This is Part 2 of a two-part story.
At a workshop during the 11th International Conference on HHV-6 and HHV-7, held June 23 to 26, 2019, in Quebec City, Canada, experts on human herpesviruses treated Alzheimer’s researchers to a lesson on the intricacies of viral biology, and what that might mean for the brain. Viruses disrupt mitochondria, autophagy, and even synapses, according to new data presented at the meeting. While some of the work was preliminary, it signaled the rapidly growing interest of virologists, immunologists, and neurobiologists alike in understanding the interaction of viruses and the innate immune system with the brain.
- Viruses use many tricks to enter and hijack cells to promote their own survival.
- Some of these strategies may contribute to neurodegeneration.
- New funding will speed work on role of infectious agents in AD.
How do herpesviruses access the brain? One route goes through the nose. Olfactory neurons form a bridge from the outside environment into a person’s brain. They project from the nasal mucous membrane across two synapses into AD-susceptible areas of the entorhinal cortex and hippocampus, with no blood-brain barrier in the way. Olfactory neurons can serve as a conduit into the brain for pathogens, including herpesvirus and oral bacteria, that have been implicated in AD and Parkinson’s disease (Jan 2019 news; May 2017 news).
Lavinia Alberi Auber, University of Fribourg, Switzerland, probed the olfactory system for signs of HHV-6. Auber studies age-related decline in olfaction, a nonspecific harbinger of future AD, PD, or dementia with Lewy bodies (DLB). Auber has seen neuropathological changes that may underlie this gradual loss. Studying postmortem olfactory tissue from 38 people who had symptoms ranging from mild cognitive impairment to severe AD, Auber noticed a prominent and progressive tauopathy in olfactory neurons. She also detected a weak amyloid pathology in the synaptic terminal in the olfactory bulb that looked to her like intracellular amyloid deposits. These could be a response to nerve damage or infection, she believes.
Using immunohistochemistry, Auber also detected certain viral proteins, which indicate a new infection or viral reactivation, only in AD patients. In people with early AD, these antigens were limited to a few neurons. In more advanced cases, staining appeared as puncta scattered across the whole olfactory tract, and co-localized with phosphorylated tau. One synapse away, in the olfactory region of the entorhinal cortex, the viral antigens co-localized with amyloid plaques. In several autopsy cases, she found neurons and microglia that tested positive for HHV6A scattered throughout the entorhinal cortex.
The results suggest that HHV-6A, and -6B, may spread from the nose through the olfactory circuit into the brain. It remains to be seen if this contributes to, or is an effect of, AD progression in these patients. One complication of the study is that herpesviruses can reactivate after death, and Auber agreed that postmortem delay may have precipitated this in some samples. The work needs to be confirmed and replicated in more subjects, she said.
Conveniently, olfactory neurons can be biopsied in living people. Auber would like to do this to screen for HHV-6 infection. She is evaluating saliva as a less invasive source for detection of specific viruses, and for monitoring changes in the saliva microbiome over the course of Alzheimer’s disease.
Robert Moir of Massachusetts General Hospital in Boston reviewed his work with Rudy Tanzi on the anti-microbial function of amyloid peptides. Soluble Aβ oligomers bind to bacteria, fungi, viruses, then fibrillize, trapping invaders in a sticky amyloid net that stops their spread, they’ve shown (May 2016 conference news). In this way, viruses and other microbes instigate and accelerate the formation of amyloid plaques (Jun 2018 news; May 2019 news). Ruth Itzhaki’s group found HSV-1 DNA in amyloid plaques, but no one has looked for HHV-6 yet, Moir said (Wozniak et al., 2009). Fibrils not only sequester pathogens, they kill them by piercing their membranes and complexing with copper to create an oxidative burst, the researchers postulate. Moir said his group can watch antigens disappear over a few days in their in vitro systems, as plaque-produced oxidizers destroy the epitopes. That could make it harder to identify pathogens in autopsy tissue, he said.
“Amyloidosis and AD pathology in general are emerging from this interface between pathogens and the innate immune system,” Moir said. “Aβ doesn’t spontaneously aggregate; it needs a seed, and microbes provide that seed. It’s likely not one pathogen, but several that can seed.” The waning of adaptive immunity as people age could explain why AD arises late in life, even though many viral infections begin early. “The blood-brain barrier loses integrity with age, and we become susceptible to neuroinflammation and new infections. As we pass age 40, adaptive immunity starts to decline and innate immunity takes up the slack,” Moir believes.
Roberta Rizzo, an immunologist and microbiologist at the University of Ferrara, Italy, takes a similar view. “I don’t think a primary infection is causing dementia. If so, we should find viremia or antibody titers in AD, but this is not the case. We don’t find huge infections in the brain. Perhaps reactivation of a latent infection is enough to aggravate the immune system in a brain that is no longer able to control inflammation,” she told Alzforum. She recently reported that HHV-6 infection of peripheral, immune, natural-killer T cells induces expression of ApoE, a gene expressed in the brain mostly by astrocytes under normal conditions (Rizzo et al. 2019). She is now studying the response of microglia to HHV-6 infection in vitro.
For neurons, HHV-6 infection affects the structure of synapses, according to work presented by Margot Mayer-Proschel, a neuroscientist at University of Rochester, New York. Her lab also established that expression of the HHV-6A latency-associated transcript U94A in human oligodendrocyte precursor cells disables their migration to sites of axonal injury (Campbell et al., 2017). This could contribute to demyelination in multiple sclerosis. Mayer-Proschel traced the migration defect to U94A’s ability to induce cytoskeletal dysfunction.
The cytoskeleton also supports dendritic structures more generally. When Mayer-Proschel expressed U94A in purified cortical neurons, the cells’ dendritic arborization diminished. They did not lose material for dendrites; they expressed all the building blocks but did not assemble them. In these U94A-expressing neurons, otherwise sub-toxic doses of soluble Aβ assemblies induced beads to form in dendrites and reduced the complexity of dendritic branching. To Mayer-Proschel, this suggests HHV-6A infection could modify disease by destabilizing neuronal dendrites. This raises the possibility that individual differences in the brain virome could contribute to variation in disease progression in the same way a person’s genetic profile does, Mayer-Proschel speculated.
Herpesviruses hijack other cellular pathways implicated in AD, too. For example, many viruses antagonize autophagy though the cellular regulator Beclin 1 (Munz, 2011). Autophagy slows the buildup of amyloid but the process is impaired in mouse models of amyloidosis (Rocchi et al., 2017; Pickford et al., 2008). In Quebec, Xiaonan Dong, an immunologist at the University of Southwestern Texas Medical Center, Dallas, showed that HSV-1 also suppresses autophagy, and that this inhibition is required for the virus to survive and replicate.
Mitochondrial insufficiency is a common theme in neurodegeneration and again, viruses may play a role. In human T cells, HHV-6 infection degrades the mitochondrial membrane potential, weakens oxidative phosphorylation, and fragments the mitochondrial network. Those results, presented by Christine Birdwell, Texas A&M Health Science Center, College Station, suggest the virus damages mitochondria to ensure its own replication. Whether mitochondrial distress occurs, and might help drive AD pathogenesis, in neurons or microglia infected with HHV-6 remains to be seen.
Do animal models of HHV-6 offer insight into its potential role in AD? Because the virus does not infect mouse cells, researchers have taken two approaches to study the virus in vivo. In one, they added the human gene for the HHV-6 receptor, CD46, into the mice (Reynaud et al., 2014). Cynthia Liefer, an immunologist at the Cornell University College of Veterinary Medicine, Ithaca, New York, described those mice. After an intracranial injection of virus into the right hemisphere, the animals developed persistent infection and chromosomal integration of the virus, most prominently in Purkinje cells in the cerebellum and in serotonergic neurons of the Raphe nuclei in the brain stem. The mice develop behavior changes reminiscent of anxiety and depression.
The second option is to use the murine roseolovirus, a natural mouse pathogen genetically related to HHV-6. Tarin Bigley of Washington University, St. Louis, described neonatal infection of wild-type mice, which led to autoimmune disease in adults. Bigley told Alzforum that he is collaborating with David Holtzman’s lab at WashU on studies to infect 5XFAD mice with this virus.
Finally, marmoset monkeys can be infected with HHV-6 and they mount an immune response to it. Steven Jacobson, National Institute of Neurological Disorders and Stroke, Bethesda, Maryland, has used these primates to study the role HHV-6 in MS. His group found that monkeys infected with HHV-6A did not get MS, but did get severe experimental autoimmune encephalomyelitis (EAE). In HHV-6A infected monkeys, brain lesions developed faster, along with a burst of virus production in response to the EAE stimulus. The animals with HHV-6A also died sooner (Leibovitch et al., 2018). Jacobson considers this the strongest evidence that this virus can trigger MS. He told Alzforum he is starting a collaboration to study HHV-6 infection in a marmoset model of AD.
At the end of the day, the virologists urged AD researchers in the room to adopt an attitude of healthy skepticism about the role of HHV-6 in AD, and to continue to research the question. Those efforts are getting some backing, as the NIH recently designated infectious etiology of AD as a high-priority research topic. This makes some funds available to tackle the many outstanding questions raised at the conference, said Miroslaw Mackiewicz of the National Institute on Aging. Foreign institutions are eligible to apply.—Pat McCaffrey.
- An Antimicrobial Approach to Treating Alzheimer’s?
- Olfactory System Model Explores Antimicrobial Role for α-Synuclein
- Microbial Hypotheses Intrigue at Zilkha Alzheimer’s Meeting
- Herpes Triggers Amyloid—Could This Virus Fuel Alzheimer’s?
- When Host Proteins Coat Virus, Amyloid Fibrils Form
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