In 2020, Alzheimer’s disease will remain elusive. With decades of deep cultivation of beta-amyloid protein and tau protein seeking to find ends in the field, scientists have had to think about the causes of Alzheimer’s disease in a different way. In 2017, scientists discovered that beta amyloid is actually an anti-infective protein; in 2018, a queue study from Taiwan, China, found that herpes virus infection increases the risk of dementia; and in the same year, scientists revealed the mechanisms by which two human herpes viruses (HHVs) are involved in regulating genes associated with Alzheimer’s disease.
All this seems to suggest that Alzheimer’s disease actually has a specific pathogen, and that the culprit behind this is the very common herpes virus in life.
In a new study published recently in the journal Science Advances, scientists found that in a 3D brain model of human induced neural stem cells (hiNSC), hSV-1 infection sedifies amyloid precipitate formation, neurogel hyperplasia, neuroinflammation and decreased nerve function – a signature change in Alzheimer’s disease! 
In addition, this 3D model is the first 3D brain model of Alzheimer’s disease, and does not rely on related gene mutations associated with APP, PSEN1/2, and future research can be said to be a very powerful tool.
HSV-1 infected cells (Photo: wiki)
There are not many pathogens that are thought to be associated with Alzheimer’s disease, such as chlamydia pneumonia, Borrelia burgdorferi, smooth Candida glabrata, and so on. The most commonly mentioned is herpes virus essob strains, which are mainly type 1 herpes simplex virus (HSV1) and type 6 human herpes virus (HHV-6).
These viruses are common, and it is estimated that 90 percent of people carry HSV-1 by the age of 70, and almost every baby is infected with HHV-6 after birth, and these viruses can lurk quietly in our bodies for decades.
HSV-1, which is introduced today, is a neurotic double-stranded DNA virus that is mainly present in peripheral nerves. The activation of HSV-1 can be asymptomatic, mainly in the form of lip herpes, and in rare cases herpes simplex encephalitis (HSE) occurs – which indicates that HSV-1 can penetrate the blood-brain barrier.
Knowing herpes simplex encephalitis, one can’t help but doubt the relationship between HSV-1 and Alzheimer’s disease. By looking at the cases, we can find that the region struck by HSV-1 infection in the brain is basically the same as the brain region of Alzheimer’s disease, and that patients with encephalitis often exhibit typical cognitive and behavioral disorders.
Further studies have found hSV-1 in the brains of Alzheimer’s patients, while more than 30,000 people in Taiwan have made it clear that HSV-1 infection increases the risk of dementia by 2.564 times, while antiviral therapy reduces the risk by 90%.
If it can be revealed from scratch that HSV-1 infection induces the brain tissue to develop Alzheimer’s disease, it can be said to be a complete blow.
Herpes simplex virus (Photo: wiki)
But it’s not easy.
Currently, most in vitro human Alzheimer’s models rely primarily on cells from people with early-onset Alzheimer’s disease — in other words, three mutations in the genes associated with beta amyloid, app, PSEN1, and PSEN2. While this does reproduce the pathological process to some extent, in practice these patients make up less than 5% of all patients, which means that many underlying environmental factors and causation are ignored.
This is the advantage of the model presented in this study.
Previously, the research team has developed a gene by dermal fibroblast reprogramming human induced neural stem cells (hiNSC), which can rapidly amplify and differentiate, within four days without complex differentiation programs and media requirements, can spontaneously produce a variety of neurons and glial cells, and they are sensitive to a variety of growth factors, drugs, and easily infected with the Zika virus.
By implanting this hiNSC into a stent made of biomaterials and injecting collagen gels, it is possible to generate an in vitro 3D model of human brain tissue with representative brain structures, observing that nerve synapses, neuronal networks, and electrophysiology are not a problem.
Manufacturing process for 3D models
First, hiNSC can indeed be infected by HSV-1. In the case of multiple infection (MOI) of 1, a total of 24 hours of cultivation, almost 100% of the hiNSC are infected with HSV-1, the level of the virus increases with the duration of incubation. At the same time, infected cells can also secrete HSV-1 infection of other cells.
This suggests that low levels of viral infection can lead to high levels of infection over time, much like reactivation of HSV-1 infection in clinically patients.
Results of 24 hours of training under different MOIs
High levels of viral infection are fatal to hiNSC, the higher the MOI, the more apoptosis there is, and even if the MOI is 0.01, a large number of cells will die, so the researchers took very low MOI 0.0001 to infect the cells.
Interestingly, the researchers observed some specific changes in hiNSC. Simply put, the infected hiNSC forms a special multicellular structure similar to the condos found earlier in alzheimer’s patients’ brains. The more immature the neurons are at the time of infection, the larger the multicellular clump structure formed.
Multicellular structure caused by HSV-1 infection
Through special fluorescent staining and ELISA, the researchers found that amyloid proprogenite fibers were present in these multicellular structures, with the increase being mainly A-beta 1-42 and A-beta 1-40 unchanged, which is consistent with the pathological manifestations of Alzheimer’s disease.
After qPCR, the researchers found that HSV-1 caused a reduction in app and BACE1 expression, as well as an increase in PSEN1/2 expression.
Considering that beta amyloid precipitation is not the only pathological feature of Alzheimer’s disease, the researchers also analyzed the presence of neural fibrous tangles (NFT). As a result, there are indeed a number of Tau proteins that make up NFT near amyloid progenitor fibers in the hiNSC of SV-1 infection.
HSV-1 causes the emergence of beta amyloid progenitias
Now that the two main characteristics of beta amyloid precipitate and nerve fiber tangles are already beginning to emerge, other pathological changes in Alzheimer’s disease naturally have to be examined.
After HSV-1 infection, the expression level of markers of various glial cells in hiNSC was raised, and the neuroglial cells induced by HSV-1 exhibited a special polynuclear globular, which was also consistent with the astrocytes fusion found in the autopsy of Alzheimer’s patients.
A variety of inflammatory cytokines associated with Alzheimer’s disease, including tumor necrosis factors alpha (TNF-alpha), leukin 1 beta (IL-1 beta), IL-6, and interferon symphyliones (IFN-thium), also expressed a surge after HSV-1 infection.
As an organ-like organ, the 3D model of HSV-1 infection also showed weaker electrophysiological activity, corresponding to impaired brain function in Alzheimer’s patients.
Significant reduction in electrophysiological activity
So can antiviral treatments for HSV-1 save the “brain”?
The researchers chose the common antiviral drug VCV. The results show that VCV can reduce HSV-infection, but also greatly reduce the related pathological phenomena. It is worth noting that if the drug is given before HSV-infection, HSV-infection and pathological phenomena can almost be avoided, but after the infection, the administration can only be partially reversed.
Thus, low levels of HSV-1 infection are indeed closely related to the occurrence of Alzheimer’s disease. However, HSV-1 infections are also quite common, and more complex processes need to be explored further.