Brain aging in acquired immunodeficiency syndrome: increased ubiquitin-protein conjugate is correlated with decreased synaptic protein but not amyloid plaque accumulation

Alzheimer-Type Cerebral Amyloidosis in the Context of HIV Infection: Implications for a Proposed New Treatment Approach

Reverse transcriptase inhibitors (RTIs) are currently broadly prescribed for the treatment of HIV infection but are also thought to prevent Alzheimer's disease (AD) progression by protecting against amyloidosis. Our study evaluates the hypothesis that reverse transcriptase inhibitors protect against Alzheimer-type brain amyloidogenesis in the context of HIV infection. We compiled a case series of participants from a prospective study of the neurological consequences of HIV infection at the HIV Neurobehavioral Research Program (HNRP) who had serial neuropsychological and neurological assessments and were on RTIs. Two participants had gross and microscopic examination and immunohistochemistry of the brain at autopsy; one was assessed clinically for Alzheimer's disease by cerebrospinal fluid (CSF) analysis of phosphorylated-Tau, Total-Tau and Aβ42. Additionally, a larger cohort of 250 autopsied individuals was evaluated for presence of amyloid plaques, Tau, and related pathologies. Three older, virally suppressed individuals with HIV who had long-term treatment with RTIs were included in analyses. Two cases demonstrated substantial cerebral amyloid deposition at autopsy. The third case met clinical criteria for AD based on a typical clinical course and CSF biomarker profile. In the larger cohort of autopsied individuals, the prevalence of cerebral amyloidosis among people with HIV (PWH) was greater for those on RTIs. Our study showed that long-term RTI therapy did not protect against Alzheimer-type brain amyloidogenesis in the context of HIV infection in these patients. Given the known toxicities of RTIs, it is premature to recommend them to individuals at risk or with Alzheimer's disease who do not have HIV infection.

Neuropathogenic role of astrocyte-derived extracellular vesicles in HIV-associated neurocognitive disorders

Our previous findings demonstrated that astrocytic HIF-1α plays a major role in HIV-1 Tat-mediated amyloidosis which can lead to Alzheimer's-like pathology-a comorbidity of HIV-Associated Neurocognitive Disorders (HAND). These amyloids can be shuttled in extracellular vesicles, and we sought to assess whether HIV-1 Tat stimulated astrocyte-derived EVs (ADEVs) containing the toxic amyloids could result in neuronal injury in vitro and in vivo. We thus hypothesized that blocking HIF-1α could likely mitigate HIV-1 Tat-ADEV-mediated neuronal injury. Rat hippocampal neurons when exposed to HIV-1 Tat-ADEVs carrying the toxic amyloids exhibited amyloid accumulation and synaptodendritic injury, leading to functional loss as evidenced by alterations in miniature excitatory post synaptic currents. The silencing of astrocytic HIF-1α not only reduced the biogenesis of ADEVs, as well as amyloid cargos, but also ameliorated neuronal synaptodegeneration. Next, we determined the effect of HIV-1 Tat-ADEVs carrying amyloids in the hippocampus of naive mice brains. Naive mice receiving the HIV-1 Tat-ADEVs, exhibited behavioural changes, and Alzheimer's 's-like pathology accompanied by synaptodegeneration. This impairment(s) was not observed in mice injected with HIF-1α silenced ADEVs. This is the first report demonstrating the role of amyloid-carrying ADEVs in mediating synaptodegeneration leading to behavioural changes associated with HAND and highlights the protective role of HIF-1α.

Alzheimer-type cerebral amyloidosis in the context of HIV infection: implications for a proposed new treatment approach

Reverse transcriptase inhibitors (RTIs) are currently broadly prescribed for the treatment of HIV infection but are also thought to prevent Alzheimer's Disease (AD) progression by protecting against amyloidosis. Our study evaluates the hypothesis that reverse transcriptase inhibitors protect against Alzheimer-type brain amyloidogenesis in the context of HIV infection. We compiled a case series of participants from a prospective study of the neurological consequences of HIV infection at the HIV Neurobehavioral Research Program (HNRP) who had serial neuropsychological and neurological assessments and were on RTIs. Two participants had gross and microscopic examination and immunohistochemistry of the brain at autopsy; one was assessed clinically for Alzheimer's Disease by cerebrospinal fluid (CSF) analysis of phosphorylated-Tau, Total-Tau and Aβ42. Additionally, a larger cohort of autopsied individuals was evaluated for presence of amyloid plaques, Tau, and related pathologies. Three older, virally suppressed individuals with HIV who had long-term treatment with RTIs were included in analyses. Two cases demonstrated substantial cerebral amyloid deposition at autopsy. The third case met clinical criteria for AD based on a typical clinical course and CSF biomarker profile. In the larger cohort of autopsied individuals, the prevalence of cerebral amyloidosis among people with HIV (PWH) was greater for those on RTIs. Our study showed that long-term RTI therapy did not protect against Alzheimer-type brain amyloidogenesis in the context of HIV infection in these patients. Given the known toxicities of RTIs, it is premature to recommend them to individuals at risk or with Alzheimer's disease who do not have HIV infection.

Alzheimer's Disease Pathology in Middle Aged and Older People with HIV: Comparisons with Non-HIV Controls on a Healthy Aging and Alzheimer's Disease Trajectory and Relationships with Cognitive Function

We determined the prevalence of Alzheimer's disease (AD) pathological hallmarks, amyloid-β and phosphorylated-Tau, in autopsied brains of 49 people with HIV (PWH) (ages: 50-68; mean age = 57.0) from the National NeuroAIDS Tissue Consortium and in a comparative cohort of 55 people without HIV (PWoH) from the UC San Diego Alzheimer's Disease Research Center (17 controls, 14 mild cognitive impairment, 24 AD; ages: 70-102, mean age = 88.7). We examined how AD pathology relates to domain-specific cognitive functions in PWH overall and in sex-stratified samples. Amyloid-β and phosphorylated-Tau positivity (presence of pathology of any type/density) was determined via immunohistochemistry in AD-sensitive brain regions. Among PWH, amyloid-β positivity ranged from 19% (hippocampus) to 41% (frontal neocortex), and phosphorylated-Tau positivity ranged from 47% (entorhinal cortex) to 73% (transentorhinal cortex). Generally, AD pathology was significantly less prevalent, and less severe when present, in PWH versus PWoH regardless of cognitive status. Among PWH, positivity for AD pathology related most consistently to memory-related domains. Positivity for p-Tau pathology related to memory-related domains in women with HIV only, although the sample size of women with HIV was small (n = 10). Results indicate that AD pathology is present in a sizable portion of middle aged and older PWH, although not to the extent in older PWoH. Studies with better age-matched PWoH are needed to examine the effect of HIV status on AD pathology.

Neuropathologic Findings in Elderly HIV-Positive Individuals

The elderly HIV-positive population is growing due to the widespread use of combination antiretroviral therapy (cART), but the effects of longstanding HIV infection on brain aging are unknown. A significant proportion of HIV-positive individuals develop HIV-associated neurocognitive disorder (HAND) even on cART, but the pathogenesis of HAND is unknown. Although neuroinflammation is postulated to play an important role in aging and neurodegenerative diseases such as Alzheimer disease (AD), it is unclear whether HIV accelerates aging or increases the risk for AD. We examined the brains of 9 elderly HIV-positive subjects on cART without co-infection by hepatitis C virus compared to 7 elderly HIV-negative subjects. Microglial and astrocyte activation and AD pathologic change in association with systemic comorbidities and neurocognitive assessment were evaluated. There was no difference in microglial or astrocyte activation between our HIV-positive and HIV-negative cohorts. One HIV-positive subject and 2 HIV-negative subjects demonstrated significant amyloid deposition, predominantly in the form of diffuse senile plaques, but these individuals were cognitively normal. Neurofibrillary tangles were sparse in the HIV-positive cohort. There was a high prevalence of cardiovascular comorbidities in all subjects. These findings suggest that multiple factors likely contribute to aging and cognitive impairment in elderly HIV-positive individuals on cART.

Neurodegeneration Within the Amygdala Is Differentially Induced by Opioid and HIV-1 Tat Exposure

Opioid use disorder (OUD) is a critical problem that contributes to the spread of HIV and may intrinsically worsen neuroHIV. Despite the advent of combined antiretroviral therapies (cART), about half of persons infected with HIV (PWH) experience cognitive and emotional deficits that can be exacerbated by opioid abuse. HIV-1 Tat is expressed in the central nervous system (CNS) of PWH on cART and is thought to contribute to neuroHIV. The amygdala regulates emotion and memories associated with fear and stress and is important in addiction behavior. Notwithstanding its importance in emotional saliency, the effects of HIV and opioids in the amygdala are underexplored. To assess Tat- and morphine-induced neuropathology within the amygdala, male Tat transgenic mice were exposed to Tat for 8 weeks and administered saline and/or escalating doses of morphine twice daily (s.c.) during the last 2 weeks of Tat exposure. Eight weeks of Tat exposure decreased the acoustic startle response and the dendritic spine density in the basolateral amygdala, but not the central nucleus of the amygdala. In contrast, repeated exposure to morphine alone, but not Tat, increased the acoustic startle response and whole amygdalar levels of amyloid-β (Aβ) monomers and oligomers and tau phosphorylation at Ser396, but not neurofilament light chain levels. Co-exposure to Tat and morphine decreased habituation and prepulse inhibition to the acoustic startle response and potentiated the morphine-induced increase in Aβ monomers. Together, our findings indicate that sustained Tat and morphine exposure differentially promote synaptodendritic degeneration within the amygdala and alter sensorimotor processing.

Alzheimer's-Like Pathology at the Crossroads of HIV-Associated Neurological Disorders

Despite the widespread success of combined antiretroviral therapy (cART) in suppressing viremia, the prevalence of human immunodeficiency virus (HIV)-associated neurological disorders (HAND) and associated comorbidities such as Alzheimer’s disease (AD)-like symptomatology is higher among people living with HIV. The pathophysiology of observed deficits in HAND is well understood. However, it has been suggested that it is exacerbated by aging. Epidemiological studies have suggested comparable concentrations of the toxic amyloid protein, amyloid-β42 (Aβ42), in the cerebrospinal fluid (CSF) of HAND patients and in the brains of patients with dementia of the Alzheimer’s type. Apart from abnormal amyloid-β (Aβ) metabolism in AD, a better understanding of the role of similar pathophysiologic processes in HAND could be of substantial value. The pathogenesis of HAND involves either the direct effects of the virus or the effect of viral proteins, such as Tat, Gp120, or Nef, as well as the effects of antiretrovirals on amyloid metabolism and tauopathy, leading, in turn, to synaptodendritic alterations and neuroinflammatory milieu in the brain. Additionally, there is a lack of knowledge regarding the causative or bystander role of Alzheimer’s-like pathology in HAND, which is a barrier to the development of therapeutics for HAND. This review attempts to highlight the cause–effect relationship of Alzheimer’s-like pathology with HAND, attempting to dissect the role of HIV-1, HIV viral proteins, and antiretrovirals in patient samples, animal models, and cell culture model systems. Biomarkers associated with Alzheimer’s-like pathology can serve as a tool to assess the neuronal injury in the brain and the associated cognitive deficits. Understanding the factors contributing to the AD-like pathology associated with HAND could set the stage for the future development of therapeutics aimed at abrogating the disease process.

Mechanisms of neuronal dysfunction in HIV-associated neurocognitive disorders

HIV-associated neurocognitive disorder (HAND) is characterized by cognitive and behavioral deficits in people living with HIV. HAND is still common in patients that take antiretroviral therapies, although they tend to present with less severe symptoms. The continued prevalence of HAND in treated patients is a major therapeutic challenge, as even minor cognitive impairment decreases patient’s quality of life. Therefore, modern HAND research aims to broaden our understanding of the mechanisms that drive cognitive impairment in people with HIV and identify promising molecular pathways and targets that could be exploited therapeutically. Recent studies suggest that HAND in treated patients is at least partially induced by subtle synaptodendritic damage and disruption of neuronal networks in brain areas that mediate learning, memory, and executive functions. Although the causes of subtle neuronal dysfunction are varied, reversing synaptodendritic damage in animal models restores cognitive function and thus highlights a promising therapeutic approach. In this review, we examine evidence of synaptodendritic damage and disrupted neuronal connectivity in HAND from clinical neuroimaging and neuropathology studies and discuss studies in HAND models that define structural and functional impairment of neurotransmission. Then, we report molecular pathways, mechanisms, and comorbidities involved in this neuronal dysfunction, discuss new approaches to reverse neuronal damage, and highlight current gaps in knowledge. Continued research on the manifestation and mechanisms of synaptic injury and network dysfunction in HAND patients and experimental models will be critical if we are to develop safe and effective therapies that reverse subtle neuropathology and cognitive impairment.

HIV-1 Tat-mediated astrocytic amyloidosis involves the HIF-1α/lncRNA BACE1-AS axis

Increased life expectancy of patients diagnosed with HIV in the current era of antiretroviral therapy is unfortunately accompanied with the prevalence of HIV-associated neurocognitive disorders (HANDs) and risk of comorbidities such as Alzheimer-like pathology. HIV-1 transactivator of transcription (Tat) protein has been shown to induce the production of toxic neuronal amyloid protein and also enhance neurotoxicity. The contribution of astrocytes in Tat-mediated amyloidosis remains an enigma. We report here, in simian immunodeficiency virus (SIV)+ rhesus macaques and patients diagnosed with HIV, brain region-specific up-regulation of amyloid precursor protein (APP) and Aβ (40 and 42) in astrocytes. In addition, we find increased expression of β-site cleaving enzyme (BACE1), APP, and Aβ in human primary astrocytes (HPAs) exposed to Tat. Mechanisms involved up-regulation of hypoxia-inducible factor (HIF-1α), its translocation and binding to the long noncoding RNA (lncRNA) BACE1-antisense transcript (BACE1-AS), resulting, in turn, in the formation of the BACE1-AS/BACE1 RNA complex, subsequently leading to increased BACE1 protein, and activity and generation of Aβ-42. Gene silencing approaches confirmed the regulatory role of HIF-1α in BACE1-AS/BACE1 in Tat-mediated amyloidosis. This is the first report implicating the role of the HIF-1α/lncRNABACE1-AS/BACE1 axis in Tat-mediated induction of astrocytic amyloidosis, which could be targeted as adjunctive therapies for HAND-associated Alzheimer-like comorbidity.

Cerebrospinal Fluid Concentrations of the Synaptic Marker Neurogranin in Neuro-HIV and Other Neurological Disorders

Purpose of Review

The aim of this study was to examine the synaptic biomarker neurogranin in cerebrospinal fluid (CSF) in different stages of HIV infection and in relation to what is known about CSF neurogranin in other neurodegenerative diseases.

Recent Findings

CSF concentrations of neurogranin are increased in Alzheimer’s disease, but not in other neurodegenerative disorder such as Parkinson’s disease, frontotemporal dementia, and Lewy body dementia. Adults with HIV-associated dementia have been found to have decreased levels of neurogranin in the frontal cortex, which at least to some extent, may be mediated by the proinflammatory cytokines IL-1β and IL-8.

Summary

CSF neurogranin concentrations were in the same range for all groups of HIV-infected individuals and uninfected controls. This either indicates that synaptic injury is not an important part of HIV neuropathogenesis or that CSF neurogranin is not sensitive to the type of synaptic impairment present in HIV-associated neurocognitive disorders.

The impact of HIV central nervous system persistence on pathogenesis

: The persistence of HIV in the central nervous system is somewhat controversial particularly in the context of HIV viral suppression from combined antiretroviral therapy. Further, its significance in relation to HIV pathogenesis in the context of HIV-associated neurocognitive disorders, systemic HIV pathogenesis, and eradication in general, but especially from the brain, are even more contentious. This review will discuss each of these aspects in detail, highlighting new data, particularly from recent conference presentations.

Risk of developing cerebral β-amyloid plaques with posttranslational modification among HIV-infected adults

OBJECTIVES

Evidence of accelerated brain aging among HIV-infected adults argues for the increased risk of developing cerebral β-amyloid (Aβ) plaques. We compared the frequency of Aβ plaque-bearing cases in our HIV cohort with that in a general cohort reported by Braak et al. We explored posttranslationally modified Aβ forms (N3pE, E22P, phospho-Ser8) in plaques and E22P-Aβ in the postmortem cerebrospinal fluid (CSF) in the HIV cohort.

DESIGN

Clinicopathological study of HIV-infected adults.

METHODS

To assess frontal Aβ plaque deposition, we conducted immunohistochemistry for generic Aβ (4G8) and three modified Aβ forms. We determined CSF E22P-Aβ levels by ELISA.

RESULTS

We found 4G8-Aβ plaques in 29% of 279 HIV-infected cases. Within the age range of 31-70 years, the frequency of 4G8-Aβ plaque-bearing cases was higher in our HIV cohort (n = 273) compared with the general cohort (n = 1110) overall (29.3 vs. 25.8%) and across four age groups by decade (odds ratio 2.35, P < 0.0001). In HIV-infected cases with (n = 37) and without (n = 12) 4G8-Aβ plaques, modified Aβ forms occurred in order: N3pE, E22P, and phospho-Ser8. In CSF assays of HIV-infected cases with (n = 27; 17 focal, 10 widespread) and without (n = 11) 4G8-Aβ plaques, the median E22P-Aβ/Aβ40 ratio was higher among cases with widespread plaques than in cases with focal or absent plaques (P = 0.047).

CONCLUSION

Our findings suggest HIV-infected adults are at increased risk of developing cerebral Aβ plaques. The occurrence of modified Aβ forms in order suggests the progression stages of Aβ plaque deposition. The potential for E22P-Aβ as a CSF biomarker of cerebral Aβ plaques should be investigated.

Associations of regional amyloid-β plaque and phospho-tau pathology with biological factors and neuropsychological functioning among HIV-infected adults

With increasing age, the general population is increasingly vulnerable to the development of cerebral amyloid-β (Aβ) plaque and neuronal phospho-tau (p-tau) pathology. In HIV disease, prior studies of these neuropathologic changes were relatively limited. Here, we characterized Aβ plaques and p-tau lesions by immunohistochemistry in relevant brain regions (prefrontal neocortex, putamen, basal-temporal neocortex, and hippocampus) of HIV-infected adults. We used multivariable logistic regression to predict regional Aβ plaque or p-tau pathology based on demographic factors, apolipoprotein E (APOE) genotypes, HIV disease-related factors, and regional gliosis. We used multiple linear regression to predict T-scores in neuropsychological domains based on regional Aβ plaque or p-tau pathology. We found that APOE ε4 alleles, older age, and higher plasma HIV-1 RNA predicted prefrontal Aβ plaques (odds ratio (OR) 5.306, 1.045, and 0.699, respectively, n = 168). Older age predicted putamen Aβ plaques (OR 1.064, n = 171). APOE ε4 alleles, hepatitis C virus seropositivity, and higher plasma HIV-1 RNA predicted hippocampus Aβ plaques (OR 6.779, 6.138, and 0.589, respectively, n = 56). The p-tau lesions were sparse in the vast majority of affected cases. Lifetime substance use disorder and higher plasma HIV-1 RNA predicted putamen p-tau lesions (OR 0.278 and 0.638, respectively, n = 67). Older age and gliosis predicted hippocampus p-tau lesions (OR 1.128 and 0.592, respectively, n = 59). Prefrontal Aβ plaques predicted lower speed of information processing (n = 159) and putamen Aβ plaques predicted lower levels of attention and working memory (n = 88). Regional p-tau lesions were not significantly predictive of any neuropsychological domains. In conclusion, Aβ plaque or p-tau pathology in different brain regions was predicted by different sets of biological factors. Aβ plaques in prefrontal neocortex and putamen predicted poorer functioning in cognitive domains relevant to these brain regions. The absence of significant impact of regional p-tau lesions on neuropsychological functioning might be explained by the subthreshold burden of p-tau lesions.

The current understanding of overlap between characteristics of HIV-associated neurocognitive disorders and Alzheimer's disease

The advent of effective antiretroviral medications (ARVs) has led to an aging of the HIV population with approximately 50% of people with HIV (PWH) being over the age of 50 years. Neurocognitive complications, typically known as HIV-associated neurocognitive disorders (HAND), persist in the era of ARVs and, in addition to risk of HAND, older PWH are also at risk for age-associated, neurodegenerative disorders including Alzheimer's disease (AD). It has been postulated that risk for AD may be greater among PWH due to potential compounding effects of HIV and aging on mechanisms of neural insult. We are now faced with the challenge of disentangling AD from HAND, which has important prognostic and treatment implications given the more rapidly debilitating trajectory of AD. Herein, we review the evidence to date demonstrating both parallels and differences in the profiles of HAND and AD. We specifically address similarities and difference of AD and HAND as it relates to (1) neuropsychological profiles (cross-sectional/longitudinal), (2) AD-associated neuropathological features as evidenced from neuropathological, cerebrospinal fluid and neuroimaging assessments, (3) biological mechanisms underlying cortical amyloid deposition, (4) parallels in mechanisms of neural insult, and (5) common risk factors. Our current understanding of the similarities and dissimilarities of AD and HAND should be further delineated and leveraged in the development of differential diagnostic methods that will allow for the early identification of AD and more suitable and effective treatment interventions among graying PWH.

Diagnostic and prognostic biomarkers for HAND

In 2007, the nosology for HIV-1-associated neurocognitive disorders (HAND) was updated to a primarily neurocognitive disorder. However, currently available diagnostic tools lack the sensitivity and specificity needed for an accurate diagnosis for HAND. Scientists and clinicians, therefore, have been on a quest for an innovative biomarker to diagnose (i.e., diagnostic biomarker) and/or predict (i.e., prognostic biomarker) the progression of HAND in the post-combination antiretroviral therapy (cART) era. The present review examined the utility and challenges of four proposed biomarkers, including neurofilament light (NFL) chain concentration, amyloid (i.e., sAPPα, sAPPβ, amyloid β) and tau proteins (i.e., total tau, phosphorylated tau), resting-state functional magnetic resonance imaging (fMRI), and prepulse inhibition (PPI). Although significant genotypic differences have been observed in NFL chain concentration, sAPPα, sAPPβ, amyloid β, total tau, phosphorylated tau, and resting-state fMRI, inconsistencies and/or assessment limitations (e.g., invasive procedures, lack of disease specificity, cost) challenge their utility as a diagnostic and/or prognostic biomarker for milder forms of neurocognitive impairment (NCI) in the post-cART era. However, critical evaluation of the literature supports the utility of PPI as a powerful diagnostic biomarker with high accuracy (i.e., 86.7-97.1%), sensitivity (i.e., 89.3-100%), and specificity (i.e., 79.5-94.1%). Additionally, the inclusion of multiple CSF and/or plasma markers, rather than a single protein, may provide a more sensitive diagnostic biomarker for HAND; however, a pressing need for additional research remains. Most notably, PPI may serve as a prognostic biomarker for milder forms of NCI, evidenced by its ability to predict later NCI in higher-order cognitive domains with regression coefficients (i.e., r) greater than 0.8. Thus, PPI heralds an opportunity for the development of a brief, noninvasive diagnostic and promising prognostic biomarker for milder forms of NCI in the post-cART era.

Association of antiretroviral therapy with brain aging changes among HIV-infected adults

OBJECTIVE

Antiretroviral therapy (ART) is currently recommended for all persons living with HIV (PLWH), regardless of their CD4 T-cell count, and should be continued throughout life. Nonetheless, vigilance of the safety of ART, including its neurotoxicity, must continue. We hypothesized that use of certain ART drugs might be associated with aging-related cerebral degenerative changes among PLWH.

DESIGN

Clinicopathological study of PLWH who were using ART drugs at the last clinical assessment.

METHODS

Using multivariable logistic regression, we tested associations between use of each specific ART drug (with reference to use of other ART drugs) and cerebral degenerative changes including neuronal phospho-tau lesions, β-amyloid plaque deposition, microgliosis, and astrogliosis in the frontal cortex and putamen (immunohistochemistry), as well as cerebral small vessel disease (CSVD) in the forebrain white matter (standard histopathology), with relevant covariates being taken into account. The Bonferroni adjustment was applied.

RESULTS

Darunavir use was associated with higher likelihood of neuronal phospho-tau lesions in the putamen [odds ratio (OR) 15.33, n = 93, P = 0.005]. Ritonavir use was associated with marked microgliosis in the putamen (OR 4.96, n = 101, P = 0.023). On the other hand, use of tenofovir disoproxil fumarate was associated with lower likelihood of β-amyloid plaque deposition in the frontal cortex (OR 0.13, n = 102, P = 0.012). There was a trend toward an association between emtricitabine use and CSVD (OR 13.64, n = 75, P = 0.099).

CONCLUSION

Our findings suggest that PLWH treated with darunavir and ritonavir may be at increased risk of aging-related cerebral degenerative changes.

Differentiating HIV-Associated Neurocognitive Disorders From Alzheimer's Disease: an Emerging Issue in Geriatric NeuroHIV

PURPOSE OF REVIEW

The purpose of this review was to examine characteristics that may distinguish HIV-associated neurocognitive disorder (HAND) from early Alzheimer's disease (AD).

RECENT FINDINGS

Cerebrospinal fluid (CSF) AD biomarkers are perturbed in HIV, yet these alterations may be limited to settings of advanced dementia or unsuppressed plasma HIV RNA. Neuropsychological testing will require extensive batteries to maximize utility. Structural imaging is limited for early AD detection in the setting of HIV, but proper studies are absent. While positron-emission tomography (PET) amyloid imaging has altered the landscape of differential diagnosis for age-associated neurodegenerative disorders, costs are prohibitive. Risk for delayed AD diagnosis in the aging HIV-infected population is now among the most pressing issues in geriatric neuroHIV. While clinical, imaging, and biomarker characterizations of AD are extensively defined, fewer data define characteristics of HIV-associated neurocognitive disorder in the setting of suppressed plasma HIV RNA. Data needed to inform the phenotype of AD in the setting of HIV are equally few.

HIV-associated neurocognitive disorder

Human immunodeficiency virus (HIV)-associated neurocognitive disorder (HAND) affects roughly half the HIV-positive population. The symptoms of cognitive slowing, poor concentration, and memory problems can impact on everyday life. Its diagnosis is validated where possible by identifying deficits in two cognitive domains on neuropsychologic testing in patients either with or without symptoms. Corroborating evidence may be found on imaging, blood tests, and cerebrospinal fluid analysis, though sensitive and specific biomarkers are currently lacking. The introduction of combined antiretroviral therapy in the 1990s has generated a therapeutic paradox whereby the number of severe cases of HAND has fallen, yet milder forms continue to rise in prevalence. New emphasis has been placed on identifying the cause of apparent ongoing HIV infection and inflammation of the central nervous system (CNS) in the face of durable systemic viral suppression, and how this equates to the neuronal dysfunction underlying HAND. The interaction with aging and comorbidities is becoming increasingly common as the HIV-positive population enters older adulthood, with neurodegenerative, metabolic, and vascular causes of cognitive impairment combining and probably accelerating in the context of chronic HIV infection. Therapies targeted to the CNS, but without neurotoxic side-effects, are being investigated to attempt to reduce the likelihood of developing, and improving, HAND.

Differential Effects of Antiretroviral Drugs on Neurons In Vitro: Roles for Oxidative Stress and Integrated Stress Response

Mounting evidence suggests that antiretroviral drugs may contribute to the persistence of HIV-associated neurocognitive disorders (HAND), which impact 30%-50% of HIV-infected patients in the post-antiretroviral era. We previously reported that two first generation HIV protease inhibitors, ritonavir and saquinavir, induced oxidative stress, with subsequent neuronal death in vitro, which was reversed by augmentation of the endogenous antioxidant response by monomethyl fumarate. We herein determined whether two newer-generation PIs, darunavir and lopinavir, were deleterious to neurons in vitro. Further, we expanded our assessment to include three integrase strand transfer inhibitors, raltegravir, dolutegravir, and elvitegravir. We found that only lopinavir and elvitegravir were neurotoxic to primary rat neuroglial cultures as determined by the loss of microtubule-associated protein 2 (MAP2). Intriguingly, lopinavir but not elvitegravir led to oxidative stress and induced the endogenous antioxidant response (EAR). Furthermore, neurotoxicity of lopinavir was blocked by pharmacological augmentation of the endogenous antioxidant heme oxygenase-1 (HO-1), expanding our previous finding that protease inhibitor-induced neurotoxicity was mediated by oxidative stress. Conversely, elvitegravir but not lopinavir led to increased eIF2α phosphorylation, indicating the activation of a common adaptive pathway termed the integrated stress response (ISR), and elvitegravir-mediated neurotoxicity was partially alleviated by the ISR inhibitor trans-ISRIB, suggesting ISR as a promoter of elvitegravir-associated neurotoxicity. Overall, we found that neurotoxicity was induced only by a subset of protease inhibitors and integrase strand transfer inhibitors, providing evidence for class- and drug-specific neurotoxic effects of antiretroviral drugs. Future in vivo studies will be critical to confirm the neurotoxicity profiles of these drugs for incorporation of these findings into patient management. The EAR and ISR pathways are potential access points for the development of adjunctive therapies to complement antiretroviral therapies and limit their contribution to HAND persistence.

Accelerated and accentuated neurocognitive aging in HIV infection

There is debate as to whether the neurocognitive changes associated with HIV infection represent an acceleration of the typical aging process or more simply reflect a greater accentuated risk for age-related declines. We aimed to determine whether accelerated neurocognitive aging is observable in a sample of older HIV-infected individuals compared to age-matched seronegatives and older old (i.e., aged ≥65) seronegative adults. Participants in a cross-sectional design included 48 HIV-seronegative (O-) and 40 HIV-positive (O+) participants between the ages of 50-65 (mean ages = 55 and 56, respectively) and 40 HIV-seronegative participants aged ≥65 (OO-; mean age = 74) who were comparable for other demographics. All participants were administered a brief neurocognitive battery of attention, episodic memory, speeded executive functions, and confrontation naming (i.e., Boston Naming Test). The O+ group performed more poorly than the O- group (i.e., accentuated aging), but not differently from the OO- on digit span and initial recall of a supraspan word list, consistent with an accelerating aging profile. However, the O+ group's performance was comparable to the O- group on all other neurocognitive tests (ps > 0.05). These data partially support a model of accelerated neurocognitive aging in HIV infection, which was observed in the domain of auditory verbal attention, but not in the areas of memory, language, or speeded executive functions. Future studies should examine whether HIV-infected adults over 65 evidence accelerated aging in downstream neurocognitive domains and subsequent everyday functioning outcomes.

Caffeine Blocks HIV-1 Tat-Induced Amyloid Beta Production and Tau Phosphorylation

The increased life expectancy of people living with HIV-1 who are taking effective anti-retroviral therapeutics is now accompanied by increased Alzheimer's disease (AD)-like neurocognitive problems and neuropathological features such as increased levels of amyloid beta (Aβ) and phosphorylated tau proteins. Others and we have shown that HIV-1 Tat promotes the development of AD-like pathology. Indeed, HIV-1 Tat once endocytosed into neurons can alter morphological features and functions of endolysosomes as well as increase Aβ generation. Caffeine has been shown to have protective actions against AD and based on our recent findings that caffeine can inhibit endocytosis in neurons and can prevent neuronal Aβ generation, we tested the hypothesis that caffeine blocks HIV-1 Tat-induced Aβ generation and tau phosphorylation. In SH-SY5Y cells over-expressing wild-type amyloid beta precursor protein (AβPP), we demonstrated that HIV-1 Tat significantly increased secreted levels and intracellular levels of Aβ as well as cellular protein levels of phosphorylated tau. Caffeine significantly decreased levels of secreted and cellular levels of Aβ, and significantly blocked HIV-1 Tat-induced increases in secreted and cellular levels of Aβ. Caffeine also blocked HIV-1 Tat-induced increases in cellular levels of phosphorylated tau. Furthermore, caffeine blocked HIV-1 Tat-induced endolysosome dysfunction as indicated by decreased protein levels of vacuolar-ATPase and increased protein levels of cathepsin D. These results further implicate endolysosome dysfunction in the pathogenesis of AD and HAND, and by virtue of its ability to prevent and/or block neuropathological features associated with AD and HAND caffeine might find use as an effective adjunctive therapeutic agent.

Verbal list learning and memory profiles in HIV-infected adults, Alzheimer's disease, and Parkinson's disease: An evaluation of the "cortical hypothesis" of NeuroAIDS

HIV+ population is getting older because of progress in treatments. Yet, there are concerns that Older HIV+ individuals (OHIV+) may be more vulnerable for developing a "cortical" dementia such as Alzheimer Disease (AD). Our aim was to explore the hypothesis that the cognitive deficit extends to ''cortical'' functions in OHIV+ by comparing serial position effects (SPE) in different groups of participants affected by "cortical" or "subcortical" damage. We enrolled a total of 122 subjects: 22 OHIV+ (≥60 years of age), 31 Younger HIV+ (YHIV+) (<60 years of age), 18 participants with AD, 23 subjects with Parkinson Disease (PD), and 28 healthy subjects. All subjects performed verbal learning tasks (VLT) to explore SPE. Factorial analysis of covariance showed a significant effect of "group" (p < 0.001) and "task" (Primacy vs Recency) (p < 0.001), but no significant group*task (p = 0.257) interaction. Compared with healthy subjects (p = 0.003), AD had the most severe reduction of Primacy, confirming a primary "encoding deficit," while PD confirmed a "frontal pattern." OHIV+ showed a memory profile similar to that of PD with a worsening of the cognitive performance in comparison with YHIV+. In conclusion, we did not confirm the "cortical" hypothesis in OHIV+, at least in terms of learning and memory functions.

Differing roles of autophagy in HIV-associated neurocognitive impairment and encephalitis with implications for morphine co-exposure

We investigated the role of autophagy in HIV-infected subjects with neurocognitive impairment (NCI) ± HIV encephalitis (HIVE), many of which had a history of polysubstance abuse/dependence, using post-mortem brain tissues to determine whether differences in autophagy related factors may be more associated with NCI or NCI-encephalitis. Using qRT-PCR, we detected significant differences in gene expression levels with SQSTM1, LAMP1 higher in HIV-infected subjects without NCI while ATG5, SQSTM1 were then lower in HIV infection/NCI and ATG7, SQSTM1 being higher in NCI-HIVE. Immunohistochemical labeling of these autophagy associated proteins (also including Beclin 1 and LC3B) in Iba1-positive microglial cells showed generally higher immunoreactivity in the NCI and NCI-HIVE groups with more focal vs. diffuse patterns of expression in the NCI-HIVE group. Furthermore, analysis of microarray data from these same subjects found significantly higher levels of LAMP1 in NCI-HIVE compared to uninfected subjects in the basal ganglia. Finally, we tested the effect of supernatant from HIV-1-infected microglia and HIV-1 Tat protein in combination with morphine on neurons in vitro and found opposing events with both significant inhibition of autophagic flux and reduced dendrite length for morphine and supernatant treatment while Tat and morphine exposure resulted in lower autophagic activity at an earlier time point and higher levels in the later. These results suggest autophagy genes and their corresponding proteins may be differentially regulated at the transcriptional, translational, and post-translational levels in the brain during various stages of the HIV disease and that infected individuals exposed to morphine can experience mixed signaling of autophagic activity which could lead to more severe NCI than those without opioid use.

Cerebrospinal fluid (CSF) neuronal biomarkers across the spectrum of HIV infection: hierarchy of injury and detection

The character of central nervous system (CNS) HIV infection and its effects on neuronal integrity vary with evolving systemic infection. Using a cross-sectional design and archived samples, we compared concentrations of cerebrospinal fluid (CSF) neuronal biomarkers in 143 samples from 8 HIV-infected subject groups representing a spectrum of untreated systemic HIV progression and viral suppression: primary infection; four groups of chronic HIV infection neuroasymptomatic (NA) subjects defined by blood CD4+ T cells of >350, 200–349, 50–199, and <50 cells/µL; HAD; treatment-induced viral suppression; and ‘elite’ controllers. Samples from 20 HIV-uninfected controls were also examined. The neuronal biomarkers included neurofilament light chain protein (NFL), total and phosphorylated tau (t-tau, p-tau), soluble amyloid precursor proteins alpha and beta (sAPPα, sAPPβ) and amyloid beta (Aβ) fragments 1–42, 1–40 and 1–38. Comparison of the biomarker changes showed a hierarchy of sensitivity in detection and suggested evolving mechanisms with progressive injury. NFL was the most sensitive neuronal biomarker. Its CSF concentration exceeded age-adjusted norms in all HAD patients, 75% of NA CD4<50, 40% of NA CD4 50–199, and 42% of primary infection, indicating common neuronal injury with untreated systemic HIV disease progression as well as transiently during early infection. By contrast, only 75% of HAD subjects had abnormal CSF t-tau levels, and there were no significant differences in t-tau levels among the remaining groups. sAPPα and β were also abnormal (decreased) in HAD, showed less marked change than NFL with CD4 decline in the absence of HAD, and were not decreased in PHI. The CSF Aβ peptides and p-tau concentrations did not differ among the groups, distinguishing the HIV CNS injury profile from Alzheimer's disease. These CSF biomarkers can serve as useful tools in selected research and clinical settings for patient classification, pathogenetic analysis, diagnosis and management.

Activation of TRPML1 clears intraneuronal Aβ in preclinical models of HIV infection

Antiretroviral therapy extends the lifespan of human immunodeficiency virus (HIV)-infected patients, but many survivors develop premature impairments in cognition. These residual cognitive impairments may involve aberrant deposition of amyloid β-peptides (Aβ). By unknown mechanisms, Aβ accumulates in the lysosomal and autophagic compartments of neurons in the HIV-infected brain. Here we identify the molecular events evoked by the HIV coat protein gp120 that facilitate the intraneuronal accumulation of Aβ. We created a triple transgenic gp120/APP/PS1 mouse that recapitulates intraneuronal deposition of Aβ in a manner reminiscent of the HIV-infected brain. In cultured neurons, we found that the HIV coat protein gp120 increased the transcriptional expression of BACE1 through repression of PPARγ, and increased APP expression by promoting interaction of the translation-activating RBP heterogeneous nuclear ribonucleoprotein C with APP mRNA. APP and BACE1 were colocalized into stabilized membrane microdomains, where the β-cleavage of APP and Aβ formation were enhanced. Aβ-peptides became localized to lysosomes that were engorged with sphingomyelin and calcium. Stimulating calcium efflux from lysosomes with a TRPM1 agonist promoted calcium efflux, luminal acidification, and cleared both sphingomyelin and Aβ from lysosomes. These findings suggest that therapeutics targeted to reduce lysosomal pH in neurodegenerative conditions may protect neurons by facilitating the clearance of accumulated sphingolipids and Aβ-peptides.

Role of HIV in amyloid metabolism

HIV infection has changed from an acute devastating disease to a more chronic illness due to combination anti-retroviral treatment (cART). In the cART era, the life expectancy of HIV-infected (HIV+) individuals has increased. More HIV + individuals are aging with current projections suggesting that 50% of HIV + individuals will be over 50 years old by 2015. With advancing age, HIV + individuals may be at increased risk of developing other potential neurodegenerative disorders [especially Alzheimer's disease (AD)]. Pathology studies have shown that HIV increases intra and possibly extracellular amyloid beta (Aβ42), a hallmark of AD. We review the synthesis and clearance of Aβ42; the effects of HIV on the amyloid pathway; and contrast the impact of AD and HIV on Aβ42 metabolism. Biomarker studies (cerebrospinal fluid AB and amyloid imaging) in HIV + participants have shown mixed results. CSF Aβ42 has been shown to be either normal or diminished in with HIV associated neurocognitive disorders (HAND). Amyloid imaging using [(11)C] PiB has also not demonstrated increased extracellular amyloid fibrillar deposits in HAND. We further demonstrate that Aβ42 deposition is not increased in older HIV + participants using [(11)C] PiB amyloid imaging. Together, these results suggest that HIV and aging each independently affect Aβ42 deposition with no significant interaction present. Older HIV + individuals are probably not at increased risk for developing AD. However, future longitudinal studies of older HIV + individuals using multiple modalities (including the combination of CSF markers and amyloid imaging) are necessary for investigating the effects of HIV on Aβ42 metabolism.

Endolysosome involvement in HIV-1 transactivator protein-induced neuronal amyloid beta production

The increased life expectancy of people living with HIV-1/AIDS is accompanied by increased prevalence of HIV-1-associated neurocognitive disorder. As well, these individuals are increasingly experiencing Alzheimer's disease (AD)-like neurocognitive problems and neuropathological features such as increased deposition of amyloid beta (Aβ) protein. Findings that Aβ production occurs largely in endolysosomes, that HIV-1 transactivator protein (Tat) disrupts endolysosome function-an early pathological feature of AD-and that HIV-1 Tat can increase Aβ levels prompted us to test the hypothesis that endolysosome dysfunction is associated with HIV-1 Tat-induced increases in neuronal Aβ generation. Using primary cultured rat hippocampal neurons, we found that treatment with HIV-1 Tat caused such morphological changes as enlargement of endolysosomes identified with LysoTracker dye and such functional changes as elevated endolysosome pH measured ratiometrically with LysoSensor dye. The HIV-1 Tat-induced changes in endolysosome function preceded temporally HIV-1 Tat-induced increases in Aβ generation measured using enzyme-linked immunosorbent assay. In addition, we demonstrated that HIV-1 Tat increased endolysosome accumulation of Aβ precursor protein and Aβ identified using immunostaining with 4G8 antibodies. Furthermore, we demonstrated that treatment of neurons with HIV-1 Tat increased endolysosome accumulation of beta amyloid-converting enzyme, the rate-limiting enzymatic step for Aβ production, and enhanced beta amyloid-converting enzyme activity. Together, our findings suggest that HIV-1 Tat increases neuronal Aβ generation and thereby contributes to the development of AD-like pathology in HIV-1-infected individuals by disturbing endolysosome structure and function.

Elevated amyloid β production in senescent retinal pigment epithelium, a possible mechanism of subretinal deposition of amyloid β in age-related macular degeneration

Age-related macular degeneration (AMD) is the most common cause of legal blindness in the elderly individuals in developed countries. Subretinally-deposited amyloid β (Aβ) is a main contributor of developing AMD. However, the mechanism causing Aβ deposition in AMD eyes is unknown. Aging is the most significant risk of AMD, thus, we examined the effect of aging on subretinal Aβ deposition. mRNAs and cell lysates were isolated from retinal pigment epithelial (RPE) cells derived from 24-month-old (24M RPE) and 2-month-old (2M RPE) C57BL/6 mice. Aβ concentration in culture supernatants was measured by ELISA. Activity and expression of proteins that regulate Aβ level were examined by activity assay and real time PCR. Effect of β-secretase (BACE) on Aβ production was examined by siRNA silencing. Aβ amounts in supernatants of 24M RPE were significantly higher than 2M RPE. Activity and mRNA levels of neprilysin, an Aβ degrading enzyme, were significantly decreased in 24M RPE compared to 2M RPE. PCR analysis found that BACE2 was significantly more abundantly expressed than BACE1 in RPE cells, however, inactivation of BACE2 gene did not affect Aβ production. BACE1 protein amounts did not differ between 24M and 2M RPE, however, BACE1 activity was significantly higher in 24M RPE compared to 2M RPE. There were no significant changes in the activities of α- or γ-secretase between 2M and 24M RPE. In conclusion, RPE cells produce more amounts of Aβ when they are senescent, and this is probably caused by a decrease in Aβ degradation due to a reduction in the expression and activity of neprilysin and an increase in Aβ synthesis due to increased activity of BACE1.

Substance Abuse, Hepatitis C, and Aging in HIV: Common Cofactors that Contribute to Neurobehavioral Disturbances

Although the prevalence of neurocognitive disturbances among individuals with HIV has decreased in recent years, rates of impairment still remain high. This review presents findings from comorbid conditions that may contribute to further neurocognitive impairments in this already vulnerable population. We will focus on three co-factors that have received substantial attention in the neuroAIDS literature: drug use, hepatitis C co-infection (HCV), and aging. All three conditions commonly co-occur with HIV and likely interact with HIV in complex ways. Collectively, the extant literature suggests that drug use, HCV, and aging serve to worsen the neurocognitive profile of HIV through several overlapping mechanisms. A better understanding of how specific comorbidities interact with HIV may reveal specific phenotypes of HIV-associated neurocognitive disorder that may aid in the development of more targeted behavioral and pharmacological treatment efforts.

APOE ε 4 allele and CSF APOE on cognition in HIV-infected subjects

The significance of the cerebrospinal fluid (CSF) Apolipoprotein E (APOE) level and whether it might have differential effects on brain function due to the presence of APOE ε 4 allele(s) in HIV-infected patients are unknown. However, APOE ε 4 allele has been associated with greater incidence of HIV-associated dementia and accelerated progression of HIV infection. Here, we show further evidence for the role of APOE ε 4 in promoting cognitive impairment. We measured the APOE levels in the CSF of HIV-infected individuals. HIV+ subjects showed lower CSF APOE proteins than SN controls (-19%, p= 0.03). While SN subjects with or without ε 4 allele showed no difference in CSF APOE levels, ε 4+ HIV+ subjects had similar levels to the SN subjects but higher levels than ε 4- HIV+ subjects (+34%, p= 0.01). Furthermore, while HIV+ subjects with ε 2 or ε 3 allele(s) showed a positive relationship between their CSF APOE levels and cognitive performance on the speed of processing domain (r= +0.35, p= 0.05), ε 4+ HIV+ subjects, in contrast, exhibited a negative relationship such that those with higher levels of CSF APOE(4) performed worse on the HIV Dementia Scale (r= -0.61, p= 0.02), had lower Global Cognitive Scores (r= -0.57, p= 0.03), and had poorer performance on tests involving learning (ε 4 allele x [APOE] interaction, p = 0.01). Our findings also suggest that the relatively higher levels of CSF APOE in ε 4+ HIV+ (having primarily APOE4 isoforms) may negatively impact the brain and lead to poorer cognitive outcomes, while those individuals without the ε 4 allele (with primarily APOE2 or APOE3 isoforms) may show compensatory responses that lead to better cognitive performance.

Neurocognitive consequences of HIV infection in older adults: an evaluation of the "cortical" hypothesis

The incidence and prevalence of older adults living with HIV infection is increasing. Recent reports of increased neuropathologic and metabolic alterations in older HIV+ samples, including increased cortical beta-amyloid, have led some researchers to suggest that aging with HIV may produce a neuropsychological profile akin to that which is observed in “cortical” dementias (e.g., impairment in memory consolidation). To evaluate this possibility, we examined four groups classified by HIV serostatus and age (i.e., younger ≤40 years and older ≥50 years): (1) Younger HIV− (n = 24); (2) Younger HIV+ (n = 24); (3) Older HIV− (n = 20); and (4) Older HIV+ (n = 48). Main effects of aging were observed on episodic learning and memory, executive functions, and visuoconstruction, and main effects of HIV were observed on measures of verbal learning and memory. The interaction of age and HIV was observed on a measure of verbal recognition memory, which post hoc analyses showed to be exclusively attributed to the superior performance of the younger HIV seronegative group. Thus, in this sample of older HIV-infected individuals, the combined effects of HIV and aging do not appear to result in a “cortical” pattern of cognitive deficits.

Ubiquitin-proteasome pathway and cellular responses to oxidative stress

The ubiquitin-proteasome pathway (UPP) is the primary cytosolic proteolytic machinery for the selective degradation of various forms of damaged proteins. Thus, the UPP is an important protein quality control mechanism. In the canonical UPP, both ubiquitin and the 26S proteasome are involved. Substrate proteins of the canonical UPP are first tagged by multiple ubiquitin molecules and then degraded by the 26S proteasome. However, in noncanonical UPP, proteins can be degraded by the 26S or the 20S proteasome without being ubiquitinated. It is clear that a proteasome is responsible for selective degradation of oxidized proteins, but the extent to which ubiquitination is involved in this process remains a subject of debate. Whereas many publications suggest that the 20S proteasome degrades oxidized proteins independent of ubiquitin, there is also solid evidence indicating that ubiquitin and ubiquitination are involved in degradation of some forms of oxidized proteins. A fully functional UPP is required for cells to cope with oxidative stress and the activity of the UPP is also modulated by cellular redox status. Mild or transient oxidative stress up-regulates the ubiquitination system and proteasome activity in cells and tissues and transiently enhances intracellular proteolysis. Severe or sustained oxidative stress impairs the function of the UPP and decreases intracellular proteolysis. Both the ubiquitin-conjugating enzymes and the proteasome can be inactivated by sustained oxidative stress, especially the 26S proteasome. Differential susceptibilities of the ubiquitin-conjugating enzymes and the 26S proteasome to oxidative damage lead to an accumulation of ubiquitin conjugates in cells in response to mild oxidative stress. Thus, increased levels of ubiquitin conjugates in cells seem to be an indicator of mild oxidative stress.

The role of the ubiquitin proteasome system in Alzheimer's disease

Today, Alzheimer's disease (AD) is one of the most important age-related neurodegenerative diseases, but its etiology remains still unknown. Since the discovery that the hallmark structures of this disease i.e. the formation of amyloid fibers could be the product of ubiquitin-mediated protein degradation defects, it has become clear that the ubiquitin–proteasome system (UPS), usually essential for protein repair, turnover and degradation, is perturbed in this disease. Different aspects of normal and pathological aging are discussed with respect to protein repair and degradation via the UPS, as well as consequences of a deficit in the UPS in AD. Selective protein oxidation may cause protein damage, or protein mutations may induce a dysfunction of the proteasome. Such events eventually lead to activation of cell death pathways and to an aberrant aggregation or incorporation of ubiquitinated proteins into hallmark structures. Aggresome formation is also observed in other neurodegenerative diseases, suggesting that an activation of similar mechanisms must occur in neurodegeneration as a basic phenomenon. It is essential to discuss therapeutic ways to investigate the UPS dysfunction in the human brain and to identify specific targets to hold or stop cell decay.

Tryptophan, Neurodegeneration and HIV-Associated Neurocognitive Disorder

This review presents an up-to-date assessment of the role of the tryptophan metabolic and catabolic pathways in neurodegenerative disease and HIV-associated neurocognitive disorder. The kynurenine pathway and the effects of each of its enzymes and products are reviewed. The differential expression of the kynurenine pathway in cells within the brain, including inflammatory cells, is explored given the increasing recognition of the importance of inflammation in neurodegenerative disease. An overview of common mechanisms of neurodegeneration is presented before a review and discussion of the evidence for a pathogenetic role of the kynurenine pathway in Alzheimer's disease, HIV-associated neurocognitive disorder, Huntington's disease, motor neurone disease, and Parkinson's disease.

Persistent hijacking of brain proteasomes in HIV-associated dementia

Immunoproteasome induction sustains class 1 antigen presentation and immunological vigilance against HIV-1 in the brain. Investigation of HIV-1-associated alterations in brain protein turnover by the ubiquitin-proteasome system was performed by (1) determining proteasome subunit changes associated with persistent brain inflammation due to HIV-1; (2) determining whether these changes are related to HIV-1 neurocognitive disturbances, encephalitis, and viral loads; and (3) localizing proteasome subunits in brain cells and synapses. On the basis of neurocognitive performance, virological, and immunological measurements obtained within 6 months before death, 153 autopsy cases were selected. Semiquantitative immunoblot analysis performed in the dorsolateral prefrontal cortex revealed up to threefold induction of immunoproteasome subunits LMP7 and PA28alpha in HIV-1-infected subjects and was strongly related to diagnoses of neuropsychological impairment and HIV encephalitis. Low performance on neurocognitive tests specific for dorsolateral prefrontal cortex functioning domains was selectively correlated with immunoproteasome induction. Immunohistochemistry and laser confocal microscopy were then used to localize immunoproteasome subunits to glial and neuronal elements including perikarya, dystrophic axons, and synapses. In addition, HIV loads in brain tissue, cerebrospinal fluid, and blood plasma were robustly correlated to immunoproteasome levels. This persistent "hijacking" of the proteasome by HIV-1-mediated inflammatory response and immunoproteasome induction in the brain is hypothesized to impede turnover of folded proteins in brain cells. This would disrupt neuronal and synaptic protein dynamics, contributing to HIV-1 neurocognitive disturbances.

Amyloid and tau cerebrospinal fluid biomarkers in HIV infection

Background

Because of the emerging intersections of HIV infection and Alzheimer's disease, we examined cerebrospinal fluid (CSF) biomarkers related of amyloid and tau metabolism in HIV-infected patients.

Methods

In this cross-sectional study we measured soluble amyloid precursor proteins alpha and beta (sAPPα and sAPPβ), amyloid beta fragment 1-42 (Aβ_1-42), and total and hyperphosphorylated tau (t-tau and p-tau) in CSF of 86 HIV-infected (HIV+) subjects, including 21 with AIDS dementia complex (ADC), 25 with central nervous system (CNS) opportunistic infections and 40 without neurological symptoms and signs. We also measured these CSF biomarkers in 64 uninfected (HIV-) subjects, including 21 with Alzheimer's disease, and both younger and older controls without neurological disease.

Results

CSF sAPPα and sAPPβ concentrations were highly correlated and reduced in patients with ADC and opportunistic infections compared to the other groups. The opportunistic infection group but not the ADC patients had lower CSF Aβ_1-42 in comparison to the other HIV+ subjects. CSF t-tau levels were high in some ADC patients, but did not differ significantly from the HIV+ neuroasymptomatic group, while CSF p-tau was not increased in any of the HIV+ groups. Together, CSF amyloid and tau markers segregated the ADC patients from both HIV+ and HIV- neuroasymptomatics and from Alzheimer's disease patients, but not from those with opportunistic infections.

Conclusions

Parallel reductions of CSF sAPPα and sAPPβ in ADC and CNS opportunistic infections suggest an effect of CNS immune activation or inflammation on neuronal amyloid synthesis or processing. Elevation of CSF t-tau in some ADC and CNS infection patients without concomitant increase in p-tau indicates neural injury without preferential accumulation of hyperphosphorylated tau as found in Alzheimer's disease. These biomarker changes define pathogenetic pathways to brain injury in ADC that differ from those of Alzheimer's disease.

A coat of many colors: neuroimmune crosstalk in human immunodeficiency virus infection

The use of antiretroviral therapy has reduced mortality and increased the quality of life of HIV-1-infected people, particularly in more developed countries where access to treatment is more widespread. However, morbidities continue, which include HIV-1-associated neurocognitive disorders (HAND). Subtle cognitive abnormalities and low-level viral replication underlie disease. The balance between robust antiviral adaptive immunity, neuronal homeostatic mechanisms, and neuroprotective factors on one hand and toxicities afforded by dysregulated immune activities on the other govern disease. New insights into the pathobiological processes for neuroimmune-linked disease and ways to modulate such activities for therapeutic gain are discussed. Better understanding of the complexities of immune regulation during HAND can improve diagnosis and disease outcomes but is also relevant for the pathogenesis of a broad range of neurodegenerative disorders.

Synaptic proteins linked to HIV-1 infection and immunoproteasome induction: proteomic analysis of human synaptosomes

Infection of the central nervous system with human immunodeficiency virus type 1 (HIV-1) can produce morphological changes in the neocortical synaptodendritic arbor that are correlated with neurocognitive impairment. To determine whether HIV-1 infection influences the protein composition of human synapses, a proteomic study of isolated nerve endings was undertaken. Synaptosomes from frontal neocortex were isolated using isopyknic centrifugation from 19 human brain specimens. Purity and enrichment were assessed by measuring pre- and postsynaptic protein markers. Two-dimensional polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry was used to screen for proteins differentially expressed in HIV/AIDS. The concentrations of 31 candidate protein spots were potentially abnormal in HIV-infected decedents with HIV encephalitis and/or increased expression of immunoproteasome subunits. Immunoblots showed that the concentration of some of them was related to HIV-1 infection of the brain and immunoproteasome (IPS) induction. Synapsin 1b and stathmin were inversely related to brain HIV-1 load; 14-3-3zeta and 14-4-4epsilon proteins were higher in subjects with HIV-1 loads. Perturbed synaptosome proteins were linked with IPS subunit composition, and 14-3-3zeta was histologically colocalized with IPS subunits in stained neocortical neurons. Proteomics illustrates that certain human proteins within the synaptic compartment are involved with changes in the synaptodendritic arbor and neurocognitive impairment in HIV-1-infected people.

The comorbidity of HIV-associated neurocognitive disorders and Alzheimer's disease: a foreseeable medical challenge in post-HAART era

Although the introduction of highly active antiretroviral therapy (HAART) has led to a strong reduction of HIV-associated dementia (HAD) incidence, the prevalence of minor HIV-1-associated neurocognitive disorder (HAND) is rising among AIDS patients. HAART medication has shifted neuropathology from a subacute encephalitic condition to a subtle neurodegenerative process involving synaptic and dendritic degeneration, particularly of hippocampal neurons that are spared prior to HAART medication. Considerable neuroinflammation coupled with mononuclear phagocyte activation is present in HAART-medicated brains, particularly in the hippocampus. Accumulating evidence suggests that the resultant elevated secretion of pro-inflammatory cytokines such as interferon-gamma, tumor necrosis factor-alpha, and interleukin-1beta can increase amyloid-beta peptide (Abeta) generation and reduce Abeta clearance. Recent advancements in Alzheimer's disease (AD) research identified Abeta biogenesis and clearance venues that are potentially influenced by HIV viral infection, providing new insights into beta-amyloidosis segregation in HIV patients. Our study suggests enhanced beta-amyloidosis in ART-treated HAD and HIV-associated encephalitis brains and suppression of Abeta clearance by viral infection of human primary macrophages. A growing awareness of potential convergent mechanisms leading to neurodegeneration shared by HIV and Abeta points to a significant chance of comorbidity of AD and HAND in senile HIV patients, which calls for a need of basic studies.

HIV regulation of amyloid beta production

The use of antiretroviral therapy for HIV infection has extended the survival of individuals living with HIV. However, the effects of chronic HIV infection and aging are introducing another facet of HIV complications. HIV therapy can calm the immune system and lower viral replication to undetectable but the virus is still present. In the brain, amyloid beta (Abeta) increases during normal aging but Abeta accumulation appears to accelerate in HIV infection. HIV Tat protein inhibits the major Abeta-degrading enzyme neprilysin with the cysteine-rich domain of Tat being essential for this inhibition. In this minireview, we also include new data that the beta chemokine, CCL2/MCP-1, associated with HIV migration to the brain, also causes an increase in Abeta. These findings may explain the continued cognitive dysfunction found in HIV-infected individuals controlled on antiviral therapy.

Neurodegeneration and ageing in the HAART era

Cognitive impairment and neurodegeneration still occur despite highly active antiretroviral therapy (HAART). While there are many potential reasons for this, there is increasing evidence that such impairment occurs in the absence of a clear cause. Furthermore, there are data that some neurodegenerative diseases, especially Alzheimer's or an Alzheimer-like illness, are becoming more common in the context of HAART-treated human immunodeficiency virus (HIV) disease. This review will critically examine the evidence underpinning these observations. Potential mechanisms will be discussed with particular emphasis on the effect of ageing and how it overlaps with the effects of HIV disease itself thereby leading to neurodegeneration. The nature of this overlap will then be explored for its potential role in the facilitated expression and development of neurodegenerative diseases. Lastly, there will be a brief discussion of interventions to minimize such neurodegeneration including optimization of HAART for brain entry.

Calpain and proteasomal regulation of antiretroviral zinc finger protein OTK18 in human macrophages: visualization in live cells by intramolecular FRET

As part of the innate immune defense against HIV infection, OTK18, a zinc finger protein, is upregulated in human macrophages and reduces viral replication through suppression of viral long-terminal repeat promoter activity. Although we know that the processing products of OTK18 accumulate in the cytoplasm of brain perivascular macrophages in advanced HIV encephalitis cases, the molecular mechanisms behind its post-translational processing are still poorly understood. To characterize OTK18 processing, we assessed a panel of protease inhibitors to identify the candidates involved in the OTK18 processing using human monocyte-derived macrophages (MDM) overexpressing OTK18 by recombinant adenoviral gene transfer. Viral infection of MDM strongly increased the processing of OTK18 into its N-terminal fragment. Treatment of OTK18-expressing MDM with calpain and proteasome inhibitors significantly accumulated either full-length or processed OTK18 fragments in time- and dose-dependent manners. A series of OTK18 truncation mutants and synthetic peptides were tested to locate the calpain cleavage sites after arginine 359. Finally, we developed an enhanced cyan and yellow fluorescent protein (ECFP and EYFP)-based intramolecular fluorescent resonance energy transfer (intramolecular FRET) system to monitor the OTK18 endoproteolysis in human microglia cell line. Inhibition of proteasome activity significantly increased the intramolecular FRET signal in the nucleus. These data suggest that calpain and proteasome are involved in OTK18 endoproteolysis and degradation. Additionally, intramolecular FRET has proven to be a useful tool for monitoring the processing in live cells.

The ubiquitin-proteasome system in spongiform degenerative disorders

Spongiform degeneration is characterized by vacuolation in nervous tissue accompanied by neuronal death and gliosis. Although spongiform degeneration is a hallmark of prion diseases, this pathology is also present in the brains of patients suffering from Alzheimer’s disease, diffuse Lewy body disease, human immunodeficiency virus (HIV) infection, and Canavan’s spongiform leukodystrophy. The shared outcome of spongiform degeneration in these diverse diseases suggests that common cellular mechanisms must underlie the processes of spongiform change and neurodegeneration in the central nervous system. Immunohistochemical analysis of brain tissues reveals increased ubiquitin immunoreactivity in and around areas of spongiform change, suggesting the involvement of ubiquitin–proteasome system dysfunction in the pathogenesis of spongiform neurodegeneration. The link between aberrant ubiquitination and spongiform neurodegeneration has been strengthened by the discovery that a null mutation in the E3 ubiquitin–protein ligase mahogunin ring finger-1 (Mgrn1) causes an autosomal recessively inherited form of spongiform neurodegeneration in animals. Recent studies have begun to suggest that abnormal ubiquitination may alter intracellular signaling and cell functions via proteasome-dependent and proteasome-independent mechanisms, leading to spongiform degeneration and neuronal cell death. Further elucidation of the pathogenic pathways involved in spongiform neurodegeneration should facilitate the development of novel rational therapies for treating prion diseases, HIV infection, and other spongiform degenerative disorders.

Disruption of neuronal autophagy by infected microglia results in neurodegeneration

There is compelling evidence to support the idea that autophagy has a protective function in neurons and its disruption results in neurodegenerative disorders. Neuronal damage is well-documented in the brains of HIV-infected individuals, and evidence of inflammation, oxidative stress, damage to synaptic and dendritic structures, and neuronal loss are present in the brains of those with HIV-associated dementia. We investigated the role of autophagy in microglia-induced neurotoxicity in primary rodent neurons, primate and human models. We demonstrate here that products of simian immunodeficiency virus (SIV)-infected microglia inhibit neuronal autophagy, resulting in decreased neuronal survival. Quantitative analysis of autophagy vacuole numbers in rat primary neurons revealed a striking loss from the processes. Assessment of multiple biochemical markers of autophagic activity confirmed the inhibition of autophagy in neurons. Importantly, autophagy could be induced in neurons through rapamycin treatment, and such treatment conferred significant protection to neurons. Two major mediators of HIV-induced neurotoxicity, tumor necrosis factor-alpha and glutamate, had similar effects on reducing autophagy in neurons. The mRNA level of p62 was increased in the brain in SIV encephalitis and as well as in brains from individuals with HIV dementia, and abnormal neuronal p62 dot structures immunoreactivity was present and had a similar pattern with abnormal ubiquitinylated proteins. Taken together, these results identify that induction of deficits in autophagy is a significant mechanism for neurodegenerative processes that arise from glial, as opposed to neuronal, sources, and that the maintenance of autophagy may have a pivotal role in neuroprotection in the setting of HIV infection.

Systemic and brain macrophage infections in relation to the development of simian immunodeficiency virus encephalitis

The brains of individuals with lentiviral-associated encephalitis contain an abundance of infected and activated macrophages. It has been hypothesized that encephalitis develops when increased numbers of infected monocytes traffic into the central nervous system (CNS) during the end stages of immunosuppression. The relationships between the infection of brain and systemic macrophages and circulating monocytes and the development of lentiviral encephalitis are unknown. We longitudinally examined the extent of monocyte/macrophage infection in blood and lymph nodes of pigtailed macaques that did or did not develop simian immunodeficiency virus encephalitis (SIVE). Compared to levels in macaques that did not develop SIVE, more ex vivo virus production was detected from monocyte-derived macrophages and nonadherent peripheral blood mononuclear cells (PBMCs) from macaques that did develop SIVE. Prior to death, there was an increase in the number of circulating PBMCs following a rise in cerebrospinal fluid viral load in macaques that did develop SIVE but not in nonencephalitic macaques. At necropsy, macaques with SIVE had more infected macrophages in peripheral organs, with the exception of lymph nodes. T cells and NK cells with cytotoxic potential were more abundant in brains with encephalitis; however, T-cell and NK-cell infiltration in SIVE and human immunodeficiency virus encephalitis was more modest than that observed in classical acute herpes simplex virus encephalitis. These findings support the hypothesis that inherent differences in host systemic and CNS monocyte/macrophage viral production are associated with the development of encephalitis.

Co-factors in HIV neurobehavioural disturbances: substance abuse, hepatitis C and aging

Neurocognitive disturbances associated with HIV infection may be modulated or confounded by coexisting and comorbid conditions that reflect the changing populations affected by the disease. HIV infection is often accompanied by substance dependence and/or hepatitis C co-infection. Both of these cofactors that may lead to brain dysfunction on their own, and therefore can affect the nature and course neurocognitive functioning in HIV. Improvements in antiretroviral therapies translate into greater longevity for people infected with HIV, many of whom are now entering their 6th and 7th decade of life and beyond. The increasing proportion of older persons with HIV is also the result of new infections in this age group. As aging confers additional metabolic, neurologic, and neuropsychiatric vulnerability, it is important to understand how this constellation of changes affects neurocognitive functioning in the context of HIV.

Evolution of HIV dementia with HIV infection

Dementia remains one of the most fearsome complications of HIV infection. It also poses a significant challenge for the clinician both in terms of diagnosis and treatment. The use of antiretroviral agents has led to a decrease in the incidence of HIV dementia but the prevalence of milder forms of neurocognitive impairment has increased. Occasionally, the immune reconstitution caused by these agents may target the brain leading to a syndrome characterized by a severe, progressive and often fatal dementia. The progression of HIV dementia may also be determined by host and viral genetic factors, and the existence of co-morbid factors such as drug abuse, hepatitis C infection and aging. Oxidative stress markers appear to be predictive of active dementia. However, currently there is no specific treatment available for HIV dementia.