Is Aβ part of the body’s viral containment system? The idea has drawn plenty of attention, but a paper now reports evidence to the contrary, at least in mice. According to a preprint posted on bioRxiv on January 22, mice with a brain full of Aβ plaques fared no better than their wild-type counterparts when infected with herpes simplex virus-1 (HSV-1). Researchers led by Ilia Baskakov of the University of Maryland School of Medicine in Baltimore reported that in 5xFAD mice, HSV-1 was neither found within Aβ plaques nor did it spur them to form. The findings don’t necessarily negate a connection between viral infections and AD, the authors contend, but they do show that in these specific conditions, Aβ is not an anti-microbial peptide.
- An abundance of Aβ in the brain does not protect against HSV-1 infection.
- In 5xFAD mice, HSV-1 was absent from Aβ plaques and did not incite plaque growth.
- HSV-1 did not replicate in plaque-ridden brain regions.
The idea that microbes, especially herpesviruses, play a part in inciting AD pathogenesis was proposed decades ago, and has gradually picked up steam in the field (Jamieson et al., 1991; Jan 2020 news; Itzhaki et al., 2020). Later studies identified a potential explanation for the link: Sticky Aβ is an anti-microbial peptide, imbued with the capability to contain viral particles (Wozniak et al., 2007; Wozniak et al., 2009; May 2016 news). In the case of herpesviruses, the idea goes, Aβ peptides could theoretically spring into action every time a latent infection reawakens in the brain, ultimately inciting Aβ deposition. In support of this, researchers led by Rudolph Tanzi and the late Robert Moir reported that 5xFAD mice survived longer than did wild-type mice after a lethal dose of HSV-1, and that a sublethal infection spurred Aβ plaques to form prematurely in young mice (Jun 2018 news).
The new paper calls this into question. Baskakov, whose research centers around the biochemical nature of prion propagation, took an interest in the infectious etiology of AD a few years ago. His group’s foray into the topic started with a simple question: Does Aβ protect against viral infection? To address it, first author Olga Bocharova and colleagues took a similar approach to Tanzi and Moir: They intracerebrally infected wild-type and 5xFAD mice with HSV-1. As different viral strains exhibit unique properties, Baskakov used two—17Syn+ and McKrae—which was more lethal. The researchers infected mice with three different doses, ranging from 10-fold below to 10-fold above that predicted to be lethal for 50 percent of the mice (LD50). Regardless of the viral strain, dose, or age of the animals, 5xFAD mice survived no better than wild-type mice. Though not statistically significant, there was a whiff of a survival advantage under one circumstance: when 7- to 10-month-old 5xFAD mice were infected with the highest dose of the McKrae strain, 104 plaque-forming units (PFU), a measure of the number of active viral particles.
In a comment to Alzforum, and Tanzi and William Eimer of Massachusetts General Hospital emphasized this protective trend. In their 2018 study, 5xFAD mice better survived infection with a 10,000-fold higher dose of HSV-1—108 PFU—than did wild-type mice. “Perhaps, if the Bocharova et al. survival studies were expanded to use higher doses of virus, they would also have observed a significant degree of protection against HSV-1 infection by Aβ, as we reported,” Eimer and Tanzi wrote. However, Baskakov noted that such an exceedingly high viral dose—which ultimately killed all of the mice, regardless of genotype—seemed less physiological than the infection conditions used in his study, which hovered around the LD50. Eimer and Tanzi also used a different HSV-1 strain—a fluorescently tagged version of strain F (see de Oliviera et al., 2008, for strain description).
Richard Lathe of the University of Edinburgh agreed that multiple parameters, including viral strains and doses, and the number, sex, and genetic background of mice used in experiments, could underlie discrepancies between the two studies.
Baskakov and colleagues also investigated the idea that Aβ plaques trap viruses. Using antibodies against three different HSV-1 glycoproteins—gB, gD, and gH—the researchers were unable to detect any virus associated with Aβ plaques. One antibody—anti-gB—did appear to detect the virus within plaques; however, subsequent experiments revealed that this antibody also bound to plaques in uninfected 5xFAD mice, suggesting cross-reactivity explained the signal. Furthermore, infecting young 5xFAD mice, which had not yet developed plaques, triggered none. The scientists concluded that, at least in this model, HSV-1 neither associates with Aβ plaques nor spurs their growth.
No Overlap. HSV-1 (gD, red) was not found within plaques (green) in the hippocampi of 7- to 10-month-old 5xFAD mice. [Courtesy of Bocharova et al., 2020.]
Interestingly, Baskakov et al. found that, in plaque-ridden 5xFAD mice infected with the highest dose of the McKrae strain—i.e., the condition that hinted at a slight survival advantage—HSV-1 did not appear to replicate within regions of the brain with heavy plaque burden. This was most apparent in the plaque-loaded motor cortex, where the virus was barely detectable in 5xFAD mice despite its abundance there in wild-type mice. The researchers noted that this suppression could explain the smidgen of protection afforded to aged 5xFAD mice.
Tanzi and Eimer believe the virus’s apparent absence from plaques could be explained by methodology. They had visualized it not only with HSV-1 antibodies, but also by using a viral strain expressing a red fluorescent protein (RFP) transgene. They noted that Bocharova et al. did not test whether Aβ’s interaction with HSV-1 glycoproteins may have interfered with binding by the antibodies. Eimer and Tanzi previously reported that Aβ’s anti-viral activities may hinge upon its interactions with HSV-1 surface glycoproteins, implying that Aβ would mask antibody-binding sites on the virus within plaques.
Baskakov emphasized that his findings question Aβ’s anti-viral activity against these two strains of HSV-1 at the doses used. Importantly, acute models of infection don’t reflect the lifelong, latent infections most commonly present in people as they age. His findings leave open the possibility that viruses and other microbes influence the development of AD, via Aβ-dependent or independent mechanisms, he said.
“These studies raise an obvious question: What is the natural target for Aβ antimicrobial action?” asked Lathe. “Antimicrobial proteins differ widely in their activity against different pathogens, and the marginal in vivo protection against HSV-1 in 5xFAD mice (Eimer et al., 2018; and Bocharova et al.) contrasts with the reported strong protective effects against C. albicans and Salmonella,” he added. (Apr 2009 conference news; May 2016 news). “It would certainly be of interest to explore the antimicrobial activity of Aβ against a much wider range of microbes.”—Jessica Shugart
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