Viral infections represent a major challenge to public health, as evidenced by the COVID-19 pandemic (Morens and Fauci, 2020; Hu et al., 2021). Herpesviruses are ubiquitous human pathogens associated with a broad spectrum of acute and chronic diseases, including genital herpes, lymphoma, Kaposi’s sarcoma, encephalitis, stomach and nasopharyngeal cancer, coronary heart diseases, atherosclerosis, and neurodegeneration (Ashley and Wald, 1999; Whitley and Roizman, 2001; Leinonen and Saikku, 2002; Gupta et al., 2007; Dittmer and Damania, 2016; Readhead et al., 2018; Marcocci et al., 2020; Casanova and Abel, 2021). Herpes simplex virus 1 (HSV-1), also known as human herpesvirus 1 (HHV-1), establishes a life-long persistent infection in humans (Davison, 2010; Wilson and Mohr, 2012; Grinde, 2013), particularly as latent infection in neurons. HSV-1 infection results in acute lytic replication in oral epithelium and undergoes latent infection primarily in trigeminal ganglia. HSV-1 can transport through synaptically connected neurons to the central nervous system (CNS), and latent infection in the brain stem was reported in mice (Doll et al., 2019; Zhang S. et al., 2022). In addition, HSV-1 infection alters the integrity and permeability of the blood–brain barrier (BBB), which allows other infectious agents to transport into the brain parenchyma, exacerbating infection and provoking inflammation (Sweeney et al., 2018; Liu H. et al., 2019). Recent estimate of HSV-1 prevalence in the US, reported by the Centers for Disease Control and Prevention, was approximately 47.8% in the general population and increased linearly with age (McQuillan et al., 2018). Furthermore, aging and aging-associated stress conditions can trigger HSV-1 reactivation in infected cells (Stowe et al., 2012). As an obligate intracellular pathogen, HSV-1 replication relies on cellular machinery to produce viral structural components, assemble virion particles, and to support maturation and egress that yield infectious viral progeny dissemination and infection (Figure 1A; Mettenleiter, 2002). HSV-1 virion consists of a linear double-stranded DNA genome wrapped by an icosahedral capsid, which is encircled by both tegument and viral envelope (Bauer et al., 2013; Liu Y. T. et al., 2019). During a series of viral replication steps, the intimate interactions between HSV-1 and neuronal cells alter cellular homeostasis pathways, such as DNA damage and repair, metabolism, and immune response, which increase the risk of neuronal damage and cognitive impairment, instigating or exacerbating the aging-associated neurodegeneration underpinning Alzheimer’s disease (AD) (Conrady et al., 2010; Katan et al., 2013; Doll et al., 2019; Laval and Enquist, 2021; Sawtell and Thompson, 2021). Accruing evidence has suggested that microbial infections accelerate the pathogenesis of AD, and HSV-1 is the most highly suspected culprit (Heneka et al., 2015a; Ezzat et al., 2019). The presence of HSV-1 in the brain of AD patients was unambiguously confirmed by standard molecular detection methods such as PCR and in situ hybridization. Moreover, big data mining and association analysis provide molecular insights into how HSV-1 promotes neurodegeneration and AD progression (Readhead et al., 2018).

Alzheimer’s Disease, the most common form of dementia, is a debilitating neurodegenerative disorder resulting in cognitive impairments, and it is incurable (Knopman et al., 2021). From 1990 to 2019, the incidence of AD increased about 2.5-fold to 7.24 million cases worldwide. In addition, the estimated number of AD cases had reached 13.6 million in East Asia, 7.3 million in Western Europe, and 5.4 million in North America (Li et al., 2022). This rapidly increasing prevalence of AD highlights the urgency and importance of investigating core mechanisms of and developing innovative therapeutics for AD. The major pathology of AD is the accumulative senile plaques (Aβ₄₂ aggregate) and neurofibrillary tangles, that induce neuronal dysfunction and eventual cell death. Over the past decade, key biological features of the AD brain have been continuously identified, including neuroinflammation, metabolic dysregulation (Heppner et al., 2015), mitochondria dysfunction, oxidative stress, proteostasis perturbation and lipid abnormalities (Yin, 2023). Given the underlying causes and the pathological prognosis of AD are multifactorial, more risk factors, such as genetic mutations (Satizabal et al., 2019; Tasaki et al., 2019), traumatic brain injury (TBI) (Gardner and Yaffe, 2015), Down syndrome (Hartley et al., 2015), proinflammatory immune stimulation (Golde, 2019), and microbial infections (Readhead et al., 2018; Dominy et al., 2019) are emerging as prominent risk factors for neurodegeneration underpinning AD. Examining their contribution will better our understanding in the molecular pathogenesis of AD. Despite extensive clinical trials for AD treatment, no success has been reported until the most recent anti-Aβ antibody trial and a tau-targeting antisense oligonucleotide (MAPTRx) trial that showed promising response in some patients with significant side effect in others (Mummery et al., 2023; van Dyck et al., 2023). Thus, these observations suggest that current understanding of AD pathogenesis is inadequate.

There is an increasing appreciation of microbial infection in neurological disorders and chronic neurodegenerative diseases (Duggan et al., 2020; Athanasiou et al., 2022). Epstein–Barr virus (EBV) infection was found to greatly increase the risk of multiple sclerosis (Lanz et al., 2022). Influenza and pneumonia are associated with five neurodegenerative diseases, including AD, amyotrophic lateral sclerosis (ALS), generalized dementia (DEM), vascular dementia (VAS) and Parkinson’s disease (PD) (Levine et al., 2023). HSV-1 and HSV-2 infection accelerate the development of AD and AD-like neurodegeneration via instigating amyloid aggregation and neuroinflammation (Berger and Houff, 2008; Kristen et al., 2015; Zhang Y. et al., 2022). Moreover, persistent viral infection, particularly that of HSV-1 and -2, usually occurs early in life and far prior to the onset or diagnosis of neurodegenerative diseases (Mangold and Szpara, 2019; Zhu and Viejo-Borbolla, 2021). The reactivation of latently infected viruses during aging may increase the risk of brain diseases. As such, it is of importance to summarize current findings of how HSV-1 interacts with key components that drive the progression of AD. In this review, we summarize the recent advances concerning the roles of HSV-1 in AD pathogenesis, with focus on genetic risk factors, AD pathologies, inflammation, and metabolism (Figure 1B).

The Apolipoprotein E (APOE) gene exists in three common isoforms, i.e., APOE2, APOE3 and APOE4. These apolipoproteins E are the building blocks of many lipoproteins and can assist with their transport across the plasma membrane (Liu et al., 2013). The APOE4 isoform is a primary genetic risk factor of AD, which has been shown in the population-based cohort studies (Farrer et al., 1997; Linard et al., 2021) and genome-wide meta-genomic analysis (Lambert et al., 2009). The frequency of HSV-1 reactivation is corelated with the development of AD among APOE4 carriers from a cohort study (Linard et al., 2020). As reported, a 3-fold increase of AD incidence, calculated with adjusted hazard ratio, was observed in the patients with positive immunoglobulin M (IgM) or elevated levels of IgG against HSV-1. In the study using APOE4 knockout mice, Burgos et al. (2006) detected significant lower HSV-1 genomes in the nervous system than that in the wildtype mice, suggesting that APOE4 facilitates HSV-1 invasion and latency establishment in the brain. Later, by examining key steps of HSV-1 lytic replication, Liu et al. (2023) surprisingly demonstrated that APOE4 protein inhibits HSV-1 attachment but not viral entry, replication, assembly, or intracellular transportation of lytic replication, which may contribute to the enhanced latency. Paradoxically, APOE4 can be incorporated into HSV-1 virions to promote viral release from cell membrane (Liu et al., 2023). These findings indicate that APOE4 has distinct function in various steps of HSV-1 lytic replication, although the discrepancies remain unexplained. Aligning HSV-1-encoded proteins to human proteome identified viral protein sequences that mimic several human proteins (APOE4, CR1, PICALM, BACE1, BACE2 and gamma-secretase components), which are expressed by major AD genetic risk loci (Carter, 2010a). These host proteins were found to play regulatory roles in HSV-1 infection and replication (Table 1; Harris and Harris, 2018; Readhead et al., 2018), For example, PICALM regulates the transmembrane protein cation-independent mannose-6-phosphate IGF2 receptor (IGF2R) that is responsible for HSV-1 entry and cell-to-cell transmission, thus impacting HSV-1 replication and spread in the brain (Carter, 2011). Together, these findings highlight the synergistic interaction between human genetic makeup and microbial infection in shaping the AD pathogenesis. Genetic variants linked to the immune system, e.g., ADAM17, also known as TNF-α-converting enzyme and TNIP1, and LUBAC (involved in NLRP3 inflammasome activation) participate in immune signaling, such as TNF-α signaling pathways (Bellenguez et al., 2022). However, how HSV-1 infection interacts with these inflammatory genes is poorly understood. Perhaps, this may represent a sprouting area that human genetic variation and environmental factors synergize to accelerate the aging-associated neurodegeneration, leading to diverse forms of dementia including AD.

The initial observation implicating HSV-1 in AD pathogenesis was the frequent detection of the HSV-1 genome in the brain of AD patients, although it was also present in some controls. To further determine the role of HSV-1 in AD, studies involving cell cultures and animal models were routinely performed to determine whether and how HSV-1 actively contributes to AD pathogenesis. One of the well-accepted mechanisms is that Aβ functions as an antiviral peptide and is induced by HSV-1 infection (Soscia et al., 2010; Eimer et al., 2018a). The colocalization of Aβ plaques and viral proteins (e.g., glycoprotein B) is observed via immunostaining with antibodies against HSV-1 and Aβ (Eimer et al., 2018b; Ezzat et al., 2019). To exert its antiviral activity, Aβ deposits on HSV-1 virion particles such that HSV-1 induces the oligomerization and fibrilization of Aβ through physical interaction with its surface glycoprotein B, a ligand mediating HSV-1 entry (Bourgade et al., 2022; Yirün et al., 2023). Recently, the mechanistic connections between HSV-1 and Aβ pathology have been extensively studied, and these findings collectively support a causative role of HSV-1 in the neurodegeneration of AD. Specifically, HSV-1 was shown to directly induce the accumulation of Aβ₄₂ even at low multiplicity of infection (MOI = 0.3) in the neuronal cells and human induced pluripotent stem cells (hiPSCs) derived from healthy subjects (Wozniak et al., 2011; Abrahamson et al., 2021). Furthermore, increased intracellular Ca²⁺ was observed in the HSV-1-infected rat cortical neurons, leading to the Ca²⁺-dependent phosphorylation of APP and intracellular accumulation of Aβ₄₂ (Piacentini et al., 2011). Albaret et al. (2023) established a model of murine neuronal cells for HSV-1 infection and determined a caspase-dependent production of Aβ₄₂ oligomers. These Aβ₄₂ oligomers formed aggresomes with caspase 3A, impairing neuronal apoptosis and fueling Aβ₄₂ accumulation. However, in the study using 3D brain organoids of hiPSC infected with HSV-1, Aβ₄₂ was more abundantly found in bystander uninfected cells (Abrahamson et al., 2021), suggesting that the interaction between HSV-1 and the host cell is more complicated and likely associated with other cellular events, such as cell-to-cell communication and signal transduction. Although varying in distinct experimental settings, these studies nevertheless uncover a plethora of roles that HSV-1 can play in AD pathogenesis and suggest that more physiological models are direly needed to dissect the molecular pathogenesis of HSV-1-induced neurodegeneration.

Neurofibrillary tangles, commonly known as tauopathy, is another pathological feature of neurodegenerative AD and strongly correlates with the progression to late stages of the disease (Iqbal et al., 2005). Tau hyperphosphorylation is the known culprit of tauopathy (Martin et al., 2013). Studies using monolayer cell culture, 3D bioengineered brain model and rodent animal models demonstrated the induction of tau phosphorylation in response to HSV-1 infection (de Chiara et al., 2019; Cairns et al., 2020; Powell-Doherty et al., 2020). Multiple tau phosphorylation sites were identified upon HSV-1 infection in neuronal cells, including serine 202, threonine 212, serine 214, serine 396 and serine 404. Kinases such as glycogen synthase kinase 3β (GSK3β) and protein kinase A were reported to mediate these phosphorylations (Wozniak et al., 2009). Later, Alvarez et al. (2012) observed hyperphosphorylated tau in the nucleus of HSV-1-infected neuronal cells, and phosphorylated tau was associated with viral DNA replication. The re-distribution and phosphorylation of tau induced by HSV-1 increased host DNA damage (Sait et al., 2021), which contributes to the neuronal cell death and potentially accelerates AD progression. In addition, anti-herpetic drugs, as alternative treatment strategy, were used in the laboratory and clinical settings to reduce the development of AD, supporting the active roles of HSV-1 in AD pathogenesis. For example, antiviral agent acyclovir (ACV) ameliorates tauopathy by inhibiting HSV-1 DNA replication (Wozniak et al., 2011). Besides the cellular kinases such as GSK3β that are activated by HSV-1 to phosphorylate Tau, HSV-1-encode kinases, i.e., serine/threonine-protein kinase UL13 and US3, can directly phosphorylate tau to its hyperphosphorylation form, further exacerbating tauopathy and neurodegeneration (Benetti and Roizman, 2004; Devanand, 2018). Future studies are required to test a hypothesis determine whether there are direct interactions between other viral factors and tauopathies in AD pathogenesis associated with HSV-1 infection. In addition, in vivo studies using tau mouse models to assess the roles of HSV-1 infection in the tauopathy are missing and beg for immediate investigation.

In response to viral invasion in the central nervous system (CNS), pattern recognition receptors (PRRs) recognize the pathogen-associated molecular patterns (PAMPs) and initiate series of signaling events that culminate in the production of type I interferons (IFN-I) and inflammatory cytokines to establish a potent antiviral state. In mouse models with recurrent HSV-1 infection, augmented neuroinflammatory markers, including astrogliosis, IL1β and IL6, were observed to correlate with accumulation of amyloid-β protein and tau hyperphosphorylation (de Chiara et al., 2019). The innate immune response induced by acute HSV-1 infection in the brain is considered neuroprotective in normal brain. Upon HSV-1 infection in microglia, resident macrophages of the CNS, the cyclic GMP-AMP synthase (cGAS) binds to viral dsDNA and catalyzes the synthesis of cGAMP that in turn binds to and activates the adaptor protein stimulator of IFN-I genes (STING). Activated STING travels through the ER, trans-Golgi network (TGN) and endosomes, which activates IRF3 in a TBK1-dependent manner to stimulate IFN and inflammatory cytokine production (Figure 2; Reinert et al., 2016; Fang et al., 2023; Qin et al., 2023) To assess the role of cGAS-STING pathway in HSV-1 infection, Reinert et al. (2016) used cGAS- and STING-deficient mice and found that these mice were highly susceptible to HSV-1 infection and succumbed to herpes simplex encephalitis (HSE). These findings support the protective role of innate immune response against HSV-1 infection.

Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor strictly expressed on microglia in the brain, and maintains microglial homeostasis in metabolism and immune function (Ulland and Colonna, 2018). Variants in the TREM2 gene, exemplified by the R47H mutant, have significant association with the progression of LOAD (Guerreiro et al., 2012; Jiang et al., 2013). Several studies have demonstrated that TREM2 is an anti-inflammatory receptor against microbial infection, such as SARS-CoV-2 and HSV-1 (Zhu et al., 2020; Wu et al., 2021). Elevated viral replication was observed in TREM2-depleted co-cultures and Trem2^−/− mice (Stefanie et al., 2023). Depletion of TREM2 in microglia, differentiated from iPSC, reduced innate immune response, indicated by the lower expression of IFNB, IL6, TNFA, and MX1 induced by HSV-1 infection (Stefanie et al., 2023). Similarly, Toll-like receptor 3 (TLR3), a transmembrane receptor recognizing the dsRNA produced during HSV-1 replication, can effectively limit HSV-1 replication in CNS via activating the expression of IFNB mRNA and secretion of IFNβ (Gao et al., 2021). A cell culture study using cortical neurons derived from iPSC of HSV-1 encephalitis patients, also showed that the TLR3-deficient cells failed to restrict HSV-1 replication due to the impaired TLR3-IFN immunity (Lafaille et al., 2012). Other TLRs, such as TLR2 and TLR4, also respond to HSV-1 infection in astrocytes, resulting in the upregulation of IRF3, IRF7, IFNβ and the pro-inflammatory cytokine IL6 (Villalba et al., 2012). Findings from these in vivo and in vitro studies highlight the critical role of innate immune defense downstream of TLRs in the brain against HSV-1 infection.

Prolonged inflammation in the CNS can be pathological by creating excessive cellular stress and cytotoxicity, particularly in the AD brain (Kim and Joh, 2006; Zheng et al., 2016; Blank and Prinz, 2017; Kinney et al., 2018; Burgaletto et al., 2020). Constitutive activation of the cGAS-STING pathway in human AD and aged mice were observed (Gulen et al., 2023), and cgas deficient 5xFAD (cgas^−/−; 5xFAD) mice presented a better cognitive behavior and reduced amyloid pathology (Xie et al., 2023). Consistently, Roy et al. (2020, 2022) determined the constitutively expressed IFN-I in the brain of AD mouse model, in which active IFN-I signaling induced synapse loss, leading to cognitive impairment. In the study using AD mouse model lacking IFN-I signaling (APP_SWE/PS1_ΔE9 x IFNAR1^−/−), Taylor and Crack’s research team confirmed that the loss of IFN-I signaling conferred a predominantly anti-inflammatory glial phenotype, which protects mice from cognitive decline (Minter et al., 2016). Similarly, elevated expression of TLR2 and TLR4 in peripheral blood mononuclear cells (PBMCs) was found in AD patients compared to healthy control subjects (Zhang et al., 2012), and sustained TLR2 activation has been suggested to positively contribute to the neuroinflammation and Aβ accumulation (McDonald et al., 2016). HSV-1 reactivation in the mouse brain can increase the production of interleukin 1β (IL-1β), resulting in synaptic dysfunction through the epigenetic MeCP2-HDAC4 complex (Li Puma et al., 2023). These findings strongly support the conclusion that the innate immune activation and subsequent chronic neuroinflammation exacerbate AD pathogenesis (Heneka et al., 2015b; Zheng et al., 2016; Taylor et al., 2018).

Taken together, inflammatory response can be pleiotropic and may exert adverse effects on the brain. As Jin et al. (2021) reported that the PQBP1-cGAS-STING pathway is likely shared by viral infection and that extrinsic tau 3R/4R proteins induce microglia-mediated neuroinflammation toward neurodegenerative disease. Therefore, in the context of periodic HSV-1 reactivation in the elderly, HSV-1 infection could persistently activate PRRs (TRLs, RLRs and cGAS) (Figure 2), leading to chronic neuroinflammatory response and gradual neuron loss. Conceivably, better understanding in the mechanism of how HSV-1 induces inflammation in neurodegeneration will enable neuroprotective strategies via therapeutically modulating inflammation in the brain.

Metabolic dysregulation is another hallmark of LOAD (Salminen et al., 2015; McAvoy and Kawamata, 2019; Fairley et al., 2021). Metabolism is a central cellular activity that orchestrates diverse fundamental biological processes, such as synaptic communication and immune response. Mechanisms by which microbes reprogram cellular metabolism have been long studied, particularly those obligate intracellular parasites such as HSV-1 (Zhang et al., 2018; Polcicova et al., 2020). Upon de novo infection or reactivation, HSV-1 re-directs cellular metabolism to support virus replication, such as viral genome replication, structure protein synthesis, and lipid synthesis. To date, we have learned distinct strategies that HSV-1 evolved to thwart host immune response. However, how HSV-1 and other neurotropic viruses manipulate cellular metabolism in the brain is poorly understood, not mentioning in vivo studies using AD mouse models. Thus, how viral infection disrupts brain metabolic homeostasis in the context of AD and AD-related dementia (ADRD) deserves further investigation.

The role of HSV-1 in AD pathogenesis is emerging and is supported by mounting evidence from diverse studies. Yet, how exactly HSV-1 infection affects cellular processes that differentially impact neurodegeneration underpinning AD is largely unknown, despite the antimicrobial peptide function of Aβ. Here, we reviewed the recent advances in molecular studies on how HSV-1 infection alters key cellular activities that are relevant to AD pathogenesis. HSV-1 may provide a useful tool to probe the molecular mechanism of neurodegeneration and offers answers toward effective therapy.

SF and PF conceived of the review. SF, YZ, ZS, and ZC: draw figures. SF, YL, YZ, ZS, and PF wrote the manuscript. All authors commented on and approved the manuscript.

The Feng research group is supported by AG070904 and CA285192 from NIH, and microbial pathogenesis in AD grant from IDSA. CB research team is supported by Alfred E. Mann Family Foundation.

CB is the chief scientific advisor of ChromaDex and co-founder of Alphina Therapeutics. PF is a consultant for Marc J Bern & Partners LLP.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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