Alzheimer's Gone Viral: Could Herpes Simplex Virus Type-1 Be Stealing Your Memories?

Alzheimer's disease (AD) is one of the principal causes of disability and morbidity. It is one of the most expensive illnesses. Despite this, there are no significant data regarding its etiology and optimal treatment. This review concentrates on the viral hypothesis of AD. After a comprehensive PubMed literature search, we analyzed the studies associating herpes simplex virus type-1 (HSV1) infection to AD from the previous 10 years. Molecular mechanisms whereby HSV1 induces AD-related pathophysiology, including neuronal production and accumulation of amyloid-beta (amyloid-β), abnormal phosphorylation of tau proteins, impaired calcium homeostasis, and autophagy, are addressed. The virus also imitates the disease in other ways, showing increased neuroinflammation, oxidative stress, synaptic dysfunction, and neuronal apoptosis. Serological studies correlate HSV1 infection with AD and cognitive impairment. A causal link between HSV1 and AD raises the concept of a simple, efficient, and preventive treatment alternative. Anti-viral agents impede brain degeneration by preventing HSV1 spread and its replication, decreasing hyperphosphorylated tau and amyloid-β; thus providing an efficacious treatment for AD. We also mention brown algae, intravenous immunoglobulin (IVIG), and a synthetic drug, BAY57-1293, with anti-viral properties, as options for treating AD. We want to recommend future researchers to look for more affordable, non-invasive, and swifter techniques to identify HSV1 in the brain and assist in the early detection and prevention of AD.


Introduction And Background
"Alzheimer's disease locks all the doors and exits. There is no reprieve, no escape." -Patricia Reagen Davis.
The dominant subtype of senile dementia in the world is Alzheimer's disease (AD) [1][2][3][4][5][6]. In 2015, it accounted for 18-20 million cases globally [7][8]. In 2014, the United States (US) accounted for approximately five million cases, and epidemiologists expect a rise to nearly 14 million by 2060 [9]. Among the elderly, it is the sixth leading cause of death in the US [9]. The median annual total expenditure for one patient having advanced AD is estimated to be more than $50,000 [10]. By 2040, the cost of treating AD is expected to rise from around $215 billion to more than $500 billion annually [9]. The economic burden of AD in the US rounds up to 1.09% of global GDP [3]. Data have shown an expenditure of $604 billion worldwide in 2010 [8,11].
This review examines evidence suggesting HSV1 as a causative agent of AD. We studied the literature focusing on viral characteristics of HSV1, the mechanisms this virus uses to enter the brain and cause neuropathological changes, potential treatment alternatives, and the genetic background of AD. We used PubMed to identify relevant papers using the keywords: ("Herpes Simplex"[Mesh]) AND "Alzheimer Disease"
Itzhaki et al. [22,32,33] 2014,2017,2018  Upon reactivation, the virus can follow an anterograde or retrograde path [2,17]. In retrograde fashion, there is axonal transport of HSV1 particles, which infiltrate the locus coeruleus progressing to the temporal lobe, particularly the hippocampus and entorhinal cortex [7,17,20]. Another mechanism is the intraneuronal flow of viral particles along the trigeminal nerve branches that supply basal meninges or the olfactory pathway [14,20]. Martin et al. also provided evidence of various envelope glycoproteins like gD, gB, and gE in the transsynaptic spread of viruses [14]. Thus, the virus can access synaptically linked neural circuits [14]. The dendritic nerve terminals of olfactory receptor neurons are exposed directly and, therefore, the macromolecules enter freely and transport across the synapses [17]. These olfactory receptors further synapse onto the olfactory bulb's mitral cell neurons, which project onto the entorhinal cortex, amygdala, and hippocampus [15]. Animal studies by Harris et al. demonstrated the use of this pathway by HSV1 [15]. They also identified viral DNA in olfactory bulb samples of the human brain using polymerase chain reaction (PCR) [15]. They found impaired olfactory function associated with increased incidents of mild cognitive impairment (MCI) and AD [15]. The study also found neurodegenerative pathology in the olfactory bulb and tract in early AD [15]. Lastly, the virus spreads via the bloodstream due to the disruption of the blood-brain barrier (BBB) [3][4]11,17,41]. The upregulation of neuroinflammatory markers and early neurodegeneration accompanies viral reactivation [7]. Thus, recurrent HSV1 reactivation in the brain could lead to ADassociated neurodegenerative processes [32].
These studies demonstrate a relationship between HSV1 and AD. A physiological decline in the immune system is a common reason for infection by HSV1, which enters the brain to reactivate later and cause further damage. This literature can be explored further to identify the population at risk for HSV1-induced AD and encourage researchers to find a preventive measure.

FIGURE 1: Viral nucleic acids on HSV1 interact with TLRs on microglia, leading to microglial production of pro-inflammatory cytokines
Studies support ROS as a mediator of neuroinflammation and AD pathogenesis. Interactions between HSV1 and oxidative stress promote neurodegenerative processes found in early AD.
Another aspect highlighted by Itzhaki et al. linking HSV1 to AD, is lysosomal impairment due to interactions between HSV1 infection and oxidative stress leading to the accumulation of toxic substances, further accelerating neurodegenerative changes ( Figure 2) [33].

FIGURE 2: Neurons are particularly susceptible to lysosomal damage
Impaired lysosomal function due to HSV1 and oxidative stress leads to the accumulation of lysosomes and decreased functionality of lysosomal proteins HSV1 -Herpes simplex virus type-1 Reference: [33] The above changes are known to occur early in AD development, thus supporting the role of HSV1 in AD [33].
APOE and phosphatidylinositol binding clathrin assembly protein (PICALM) are essential susceptibility genes in AD [7,33,43]. These susceptibility genes are linked with the HSV life cycle and correlate to cellular entry, intracellular transport, and APP processing [7,43]. Some of these susceptibility genes can lead to abnormalities in autophagy [33]. HSV1 inhibits the homeostatic process involved in the turnover/elimination of cytoplasmic components, damaged organelles, and protein aggregates, thereby modulating the host autophagy [4,15]. This mechanism also contributes to the deposition of amyloid plaques within the brain [4,15,42]. Mawanda et al. state that severe, recurrent, or chronic systemic infections can permanently damage the central nervous system (CNS), ultimately manifesting as cognitive impairment or dementia [2].
P-tau occurs in the activation pathway of the apoptotic process as a requirement for all changes at the cellular level that ends with the generation of apoptotic bodies [14]. They suggest that apoptotic processes and the neurodegeneration of the cytoskeleton are closely associated and occur due to various neurotoxic stimuli [14]. They found that HSV1 induces the hyperphosphorylation of Alzheimer-type tau epitopes, presenting a close analogy to the hyperphosphorylation processes described in neurodegenerative diseases [14]. Harris et al. and De Chiara et al. list the events after P-tau as conformational alterations forming paired helical filaments (PHFs) or NFTs, associated microtubule destabilization, synaptic damage, and neurodegeneration [15,29]. Viral kinases contribute to the occurrence of P-tau due to cross-species kinase promiscuity, whereby both human and viral kinases phosphorylate both human and viral proteins [17,20]. Another reason is the amino acid homology between human tau and HSV virus protein-22, the target of kinase UL13, which phosphorylates human tau [17,20].

Amyloid Plaque
The migration of new viral particles inside an infected cell requires interactions among HSV1 capsid proteins and APP [4,8,17,22]. Interaction between amyloid-β and HSV1 protein gB leads to HSV1 infectivity impairment by preventing the virus from fusing with the plasma membrane [34,44]. In early HSV1 infection, amyloid-β production plays a protective role in limiting it [3,19,29,37,40,44]. Following repeated viral reactivations, amyloid-β production switches from being defensive to becoming neurotoxic [19,29,32]. Eimer et al. list the possible factors mediating this switch as pathogen virulence and persistence, host genetics, and environmental factors [37]. Aβ peptide is overproduced to protect against latent HSV1 infection, leading to AD progression by contributing to amyloid plaque formation [2][3]34]. Overproduced Aβ leads to synaptic dysfunction, causing cognitive impairment [38]. In HSV1 infected neurons, there is an Aβdependent reduction in the expression of the presynaptic proteins associated with the diminished synaptic transmission; Piacentini et al. have first documented this [38]. In a study by De Chiara et al. on cultured mouse cortical neurons, HSV1 induced Aβ accumulation impaired synaptic function proving to be neurotoxic [29].
Studies on mice infected with HSV1 demonstrated that HSV1 reactivations triggered amyloid-β aggregation [29]. This correlated with cognitive impairment in them [29]. A parallel investigation validated that the accumulation of AD hallmarks in the same mice's brain displayed impaired memory in behavioral tests [29]. These findings demonstrate that HSV1 infection of neuronal cells can generate multiple APP fragments with neurotoxic potentials (Figure 3) [29].

FIGURE 3: HSV1 infection increases β and γ secretase, which participate in the amyoidogenic pathway to ultimately form Aβ peptides and AICD
In the amyoidogenic pathway, α and β secretases break down APP into N-and carboxy-terminal fragments. Aβ40 and Aβ42 primarily form the classical amyloid plaques seen in AD. The non-amyoidogenic pathway is made up of α and γ secretase. Harris et al. imply that amyloid plaque results from immunologic warfare between host and HSV1 [15].

Summary of Pathological Changes
The reactivated virus causes confined local damage via inflammatory and oxidative effects [15,32]. An increase in the intracellular levels of amyloid-β and a decrease in APP and P-tau follow [4,17]. This further accelerates the deposition of amyloid plaques and NFTs, which are the main components of AD [13,17]. HSV1 infection induces early upstream events that eventually lead to Aβ deposition and P-tau and thus superimpose our speculation that HSV1 is a possible risk factor for AD [39]. Pathogen-induced inflammation and CNS accumulation of amyloid-β damage the BBB, contributing to the pathophysiology of AD [7]. Thus, a vicious cycle of uncontrolled neural inflammation and neurodegeneration ensues [7,11]. Thus, data show that reactivation of HSV1 infection causes AD [12][13]17,[20][21]25,29,42,45].
Repeated cycles of HSV1 reactivation triggers chronic inflammation causing synaptic loss, leading to cognitive deficits. HSV1 infection induces upregulation in the expression of principal amyloid-β processing components leading to its deposition in the brain. Over-production of amyloid-β to contain HSV1, combined with decreased clearance of its aggregates due to aging, is neurotoxic and results in insoluble plaques. HSV1 infection increases the expression of enzymes involved in tau phosphorylation: GSK3β and PKA, resulting in tau hyperphosphorylation in infected neurons. This leads to neuronal cytoarchitectural changes, which affect synaptic stability and cognitive function. The above data demonstrate that it is, in fact, the reactivation of the virus that is responsible for the initiation of early AD changes.

Detection of Herpes Simplex Virus Type-1 in the Brain
Several epidemiological studies ( Table 2) identified HSV1 or HSV1 DNA or HSV1 proteins or HSV1 gene sequences in the brain of Alzheimer's patients and the elderly population at high risk of developing AD.

No. Discovery Author (Reference) Year of Publication
1.
An autopsy study on brains of AD patients and healthy controls (elderly) found:
In the AD group, 90% of amyloid plaques contained HSV1 DNA. 1b.
In the AD group, 72% of HSV1 DNA was plaque-associated. 1c.
The comparison group of healthy aged brains contained fewer plaques and in the control group, only 24% of HSV1 DNA was plaque-associated (p < 0.001).

2.
In AD patients, 90% of amyloid-β plaques were found to be co-localizing with HSV1 DNA. Detection of HSV1 thymidine kinase gene sequences in a higher proportion of brain tissue samples from AD cases (14/21) than controls (9/15) (using PCR).  HSV1 DNA is detected in the cerebrospinal fluid, suggesting that replication occurs in the CNS [11]. HSV1 receptors are abundantly expressed in the hippocampus [24].
After controlling for age, gender, educational level, and APOE-ε4 status, they found that anti-HSV1 IgM antibody seropositivity was associated with a significantly increased risk of developing AD (Hazard ratio: In a study, a high level of HSV1 antibodies in patients with AD was found to correlate with cortical atrophy of the gray matter using magnetic resonance.
The hypothesis that relates HSV1 to AD's pathogenesis has gained relevance because of the detection of viral DNA and viral proteins in the brains of AD cases. Additionally, serological studies show that HSV1 reactivation is associated with an increased risk of AD. These studies prove that APOE-ε4 is a multiplying factor that increases the risk of recurrence of HSV1, invasion of CNS by HSV1 on reactivation, and developing AD in general. Thus, if HSV1 and APOE-ε4 are present together, the risk of acquiring AD increases exponentially.

Herpes simplex encephalitis and Alzheimer's disease
Another evidence that links HSV1 and Alzheimer's is the striking similarity between AD and HSE, a neurological condition caused by HSV1 (Figure 4).

FIGURE 4: Herpes simplex encephalitis (HSE) affects the same anatomical locations in the brain, as involved in Alzheimer's disease (AD)
HSE patients are known to suffer long-term cognitive and behavioral symptoms similar to those seen in AD. HSE and AD patients have increased P-tau levels in cerebrospinal fluid (CSF).

FIGURE 5: Mechanism of action of acyclovir
Reference: [7,22,27] Valacyclovir is the bio-drug of acyclovir [7][8]17,22]. Figure 6 shows the mechanism of action of valacyclovir. Valacyclovir is rapidly hydrolyzed to acyclovir via the first-pass metabolism following oral administration [7]. The sustained activity of valacyclovir suggests that it may be symptomatic in the short-term and disease-modifying in the long-term, as per Devanand et al. [17]. They also mention a new trial of valacyclovir in the treatment of AD [17]. Penciclovir and foscarnet inhibit viral DNA replication [7].

FIGURE 6: Mechanism of action of valacyclovir
Reference: [17]   By preventing HSV1 spread and its replication, anti-viral agents would provide efficacious treatment ( Table  6) [27]. A study by Lathe et al. demonstrated that treatment with AVT prevented almost 90% of cases from the development of AD [24]. While HSV1 is unlikely to be the sole cause of AD, an AVT must be evaluated, primarily due to the limited effects of existing treatments and the failure of new treatments tested in patients with AD within the last two decades [17]. A study by Lin et al. about a mixed glycoprotein HSV1 vaccine has proven valuable in reducing HSV1 in the mouse brain after peripheral infection [7,15].

TABLE 6: Effect of anti-viral drugs and IVIG (intravenous Immunoglobulin)
* Although neither study shows definitely that these viruses cause Alzheimer's disease, data from a population-wide health database in Taiwan have been used to suggest not only that HSV infection increases the risk of developing the condition, but also that people treated with antiviral drugs are 10 times less likely to develop Alzheimer's disease.
Itzhaki et al. and Wozniak et al. mention a drug, BAY57-1293, more efficient than acyclovir in inhibiting HSV1 replication and decreasing amyloid-β and P-tau formation [22,47]. It also diminishes the size of cell clusters formed during infection much more efficiently than acyclovir, suggesting that BAY57-1293 is a more effective agent for treating AD [22,47].
Sulfated fucans from five algae with anti-viral activity (Scytothamnus australis, Marginariella boryana, Papenfussiella lutea, Splachnidium rugosum, and Undaria pinnatifida) has been studied [50]. Four sulfated fucan extracts prevented the accumulation of amyloid-β and P-tau in HSV1-infected Vero cells [3,50]. The most active sulfated fucan combined with acyclovir was incredibly useful, so it may be suitable for further experimental testing to develop AD patients' treatment protocols to slow or stop disease progression [50].
The above data provide indirect evidence correlating HSV1 to AD; the similarities in the anatomical locations and the long-term symptoms seen in HSE cases and AD are too substantial to neglect. The studies demonstrating improvement in AD patients on anti-viral therapy further supports our theory that HSV1 is a potential etiological factor in AD. Acyclovir and valacyclovir have proven to be the safest option amongst anti-herpetic drugs. The above studies have shown that substances with anti-viral properties are also useful in AD. This additionally strengthens our hypothesis of viral infection being a causative factor in AD. If future researchers can provide firm evidence associating the two, newer treatment and preventive alternatives can be developed, thereby enhancing the prognosis of AD and lowering the economic burden.

Limitations
The article is a narrative review, and it, therefore, does not follow the standard Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for systematic reviews. The possibility of bias remains both within individual studies and across studies since we could not perform a full quality assessment. We were not able to access all the articles completely, so some omissions and oversimplifications are possible.

Conclusions
The deleterious consequences of HSV1 infection imitate the vital aspects of AD pathophysiology. Numerous studies have enlisted mechanisms used by HSV1 to prompt chief processes involved in the formation of unique signs of AD, namely, amyloid plaques and neurofibrillary tangles. Co-localization of viral DNA with amyloid plaques, similarities in the anatomical locations involved, and serological studies linking the reactivation of the virus to AD signify HSV1 as one of the causative agents of AD. There is substantial evidence proving the efficacy of anti-viral agents in the treatment and deferral of AD. Further research establishing a causative link between HSV1 and AD are needed. Interventional clinical trials for a human HSV1 vaccine and a precise anti-viral regime for preventing and treating HSV1-infected MCI and AD patients are warranted. Until we establish new therapies, frequent screening and vaccination are vital in preventing the infection-related decline of cognition.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.