillar amyloid beta (Aβ) peptides and intracellular neurofibrillary tangles. Post-mortem and in vivo studies implicate HSV-1 infection in the brain as a precipitating factor in disease/pathology initiation. HSV-1 infection of two-dimensional (2D) neuronal cultures causes intracellular accumulation of Aβ42 peptide, but these 2D models do not recapitulate the three-dimensional (3D) architecture of brain tissue.We employed human induced pluripotent stem cells (hiPSCs) to compare patterns of Aβ42 accumulation in HSV-1 infected 2D (neuronal monolayers) and 3D neuronal cultures (brain organoids). Akin to prior studies, HSV-1-infected 2D cultures showed Aβ42 immunoreactivity in cells expressing the HSV-1 antigen ICP4 (ICP4+). Conversely, accumulation of Aβ42 in ICP4+ cells in infected organoids was rarely observed. These results highlight the importance of considering 3D cultures to model host-pathogen interaction.IMPORTANCE The "pathogen" hypothesis of Alzheimer's disease (AD) proposes that brain HSV-1 infection could be an initial source of amyloid beta (Aβ) peptide-containing amyloid plaque development. Aβ accumulation was reported in HSV-1-infected 2D neuronal cultures and neural stem cell cultures, as well as in HSV-1-infected 3D neuronal culture models.The current study extends these findings by showing different patterns of Aβ42 accumulation following HSV-1 infection of 2D compared to 3D neuronal cultures (brain organoids). Specifically, 2D neuronal cultures showed Aβ42-immunoreactivity mainly in HSV-1-infected cells and only rarely in uninfected cells or infected cells exposed to antivirals. Conversely, 3D brain organoids showed accumulation of Aβ42 mainly in non-infected cells surrounding HSV-1-infected cells. We suggest that because brain organoids better recapitulate architectural features of a developing brain than 2D cultures, they may be a more suitable model to investigate the involvement of HSV-1 in the onset of AD pathology.
Highlights
Alzheimer's disease is a progressive neurodegenerative disease characterized neuropathologically by the presence of extracellular amyloid plaques composed of fibrillar amyloid beta (Ab) peptides and intracellular neurofibrillary tangles
When 2D cultures were infected with herpes simplex virus 1 (HSV-1) at multiplicity of infection (MOI) of 1.0, 0.7, and 0.5, intense Ab42 immunoreactivity was observed in approximately 19% of ICP41 cells but not in any of the uninfected cells in close proximity or distal to infected neurons (Fig. 1c and e)
The following questions emerge: how common is Ab accumulation in HSV-1 infected cells, and is the 2D cell monolayer model the optimal choice for studying this phenomenon? Our current immunohistochemical analysis of HSV-1-infected 2D monolayer neuronal cultures showed high frequencies of perinuclear Ab42 immunoreactivity in ICP41 cells compared to uninfected cells or infected cells exposed to antivirals 5BVdU and interferon alpha (IFN-a)
Summary
Alzheimer's disease is a progressive neurodegenerative disease characterized neuropathologically by the presence of extracellular amyloid plaques composed of fibrillar amyloid beta (Ab) peptides and intracellular neurofibrillary tangles. Involvement of infective pathogens in AD pathogenesis was first proposed by Alois Alzheimer [6] and is supported by several recent studies that suggested that Ab is an antimicrobial peptide (AMP) that plays a role in protecting neurons from infectious pathogens like bacteria and viruses [7, 8] In this regard, increased Ab production could be a protective response to infection but may lead to excessive Ab accumulation and, possibly, fibrillization and deposition as amyloid plaques. We have recently reported on the use of induced pluripotent stem cells (iPSCs) to model HSV-1 acute and latent infection in 2D monolayer neuronal cultures and 3D brain organoids [14] These in vitro models showed that iPSC-derived CNS neurons are permissive for HSV-1 infection, with reporter gene expression being detected from both immediate-early (ICP0) and strict late (gC) promoters and infectious virus being released into the medium during the acute infection period of both 2D and 3D cultures. This ability contributes to the potential superiority of 3D brain organoids to model host-pathogen interactions within the CNS
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