Molecular Mechanisms for Herpes Simplex Virus Type 1 Pathogenesis in Alzheimer's Disease

Abstract

This review focuses on research in the areas of epidemiology, neuropathology, molecular biology and genetics that implicates herpes simplex virus type 1 (HSV-1) as a causative agent in the pathogenesis of sporadic Alzheimer's disease (AD). Molecular mechanisms whereby HSV-1 induces AD-related pathophysiology and pathology, including neuronal production and accumulation of amyloid beta (Aβ), hyperphosphorylation of tau proteins, dysregulation of calcium homeostasis, and impaired autophagy, are discussed. HSV-1 causes additional AD pathologies through mechanisms that promote neuroinflammation, oxidative stress, mitochondrial damage, synaptic dysfunction, and neuronal apoptosis. The AD susceptibility genes apolipoprotein E (APOE), phosphatidylinositol binding clathrin assembly protein (PICALM), complement receptor 1 (CR1) and clusterin (CLU) are involved in the HSV lifecycle. Polymorphisms in these genes may affect brain susceptibility to HSV-1 infection. APOE, for example, influences susceptibility to certain viral infections, HSV-1 viral load in the brain, and the innate immune response. The AD susceptibility gene cholesterol 25-hydroxylase (CH25H) is upregulated in the AD brain and is involved in the antiviral immune response. HSV-1 interacts with additional genes to affect cognition-related pathways and key enzymes involved in Aβ production, Aβ clearance, and hyperphosphorylation of tau proteins. Aβ itself functions as an antimicrobial peptide (AMP) against various pathogens including HSV-1. Evidence is presented supporting the hypothesis that Aβ is produced as an AMP in response to HSV-1 and other brain infections, leading to Aβ deposition and plaque formation in AD. Epidemiologic studies associating HSV-1 infection with AD and cognitive impairment are discussed. Studies are reviewed supporting subclinical chronic reactivation of latent HSV-1 in the brain as significant in the pathogenesis of AD. Finally, the rationale for and importance of clinical trials treating HSV-1-infected MCI and AD patients with antiviral medication is discussed.

Keywords: Alzheimer’s disease; amyloid beta; dementia; herpes simplex virus; neurodegeneration; neuroinflammation; pathogen; tau.

Figure 1

Electron microscopy image showing two herpes simplex virions. The nucleocapsid is seen in the center of each virion with surrounding tegument and viral envelope. Reprinted from Kaye and Choudhary (2006), copyright 2006, with permission from Elsevier.

Figure 2

Herpes simplex virus type 1 (HSV-1) infection of neuronal cells results in accumulation of amyloid beta (Aβ). Images from confocal microscopy demonstrate human neuroblastoma cells infected by HSV-1 at 18 h post-infection. Cells shown in the middle panels were double-labeled with anti-Aβ_1–42 and anti-Aβ_1–40 antibodies. Cells shown in the lower panels were double-labeled with anti-Aβ_1–42 and anti-HSV-1 antibodies. The color of the fluorescence for each primary antibody is demonstrated in the left and middle columns. Bar graph upper right shows significant increases in intracellular and secreted extracellular Aβ_1–42 by HSV-1-infected APP695-transfected neuroblastoma cells compared to mock-infected cells by ELISA (*P < 0.05 vs. HSV-1). Figure from De Chiara et al. (2010). Reprinted under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0).

Figure 3

Co-localization of HSV-1 and abnormal tau phosphorylation as shown by immunofluorescence in HSV-1-infected cultured human glioblastoma cells. HSV-1-infected glioblastoma cells show strong staining for HSV-1 proteins (green) by anti-HSV-1 antibody and abnormally phosphorylated tau proteins (red) by anti-p-tau antibody AT100, with co-localization within cells seen on far right slide. Abnormal tau phosphorylation occurred in HSV-1-infected cells and not in bystander cells. DNA is stained blue with Hoechst solution. Reprinted from Wozniak et al. (2009a), copyright 2009, with permission from IOS Press and Ruth Itzhaki. The publication is available at IOS Press through http://dx.doi.org/10.3233/JAD-2009-0963.

Figure 4

Quantification of abnormal tau phosphorylation in HSV-1-infected and uninfected cultured human neuroblastoma cells using the enzyme-linked immunosorbent assay. HSV-1-infected cells significantly hyperphosphylated tau protein at serine 214 compared to uninfected cells (p < 0.01). Reprinted from Wozniak et al. (2009a), copyright 2009, with permission from IOS Press and Ruth Itzhaki.The publication is available at IOS Press through http://dx.doi.org/10.3233/JAD-2009-0963.

Figure 5

HSV-1 infection of human neuroblastoma cells leads to accumulation of intracellular autophagosomes. (A) Electron micrograph of mock-infected neuroblastoma cells. (B) Electron micrographs of HSV-1-infected neuroblastoma cells at a multiplicity of infection (MOI) of 10 plaque forming units per cell (pfu/cell) for 18 h. Micrographs show accumulation of autophagosomes (white arrowheads) induced by HSV-1. White arrows show phagophores. Numerous cytoplasmic viral vesicles and free cytoplasmic virions are visualized (black arrows). Black arrowheads point to four-layered membrane vesicles. Note the enlarged boxed area (far right) showing a four-layered membrane vesicle. N labels the nucleus. Scale bars = 0.5 or 1 μm. Reprinted from Santana et al. (2012a), copyright 2012, with permission from IOS Press and Jesus Aldudo. The publication is available at IOS Press through http://dx.doi.org/10.3233/JAD-2012-112000.

Figure 6

HSV-1 DNA co-localizes with amyloid plaques. In situ PCR was used to detect HSV-1 DNA in specimens from AD and elderly normal brains (brown staining, A,C, respectively). In the same tissue specimens, amyloid plaques were localized using thioflavin S (green staining, B,D). Note the strong co-localization of HSV-1 DNA (brown staining) and amyloid plaques stained for Aβ_1–42 using immunohistochemistry (blue staining) (E). Scale bar = 50 μm. Figure from Wozniak et al. (2009b). Reprinted with permission from John Wiley and Sons.

Figure 7

Cerebral amyloid plaques containing Aβ in middle frontal cortex samples from HIV-infected patients. Immunohistochemical staining with anti-Aβ antibody demonstrates scattered focal plaques (A, arrows) and widespread plaques (B) in the cortex. Scale bars = 500 μm. Figure from Soontornniyomkij et al. (2012). Color version of figure from HHS Public Access PMCID: PMC3576852. Reprinted with permission from Wolters Kluwer Health, Inc.

Figure 8

Amyloid deposits containing Aβ in brain samples from neurosyphilis patients. (A) Cortical amyloid deposits from patients diagnosed with dementia due to neurosyphilis showing positive immunoreaction with anti-Aβ 8–17 (6F/3D, DakoCytomation) antibody. (B) Aβ deposition resembling immature and mature plaques. (C) Aβ deposits seen in the arterial wall of leptomeningeal vessels in the same patient as (A). Immunohistochemical analysis of Aβ was performed using the avidin-biotine-peroxidase technique. Bar = 50 μm. Panels (A) and (C) were reproduced from Figure 2 of Miklossy et al. (2006b). Figure from Miklossy (2015). Reprinted under the terms of the Creative Commons Attribution License (CC BY).

Figure 9

Acyclovir reduces Aβ accumulation in HSV-1-infected Vero cells. Vero cells were infected with HSV-1 SC16 at a MOI of 1 for 16 h. Cells were treated with 0 μM, 50 μM, 100 μM or 200 μM acyclovir. After fixation, immunocytochemistry was used to test the slides for Aβ accumulation. Acyclovir significantly reduced HSV-1-induced Aβ accumulation. Scale bar = 50 μm. Figure from Wozniak et al. (2011). Reprinted under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0).

Figure 10

Acyclovir reduces abnormal tau phosphorylation in HSV-1-infected Vero cells. Vero cells were infected with HSV-1 SC16 at a MOI of 1 for 16 h. Cells were treated with 0 μM, 50 μM, 100 μM or 200 μM acyclovir. After fixation, immunocytochemistry was used to test the slides for abnormal tau phosphorylation. Acyclovir significantly reduced AT100 staining, indicating inhibition of HSV-1-induced abnormal tau phosphorylation. Scale bar = 50 μm. Figure from Wozniak et al. (2011). Reprinted under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0).

Figure 11

Quantification of HSV-1 proteins, β-amyloid and phosphorylated tau proteins in HSV-1-infected Vero cells following treatment with acyclovir. Vero cells were infected with HSV-1 SC16 at a MOI of 1 for 16 h. Cells were treated with 0 μM, 50 μM, 100 μM or 200 μM acyclovir. After fixation, immunocytochemistry was used to test the slides for HSV-1 proteins, Aβ accumulation, and abnormal tau phosphorylation. Values are presented as the percentage of staining detected when no acyclovir is used. Statistically significant decreases in staining for HSV-1 proteins (A) and abnormal tau phosphorylation (C) are seen with all acyclovir concentrations tested compared to cells infected but not treated with acyclovir (p < 0.0001 in both cases). Statistically significant decreases in Aβ staining (B) are seen with acyclovir concentrations of 100 μM and 200 μM (p < 0.0001). Figure from Wozniak et al. (2011). Reprinted under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0).