People living with HIV (PLWH) in the antiretroviral therapy (ART) era may lose more life-years to tobacco use than to HIV. Yet, smoking rates are more than twice as high among PLWH than the general ...population, contributing not just to mortality but to other adverse health outcomes, including neurocognitive deficits (neuroHIV). There is growing evidence that synergy with chronic inflammation and immune dysregulation that persists despite ART may be one mechanism by which tobacco smoking contributes to neuroHIV. This review will summarize the differential effects of nicotine vs tobacco smoking on inflammation in addition to the effects of tobacco smoke components on HIV disease progression. We will also discuss biomarkers of inflammation via neuroimaging as well as biomarkers of nicotine dependence (e.g., nicotine metabolite ratio). Tobacco smoking and nicotine may impact ART drug metabolism and conversely, certain ARTs may impact nicotine metabolism. Thus, we will review these bidirectional relationships and how they may contribute to neuroHIV and other adverse outcomes. We will also discuss the effects of tobacco use on the interaction between peripheral organs (lungs, heart, kidney) and subsequent CNS function in the context of HIV. Lastly, given the dramatic rise in the use of electronic nicotine delivery systems, we will discuss the implications of vaping on these processes. Despite the growing recognition of the importance of addressing tobacco use among PLWH, more research is necessary at both the preclinical and clinical level to disentangle the potentially synergistic effects of tobacco use, nicotine, HIV, cognition and immune dysregulation, as well as identify optimal approaches to reduce tobacco use.
Graphical Abstract
Proposed model of the relationships among HIV, ART, smoking, inflammation, and neurocognition. Solid lines represent relationships supported by evidence. Dashed lines represent relationships for which there is not enough evidence to make a conclusion. (a) HIV infection produces elevated levels of inflammation even among virally suppressed individuals. (b) HIV is associated with deficits in cognition function. (c) Smoking rates are higher among PLWH, compared to the general population. (d) The nicotine metabolite ratio (NMR) is associated with smoking behavior. (e) HIV and tobacco use are both associated with higher rates of psychiatric comorbidities, such as depression, and elevated levels of chronic stress. These factors may represent other mechanisms linking HIV and tobacco use. (f) The relationship between nicotine, tobacco smoking, and inflammation is complex, but it is well-established that smoking induces inflammation; the evidence for nicotine as anti-inflammatory is supported in some studies, but not others. (g) The relationship between tobacco use and neurocognition may differ for the effects of nicotine (acute nicotine use may have beneficial effects) vs. tobacco smoking (chronic use may impair cognition). (h) Elevated levels of inflammation may be associated with deficits in cognition. (i) PLWH may metabolize nicotine faster than those without HIV; the mechanism is not yet known and the finding needs validation in larger samples. We also hypothesize that if HIV-infection increases nicotine metabolism, then we should observe an attenuation effect once ART is initiated. (j) It is possible that the increase in NMR is due to ART effects on CYP2A6. (k) We hypothesize that faster nicotine metabolism may result in higher levels of inflammation since nicotine has anti-inflammatory properties.
Alzheimer's disease (AD) is characterized by cognitive dysfunction and amyloid plaques composed of the amyloid-beta peptide (Aβ). APOE is the greatest genetic risk for AD with APOE4 increasing risk ...up to ~ 15-fold compared to APOE3. Evidence suggests that levels and lipidation of the apoE protein could regulate AD progression. In glia, apoE is lipidated via cholesterol efflux from intracellular pools, primarily by the ATP-binding cassette transporter A1 (ABCA1). Therefore, increasing ABCA1 activity is suggested to be a therapeutic approach for AD. CS-6253 (CS) is a novel apoE mimetic peptide that was developed to bind and stabilize ABCA1 and maintain its localization into the plasma membrane therefore promoting cholesterol efflux. The goal of this study was to determine whether CS could modulate apoE levels and lipidation, Aβ pathology, and behavior in a model that expresses human APOE and overproduce Aβ.
In vitro, APOE3-glia or APOE4-glia were treated with CS. In vivo, male and female, E3FAD (5xFAD
/APOE3
) and E4FAD (5xFAD
/APOE4
) mice were treated with CS via intraperitoneal injection at early (from 4 to 8 months of age) and late ages (from 8 to 10 months of age). ApoE levels, ABCA1 levels and, apoE lipidation were measured by western blot and ELISA. Aβ and amyloid levels were assessed by histochemistry and ELISA. Learning and memory were tested by Morris Water Maze and synaptic proteins were measured by Western blot.
CS treatment increased apoE levels and cholesterol efflux in primary glial cultures. In young male E3FAD mice, CS treatment increased soluble apoE and lipid-associated apoE, reduced soluble oAβ and insoluble Aβ levels as well as Aβ and amyloid deposition, and improved memory and synaptic protein levels. CS treatment did not induce any therapeutic benefits in young female E3FAD and E4FAD mice or in any groups when treatment was started at later ages.
CS treatment reduced Aβ pathology and improved memory only in young male E3FAD, the cohort with the least AD pathology. Therefore, the degree of Aβ pathology or Aβ overproduction may impact the ability of targeting ABCA1 to be an effective AD therapeutic. This suggests that ABCA1-stabilizing treatment by CS-6253 works best in conditions of modest Aβ levels.
Neurologic deficits associated with human immunodeficiency virus (HIV) infection impact about 50% of persons with HIV (PWH). These disorders, termed HIV-associated neurocognitive disorders (HAND), ...possess neuropathologic similarities to Alzheimer’s disease (AD), including intra- and extracellular amyloid-beta (Aβ) peptide aggregates. Aβ peptide is produced through cleavage of the amyloid precursor protein (APP) by the beta secretase BACE1. However, this is precluded by cleavage of APP by the non-amyloidogenic alpha secretase, ADAM10. Previous studies have found that BACE1 expression was increased in the CNS of PWH with HAND as well as animal models of HAND. Further, BACE1 contributed to neurotoxicity. Yet in in vitro models, the role of ADAM10 and its potential regulatory mechanisms had not been examined. To address this, primary rat cortical neurons were treated with supernatants from HIV-infected human macrophages (HIV/MDMs). We found that HIV/MDMs decreased levels of both ADAM10 and Sirtuin1 (SIRT1), a regulator of ADAM10 that is implicated in aging and in AD. Both decreases were blocked with NMDA receptor antagonists, and treatment with NMDA was sufficient to induce reduction in ADAM10 and SIRT1 protein levels. Furthermore, decreases in SIRT1 protein levels were observed at an earlier time point than the decreases in ADAM10 protein levels, and the reduction in SIRT1 was reversed by proteasome inhibitor MG132. This study indicates that HIV-associated insults, particularly excitotoxicity, contribute to changes of APP secretases by downregulating levels of ADAM10 and its regulator.
EIF2AK3 encodes PKR-like endoplasmic reticulum (ER) kinase (PERK), which regulates ER stress response and plays a crucial role in deciding the cell’s commitment to survival/adaptation, stress ...tolerance, or apoptosis, depending on stress severity, duration, and recurrence. Conversely, common single nucleotide variants (SNVs) of EIF2AK3 may be associated with an increased risk of disorders in both the periphery and the central nervous system, albeit with small-medium effect sizes. Three exonic EIF2AK3 SNVs are in linkage disequilibrium and form the PERK-B haplotype, which occurs in 28% of the global population, compared to PERK-A, which is present in 62% of the population. The PERK-B haplotype was reported to be associated with several chronic neurodegenerative diseases, including progressive supranuclear palsy, neurocognitive impairment in people living with HIV, and Alzheimer’s. Importantly, the precise impact of these SNVs on PERK activity and function remains elusive. We have developed novel triple knock-in mice expressing the three exonic PERK-B SNVs. When exposed to acute ER stress, PERK-B mice exhibited elevated PERK kinase activity in the liver and heightened vulnerability to apoptotic cell death in the pancreas, compared to PERK-A mice. Furthermore, we used primary neuroglial cells from the PERK-B or PERK-A mice to examine cell survival, stress response, and stress tolerance. Thapsigargin-induced stress response of neuroglial cultures demonstrates differential and transient changes with PERK-B compared to PERK-A. PERK-B neuroglial cultures, but not enriched neurons, exhibited higher levels of eIF2a phosphorylation at 4-6 h, and activation of PERK signaling, specifically proapoptotic CHOP, at 12 h. However, ER stress-mediated death at 24 h was comparable between the PERK-A and PERK-B neuroglia. We also observed differential stress tolerance between the PERK-A and PERK-B neuroglial cultures, wherein PERK-B neuroglial cultures pretreated with a low-dose PERK-specific activator exhibited increased vulnerability to NMDA -induced death compared to PERK-A cultures. In summary, PERK-B was associated with transient changes in stress response and decreased stress tolerance with higher vulnerability to cellular stress. These changes, albeit transient and small/moderate sized, have clinical relevance as they might contribute to the overall genetic vulnerability in chronic diseases.
Chronic glial activation and neuroinflammation induced by the amyloid‐β peptide (Aβ) contribute to Alzheimer's disease (AD) pathology. APOE4 is the greatest AD‐genetic risk factor; increasing risk up ...to 12‐fold compared to APOE3, with APOE4‐specific neuroinflammation an important component of this risk. This editorial review discusses the role of APOE in inflammation and AD, via a literature review, presentation of novel data on Aβ‐induced neuroinflammation, and discussion of future research directions. The complexity of chronic neuroinflammation, including multiple detrimental and beneficial effects occurring in a temporal and cell‐specific manner, has resulted in conflicting functional data for virtually every inflammatory mediator. Defining a neuroinflammatory phenotype (NIP) is one way to address this issue, focusing on profiling the changes in inflammatory mediator expression during disease progression. Although many studies have shown that APOE4 induces a detrimental NIP in peripheral inflammation and Aβ‐independent neuroinflammation, data for APOE‐modulated Aβ‐induced neuroinflammation are surprisingly limited. We present data supporting the hypothesis that impaired apoE4 function modulates Aβ‐induced effects on inflammatory receptor signaling, including amplification of detrimental (toll‐like receptor 4‐p38α) and suppression of beneficial (IL‐4R‐nuclear receptor) pathways. To ultimately develop APOE genotype‐specific therapeutics, it is critical that future studies define the dynamic NIP profile and pathways that underlie APOE‐modulated chronic neuroinflammation.
In this editorial review, we present data supporting the hypothesis that impaired apoE4 function modulates Aβ‐induced effects on inflammatory receptor signaling, including amplification of detrimental (TLR4‐p38α) and suppression of beneficial (IL‐4R‐nuclear receptor) pathways, resulting in an adverse NIP that causes neuronal dysfunction. NIP, Neuroinflammatory phenotype; P.I., pro‐inflammatory; A.I., anti‐inflammatory.
In this editorial review, we present data supporting the hypothesis that impaired apoE4 function modulates Aβ‐induced effects on inflammatory receptor signaling, including amplification of detrimental (TLR4‐p38α) and suppression of beneficial (IL‐4R‐nuclear receptor) pathways, resulting in an adverse NIP that causes neuronal dysfunction. NIP, Neuroinflammatory phenotype; P.I., pro‐inflammatory; A.I., anti‐inflammatory.
HIV-associated neurocognitive disorders (HANDs) share common symptoms with Alzheimer's disease (AD), which is characterized by amyloid-β (Aβ) plaques. Plaques are formed by aggregation of Aβ ...oligomers, which may be the toxic species in AD pathogenesis, and oligomers are generated by cleavage of amyloid precursor protein (APP) by β-site amyloid precursor protein cleaving enzyme 1 (BACE1). BACE1 inhibitors reverse neuronal loss and cognitive decline in animal models of AD. Although studies have also found evidence of altered APP processing in HIV
patients, it is unknown whether increased BACE1 expression or Aβ oligomer production is a common neuropathological feature of HAND. Moreover, it is unknown whether BACE1 or APP is involved in the excitotoxic, NMDAR-dependent component of HIV-associated neurotoxicity
Herein, we hypothesize that HIV-associated neurotoxicity is mediated by NMDAR-dependent elevation of BACE1 and subsequent altered processing of APP. Supporting this, we observed elevated levels of BACE1 and Aβ oligomers in CNS of male and female HIV
patients. In a model of HIV-associated neurotoxicity in which rat neurons are treated with supernatants from HIV-infected human monocyte-derived macrophages, we observed NMDAR-dependent elevation of BACE1 protein. NMDA treatment also increased BACE1 and both pharmacological BACE1 inhibition and genetic loss of APP were partially neuroprotective. Moreover, in APP knock-out (APP
) mouse neurons, NMDA-induced toxicity was BACE1 independent, indicating that cytotoxicity of BACE1 is dependent upon APP cleavage. Our findings suggest that increased BACE1 and the resultant Aβ oligomer production may contribute to HIV-associated neuropathogenesis and inhibition of BACE1 could have therapeutic potential in HANDs.
HIV-associated neurocognitive disorders (HANDs) represent a range of cognitive impairments affecting ∼50% of HIV
individuals. The specific causes of HAND are unknown, but evidence suggests that HIV-infected macrophage infiltration into the brain may cause neuronal damage. Herein, we show that neurons treated with conditioned media from HIV-infected macrophages have increased expression of β-site amyloid precursor protein cleaving enzyme 1 (BACE1), a protein implicated in Alzheimer's disease pathogenesis. Moreover, inhibition of BACE1 prevented neuronal loss after conditioned media exposure, but had no effect on HIV-associated neurotoxicity in neurons lacking its cleavage target amyloid precursor protein. We also observed increased BACE1 expression in HIV
patient brain tissue, confirming the potential relevance of BACE1 as a therapeutic target in HANDs.
Abstract only
APO
E4 allele of apolipoprotein E (apoE) is the greatest genetic risk factor for Alzheimer's disease (AD) and is associated with accelerated accumulation of both amyloid plaques and and ...soluble oligomeric forms of the amyloid‐β peptide (oAβ), likely a proximal neurotoxin. Importantly, female
APOE4
carriers have a greater lifetime risk for developing AD, an increased rate of cognitive decline and accelerated accumulation of Aβ compared to male
APOE
4 carriers.
In vivo
progress has been limited by the lack of a tractable familial AD‐transgenic (FAD‐Tg) mouse model expressing human (h)‐ rather than mouse (m)‐
APOE.
To study the interactions among sex, h‐
APOE
and AD pathology, we developed the EFAD‐Tg mice by introducing the h‐
APOE
genotypes into the 5×FAD‐Tg mice. We previously demonstrated that in the EFAD mice, confirmed in human control and AD samples of CSF and brain, that apoE lipidation is lower and soluble Aβ levels are higher with
APOE4
vs.
APOE3
. Thus, we have developed
the mechanistic hypothesis that AD pathology and APOE4 cause a reduction in apoE lipidation, resulting in inefficient clearance of soluble A
β
synaptic loss, memory/cognitive deficits, and dementia.
Novel preliminary data demonstrate that the differences in female vs. male E4FAD mice mimic these established
APOE4
vs.
APOE3
differences in Aβ aggregation/accumulation, and apoE levels/lipidation. In addition, amyloid deposition, neuroinflammation and tau pathology is significantly greater in the EFAD females compared to males. These data suggest that sex profoundly influences
APOE
genotype‐specific effects on AD pathology, with apoE lipidation and oAβ levels possibly at the intersection between these two AD risk factors. Together, these data will help to address the critical need for treatment options for women at high risk for AD from the
APOE4
allele.
Support or Funding Information
Current LaDu lab funding includes NIH (NIA) P01AG044682, R21AG030128, R21AG048498, R21AG051233, CCTS (UL1RR029879)
Abstract only
APOE4
is the greatest genetic risk factor for Alzheimer's disease (AD), increasing risk up to 15‐fold compared to
APOE3
. As ɛ4 carriers can respond anomalously in clinical trials, ...there is a critical lack of therapeutics targeting mechanistic pathways of
APOE4
‐induced AD risk.
APOE4
is associated with accelerated amyloid‐β (Aβ) accumulation, both as amyloid and soluble oligomeric forms of Aβ (oAβ), the latter considered a proximal neurotoxin. Using EFAD mice, a new tractable preclinical mouse model (expressing 5 familial AD (FAD) mutations and the human
APOE
genotypes), we have developed the mechanistic hypothesis that apoE4 is poorly lipidated compared to apoE3, resulting in lower levels of apoE/Aβ complex and higher levels of soluble oAβ. ABCA1 is a promising therapeutic target for testing this hypothesis, as it is a major transporter of lipid to nascent apoE‐containing lipoproteins in the CNS. We previously demonstrated that treatment with RXR agonists increased ABCA1 levels, apoE4 lipidation, and soluble apoE4/Aβ complex levels, while lowering soluble Aβ levels. However, while these data provide target validation for ABCA1, the detrimental effects on the liver limit RXR agonist utility in even short‐term treatment protocols. Therefore, we have identified and are evaluating alternative pharmacological approaches to enhance apoE lipidation, modeling our approach on the peripheral targets for lipoprotein biogenesis and remodeling common in the cardio‐vascular field. Two therapeutic approaches of particular interest are an ABCA1 agonist (Compound A) and an Acyl‐CoA:cholesterol acyltransferase (ACAT) inhibitor (Compound B) to increase cholesterol efflux and free cholesterol availability, respectively.
In vitro,
both compounds facilitated cholesterol efflux from primary glia and increased the concentration and lipidation of apoE.
In vivo,
both compounds are brain penetrant and further analysis is on going, including treatment and prevention paradigms with read‐outs that include biochemical and AD pathology, as well as behavioral measurements.
Support or Funding Information
P01AG044682, R21AG030128, R21AG048498, R21AG051233, CCTS (UL1RR029879)
Cytosolic phospholipase A
(cPLA
) mediates oligomeric amyloid-β peptide (oAβ)-induced oxidative and inflammatory responses in glial cells. Increased activity of cPLA
has been implicated in the ...neuropathology of Alzheimer's disease (AD), suggesting that cPLA
regulation of oAβ-induced microglial activation may play a role in the AD pathology. We demonstrate that LPS, IFNγ, and oAβ increased phosphorylated cPLA
(p-cPLA
) in immortalized mouse microglia (BV2). Aβ association with primary rat microglia and BV2 cells, possibly via membrane-binding and/or intracellular deposition, presumably indicative of microglia-mediated clearance of the peptide, was reduced by inhibition of cPLA
. However, cPLA
inhibition did not affect the depletion of this associated Aβ when cells were washed and incubated in a fresh medium after oAβ treatment. Since the depletion was abrogated by NH
Cl, a lysosomal inhibitor, these results suggested that cPLA
was not involved in the degradation of the associated Aβ. To further dissect the effects of cPLA
on microglia cell membranes, atomic force microscopy (AFM) was used to determine endocytic activity. The force for membrane tether formation (F
) is a measure of membrane-cytoskeleton connectivity and represents a mechanical barrier to endocytic vesicle formation. Inhibition of cPLA
increased F
in both unstimulated BV2 cells and cells stimulated with LPS + IFNγ. Thus, increasing p-cPLA
would decrease F
, thereby increasing endocytosis. These results suggest a role of cPLA
activation in facilitating oAβ endocytosis by microglial cells through regulation of the membrane-cytoskeleton connectivity.
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