HIV-1 endocytosis in astrocytes: a kiss of death or survival of the fittest?

Astrocytes Are a Key Target for Neurotropic Viral Infection

Astrocytes are increasingly recognized as important viral host cells in the central nervous system. These cells can produce relatively high quantities of new virions. In part, this can be attributed to the characteristics of astrocyte metabolism and its abundant and dynamic cytoskeleton network. Astrocytes are anatomically localized adjacent to interfaces between blood capillaries and brain parenchyma and between blood capillaries and brain ventricles. Moreover, astrocytes exhibit a larger membrane interface with the extracellular space than neurons. These properties, together with the expression of various and numerous viral entry receptors, a relatively high rate of endocytosis, and morphological plasticity of intracellular organelles, render astrocytes important target cells in neurotropic infections. In this review, we describe factors that mediate the high susceptibility of astrocytes to viral infection and replication, including the anatomic localization of astrocytes, morphology, expression of viral entry receptors, and various forms of autophagy.

The therapeutic effect of tanshinone IIA in mouse astrocytes after treatment with Angiostrongylus cantonensis fifth-stage larval excretory-secretory products

BACKGROUND

Angiostrongylus cantonensis is an important food-borne zoonotic parasite that causes eosinophilic meningitis and meningoencephalitis in humans. Excretory-secretory products (ESPs) are valuable targets for studying host-parasite relationships. ESPs are composed of a variety of molecules that are used to penetrate defensive barriers and avoid immune attack of the host. Tanshinone IIA (TSIIA) is a vasoactive cardioprotective drug that is widely used in studies evaluating potential therapeutic mechanisms. In this study, we will evaluate the therapeutic effects of TSIIA in mouse astrocytes after A. cantonensis fifth-stage larvae (L5) ESPs treatment.

METHODS

Here, we examined the therapeutic effect of TSIIA by real-time qPCR, western blotting, activity assay, and cell viability assays.

RESULTS

First, the results showed that TSIIA can elevate cell viability in astrocytes after stimulation with ESPs. On the other hand, TSIIA downregulated the expression of apoptosis-related molecules. However, the expression of molecules related to antioxidant, autophagy, and endoplasmic reticulum stress was significantly increased. The results of antioxidant activation assays showed that the activities of superoxide dismutase (SOD), glutathione S-transferase (GST), and catalase were significantly increased. Finally, we found that cell apoptosis and oxidative stress were reduced in TSIIA-treated astrocytes by immunofluorescence staining.

CONCLUSION

The findings from this study suggest that TSIIA can reduce cellular damage caused by A. cantonensis L5 ESPs in astrocytes and clarify the related molecular mechanisms.

Astrocytes in the pathophysiology of neuroinfection

Key homeostasis providing cells in the central nervous system (CNS) are astrocytes, which belong to the class of cells known as atroglia, a highly heterogeneous type of neuroglia and a prominent element of the brain defence. Diseases evolve due to altered homeostatic state, associated with pathology-induced astroglia remodelling represented by reactive astrocytes, astroglial atrophy and astrodegeneration. These features are hallmarks of most infectious insults, mediated by bacteria, protozoa and viruses; they are also prominent in the systemic infection. The COVID-19 pandemic revived the focus into neurotropic viruses such as SARS-CoV2 (Coronaviridae) but also the Flaviviridae viruses including tick-borne encephalitis (TBEV) and Zika virus (ZIKV) causing the epidemic in South America prior to COVID-19. Astrocytes provide a key response to neurotropic infections in the CNS. Astrocytes form a parenchymal part of the blood-brain barrier, the site of virus entry into the CNS. Astrocytes exhibit aerobic glycolysis, a form of metabolism characteristic of highly morphologically plastic cells, like cancer cells, hence a suitable milieu for multiplication of infectious agent, including viral particles. However, why the protection afforded by astrocytes fails in some circumstances is an open question to be studied in the future.

Immune Functions of Astrocytes in Viral Neuroinfections

Neuroinfections of the central nervous system (CNS) can be triggered by various pathogens. Viruses are the most widespread and have the potential to induce long-term neurologic symptoms with potentially lethal outcomes. In addition to directly affecting their host cells and inducing immediate changes in a plethora of cellular processes, viral infections of the CNS also trigger an intense immune response. Regulation of the innate immune response in the CNS depends not only on microglia, which are fundamental immune cells of the CNS, but also on astrocytes. These cells align blood vessels and ventricle cavities, and consequently, they are one of the first cell types to become infected after the virus breaches the CNS. Moreover, astrocytes are increasingly recognized as a potential viral reservoir in the CNS; therefore, the immune response initiated by the presence of intracellular virus particles may have a profound effect on cellular and tissue physiology and morphology. These changes should be addressed in terms of persisting infections because they may contribute to recurring neurologic sequelae. To date, infections of astrocytes with different viruses originating from genetically distinct families, including Flaviviridae, Coronaviridae, Retroviridae, Togaviridae, Paramyxoviridae, Picomaviridae, Rhabdoviridae, and Herpesviridae, have been confirmed. Astrocytes express a plethora of receptors that detect viral particles and trigger signaling cascades, leading to an innate immune response. In this review, we summarize the current knowledge on virus receptors that initiate the release of inflammatory cytokines from astrocytes and depict the involvement of astrocytes in immune functions of the CNS.

HIV infection of non-classical cells in the brain

HIV-associated neurological disorders (HAND) affect up to 50% of people living with HIV (PLWH), even in the era of combination antiretroviral therapy (cART). HIV-DNA can be detected in the cerebral spinal fluid (CSF) of approximately half of aviremic ART-suppressed PLWH and its presence is associated with poorer neurocognitive performance. HIV DNA + and HIV RNA + cells have also been observed in postmortem brain tissue of individuals with sustained cART suppression. In this review, we provide an overview of how HIV invades the brain and HIV infection of resident brain glial cells (astrocytes and microglia). We also discuss the role of resident glial cells in persistent neuroinflammation and HAND in PLWH and their potential contribution to the HIV reservoir. HIV eradication strategies that target persistently infected glia cells will likely be needed to achieve HIV cure.

Role of Protein-Lipid Interactions in Viral Entry

The viral entry consists of several sequential events that ensure the attachment of the virus to the host cell and the introduction of its genetic material for the continuation of the replication cycle. Both cellular and viral lipids have gained a wider focus in recent years in the field of viral entry, as they are found to play key roles in different steps of the process. The specific role is summarized that lipids and lipid membrane nanostructures play in viral attachment, fusion, and immune evasion and how they can be targeted with antiviral therapies. Finally, some of the limitations of techniques commonly used for protein-lipid interactions studies are discussed, and new emerging tools are reviewed that can be applied to this field.

Astrocytes are HIV reservoirs in the brain: A cell type with poor HIV infectivity and replication but efficient cell-to-cell viral transfer

The major barrier to eradicating Human immunodeficiency virus-1 (HIV) infection is the generation of tissue-associated quiescent long-lasting viral reservoirs refractory to therapy. Upon interruption of anti-retroviral therapy (ART), HIV replication can be reactivated. Within the brain, microglia/macrophages and a small population of astrocytes are infected with HIV. However, the role of astrocytes as a potential viral reservoir is becoming more recognized because of the improved detection and quantification of HIV viral reservoirs. In this report, we examined the infectivity of human primary astrocytes in vivo and in vitro, and their capacity to maintain HIV infection, become latently infected, be reactivated, and transfer new HIV virions into neighboring cells. Analysis of human brain tissue sections obtained from HIV-infected individuals under effective and prolonged ART indicates that a small population of astrocytes has integrated HIV-DNA. In vitro experiments using HIV-infected human primary astrocyte cultures confirmed a low percentage of astrocytes had integrated HIV-DNA, with poor to undetectable replication. Even in the absence of ART, long-term culture results in latency that could be transiently reactivated with histone deacetylase inhibitor, tumor necrosis factor-alpha (TNF-α), or methamphetamine. Reactivation resulted in poor viral production but efficient cell-to-cell viral transfer into cells that support high viral replication. Together, our data provide a new understanding of astrocytes' role as viral reservoirs within the central nervous system (CNS).

Pancreatogenic Diabetes: Triggering Effects of Alcohol and HIV

Multiorgan failure may not be completely resolved among people living with HIV despite HAART use. Although the chances of organ dysfunction may be relatively low, alcohol may potentiate HIV-induced toxic effects in the organs of alcohol-abusing, HIV-infected individuals. The pancreas is one of the most implicated organs, which is manifested as diabetes mellitus or pancreatic cancer. Both alcohol and HIV may trigger pancreatitis, but the combined effects have not been explored. The aim of this review is to explore the literature for understanding the mechanisms of HIV and alcohol-induced pancreatotoxicity. We found that while premature alcohol-inducing zymogen activation is a known trigger of alcoholic pancreatitis, HIV entry through C-C chemokine receptor type 5(CCR5)into pancreatic acinar cells may also contribute to pancreatitis in people living with HIV (PLWH). HIV proteins induce oxidative and ER stresses, causing necrosis. Furthermore, infiltrative immune cells induce necrosis on HIV-containing acinar cells. When necrotic products interact with pancreatic stellate cells, they become activated, leading to the release of both inflammatory and profibrotic cytokines and resulting in pancreatitis. Effective therapeutic strategies should block CCR5 and ameliorate alcohol's effects on acinar cells.

Productive HIV infection in astrocytes can be established via a nonclassical mechanism

OBJECTIVE

Astrocytes are proposed to be a critical reservoir of HIV in the brain. However, HIV infection of astrocytes is inefficient in vitro except for cell-to-cell transmission from HIV-infected cells. Here, we explore mechanisms by which cell-free HIV bypasses entry and postentry barriers leading to a productive infection.

METHODS

HIV infection of astrocytes was investigated by a variety of techniques including transfection of CD4-expressing plasmid, treatment with lysosomotropic agents or using a transwell culture system loaded with HIV-infected lymphocytes. Infection was monitored by HIV-1 p24 in culture supernatants and integrated proviral DNA was quantified by Alu-PCR.

RESULTS

Persistent HIV infection could be established in astrocytes by transfection of proviral DNA, transduction with VSV-G-pseudotyped viruses, transient expression of CD4 followed by HIV infection, or simultaneous treatment with lysosomotropic chloroquine or Tat-HA2 peptide with HIV infection. In absence of these treatments, HIV entered via endocytosis as seen by electronmicroscopy and underwent lysosomal degradation without proviral integration, indicating endocytosis is a dead end for HIV in astrocytes. Nevertheless, productive infection was observed when astrocytes were in close proximity but physically separated from HIV-infected lymphocytes in the transwell cultures. This occurred with X4 or dual tropic R5X4 viruses and was blocked by an antibody or antagonist to CXCR4.

CONCLUSION

A CD4-independent, CXCR4-dependent mechanism of viral entry is proposed, by which immature HIV particles from infected lymphocytes might directly bind to CXCR4 on astrocytes and trigger virus--cell fusion during or after the process of viral maturation. This mechanism may contribute to the formation of brain HIV reservoirs.

In vitro models of HIV-1 infection of the Central Nervous System

Neurocognitive disorders associated with HIV-1 infection affect more than half of persons living with HIV (PLWH) under retroviral therapy. Understanding the molecular mechanisms and the complex cellular network communication underlying neurological dysfunction is critical for the development of an effective therapy. As with other neurological disorders, challenges to studying HIV infection of the brain include limited access to clinical samples and proper reproducibility of the complexity of brain networks in cellular and animal models. This review focuses on cellular models used to investigate various aspects of neurological dysfunction associated with HIV infection.

Absence of LEDGF/p75 Expression in Astrocytes May Affect HIV-1 Integration Efficiency

In spite of effective anti-retroviral therapy, HIV-1 infection may still lead to neurological impairment in patients. The underlying mechanism of neurodegeneration remains mysterious. HIV-1 does not infect neurons, but does infect microglia cells in the brain. It is controversial whether HIV-1 productively infects astrocytes, an abundant glial cell type in the brain. Thirty years of investigation have led to conflicting reports concerning the entry, infection, and production of progeny virions from astrocytes. New models from studies in primary human fetal astrocytes suggest phagocytosis of HIV-1 with little productive infection. The retroviral life cycle requires integration of the viral genome to the host genome. The host protein LEDGF/p75 is required for efficient HIV-1 integration. In the absence of LEDGF/p75, HIV-1 integration and infection efficiency is reduced ten fold. Differentiated astrocytes do not appear to express LEDGF/p75, which suggests these cells are disabled for efficient integration. Phagocytosis of HIV-1 virions and the lack of LEDGF/p75 expression in astrocytes suggest that this cell type is not efficiently infected in vivo.

Astrocytes as an HIV CNS reservoir: highlights and reflections of an NIMH-sponsored symposium

This a summary of a National Institute of Mental Health (NIMH) sponsored symposium that was focused on the role of astrocytes as a reservoir of the human immunodeficiency virus in the brain. The talks were grouped into four themes. The first theme reviewed the evidence for HIV infection of astrocytes and discussed the challenges in the use of traditional methods of immunostaining and in situ hybridization for detection of infected astrocytes. The second theme focused on mechanisms of HIV entry into astrocytes and discussed CD4 independent mechanisms, such as receptor-mediated endocytosis and transmission of HIV by cell-to-cell contact with infected lymphocytes. The third theme focused on epigenetic regulation of HIV latency in astrocytes and other factors, such as cytokines and transcriptional factors regulating HIV replication in astrocytes. The fourth theme focused on therapeutic approaches, such as gene editing to block persistently infected astrocytes. A discussion that followed was focused on major unanswered questions in the field and future directions for research.

Neuropathogenesis of human immunodeficiency virus infection

Human immunodeficiency virus (HIV)-associated neurocognitive disorders (HAND) remain a common end-organ manifestation of viral infection. Subclinical and mild symptoms lead to neurocognitive and behavioral abnormalities. These are associated, in part, with viral penetrance and persistence in the central nervous system. Infections of peripheral blood monocytes, macrophages, and microglia are the primary drivers of neuroinflammation and neuronal impairments. While current antiretroviral therapy (ART) has reduced the incidence of HIV-associated dementia, milder forms of HAND continue. Depression, comorbid conditions such as infectious liver disease, drugs of abuse, antiretroviral drugs themselves, age-related neurodegenerative diseases, gastrointestinal maladies, and concurrent social and economic issues can make accurate diagnosis of HAND challenging. Increased life expectancy as a result of ART clearly creates this variety of comorbid conditions that often blur the link between the virus and disease. With the discovery of novel biomarkers, neuropsychologic testing, and imaging techniques to better diagnose HAND, the emergence of brain-penetrant ART, adjunctive therapies, longer life expectancy, and better understanding of disease pathogenesis, disease elimination is perhaps a realistic possibility. This review focuses on HIV-associated disease pathobiology with an eye towards changing trends in the face of widespread availability of ART.

Multifunctional Nanotherapeutics for the Treatment of neuroAIDS in Drug Abusers

HIV and substance abuse plays an important role in infection and disease progression. Further, the presence of persistent viral CNS reservoirs makes the complete eradication difficult. Thus, neutralizing the drug of abuse effect on HIV-1 infectivity and elimination of latently infected cells is a priority. The development of a multi-component [antiretroviral drugs (ARV), latency reactivating agents (LRA) and drug abuse antagonist (AT)] sustained release nanoformulation targeting the CNS can overcome the issues of HIV-1 cure and will help in improving the drug adherence. The novel magneto-liposomal nanoformulation (NF) was developed to load different types of drugs (LRAs, ARVs, and Meth AT) and evaluated for in-vitro and in-vivo BBB transmigration and antiviral efficacy in primary CNS cells. We established the HIV-1 latency model using human astrocyte cells (HA) and optimized the dose of LRA for latency reversal, Meth AT in in-vitro cell culture system. Further, PEGylated magneto-liposomal NF was developed, characterized for size, shape, drug loading and BBB transport in-vitro. Results showed that drug released in a sustained manner up to 10 days and able to reduce the HIV-1 infectivity up to ~40-50% (>200 pg/mL to <100 pg/mL) continuously using single NF treatment ± Meth treatment in-vitro. The magnetic treatment (0.8 T) was able to transport (15.8% ± 5.5%) NF effectively without inducing any toxic effects due to NF presence in the brain. Thus, our approach and result showed a way to eradicate HIV-1 reservoirs from the CNS and possibility to improve the therapeutic adherence to drugs in drug abusing (Meth) population. In conclusion, the developed NF can provide a better approach for the HIV-1 cure and a foundation for future HIV-1 purging strategies from the CNS using nanotechnology platform.

Astrogliopathology in the infectious insults of the brain

Astroglia, a heterogeneous type of neuroglia, play key homeostatic functions in the central nervous system (CNS) and represent an important defence system. Impaired homeostatic capacity of astrocytes manifests in diseases and this is mirrored in various astrocyte-based pathological features including reactive astrogliosis, astrodegeneration with astroglial atrophy and pathological remodelling of astrocytes. All of these manifestations are most prominently associated with infectious insults, mediated by bacteria, protozoa and viruses. Here we focus onto neurotropic viruses such as tick-borne encephalitis (TBEV) and Zika virus (ZIKV), both belonging to Flaviviridae and both causing severe neurological impairments. We argue that astrocytes provide a route through which neurotropic infectious agents attack the CNS, since they are anatomically associated with the blood-brain barrier and exhibit aerobic glycolysis, a metabolic specialisation of highly morphologically dynamic cells, which may provide a suitable metabolic milieu for proliferation of infectious agents, including viral bodies.

Backbone resonance assignment of the human uracil DNA glycosylase-2

The HIV-1 viral protein R (Vpr) is incorporated into virus particle during budding suggesting that its presence in the mature virion is required in the early steps of the virus life cycle in newly infected cells. Vpr is released into the host cell cytoplasm to participate to the translocation of the preintegration complex (PIC) into the nucleus for integration of the viral DNA into the host genome. Actually, Vpr plays a key role in the activation of the transcription of the HIV-1 long terminal repeat (LTR), mediates cell cycle arrest in G2 to M transition, facilitates apoptosis and controls the fidelity of reverse transcription. Moreover, Vpr drives the repair enzyme uracil DNA glycosylase (UNG2) towards degradation. UNG2 has a major role in "Base excision repair" (BER) whose main function is to maintain genome integrity by controlling DNA uracilation. The interaction of Vpr with the cellular protein UNG2 is a key event in various stages of retroviral replication and its role remains to be defined. We have performed the structural study of UNG2 by NMR and we report its (¹H^{N, 15}N, ¹³C^{α, 13}C^β and ¹³C') chemical shift backbone assignment and its secondary structure in solution as predicted by TALOS-N. We aim to determine with accuracy by NMR, the residues of UNG2 interacting with Vpr, characterize their interaction and use the local structure of UNG2 and its interface with Vpr to propose potential ligands disturbing this interaction.

Astroglial vesicular network: evolutionary trends, physiology and pathophysiology

Intracellular organelles, including secretory vesicles, emerged when eukaryotic cells evolved some 3 billion years ago. The primordial organelles that evolved in Archaea were similar to endolysosomes, which developed, arguably, for specific metabolic tasks, including uptake, metabolic processing, storage and disposal of molecules. In comparison with prokaryotes, cell volume of eukaryotes increased by several orders of magnitude and vesicle traffic emerged to allow for communication between distant intracellular locations. Lysosomes, first described in 1955, a prominent intermediate of endo- and exocytotic pathways, operate virtually in all eukaryotic cells including astroglia, the most heterogeneous type of homeostatic glia in the central nervous system. Astrocytes support neuronal network activity in particular through elaborated secretion, based on a complex intracellular vesicle network dynamics. Deranged homeostasis underlies disease and astroglial vesicle traffic contributes to the pathophysiology of neurodegenerative (Alzheimer's disease, Huntington's disease), neurodevelopmental diseases (intellectual deficiency, Rett's disease) and neuroinfectious (Zika virus) disorders. This review addresses astroglial cell-autonomous vesicular traffic network, as well as its into primary and secondary vesicular network defects in diseases, and considers this network as a target for developing new therapies for neurological conditions.

Astrocytes as an HIV Reservoir: Mechanism of HIV Infection

If we have any hope of achieving a cure for HIV infection, close attention to the cell types capable of getting infected with HIV is necessary. Of these cell types, astrocytes are the most ideal cell type for the formation of such a reservoir. These are long-lived cells with a very low turnover rate and are found in the brain and the gastrointestinal tract. Although astrocytes are evidently resistant to infection of cell-free HIV in vitro, these cells are efficiently infected via cell-tocell contact by which immature HIV virions bud off lymphocytes and have the ability to directly bind to CXCR4, triggering the process of fusion in the absence of CD4. In this review, we closely examine the evidence for HIV infection of astrocytes in the brain and the mechanisms for viral entry and regulation in this cell type, and discuss an approach for controlling this viral reservoir.

HIV-1, methamphetamine and astrocytes at neuroinflammatory Crossroads

As a popular psychostimulant, methamphetamine (METH) use leads to long-lasting, strong euphoric effects. While METH abuse is common in the general population, between 10 and 15% of human immunodeficiency virus-1 (HIV-1) patients report having abused METH. METH exacerbates the severity and onset of HIV-1-associated neurocognitive disorders (HAND) through direct and indirect mechanisms. Repetitive METH use impedes adherence to antiretroviral drug regimens, increasing the likelihood of HIV-1 disease progression toward AIDS. METH exposure also directly affects both innate and adaptive immunity, altering lymphocyte numbers and activity, cytokine signaling, phagocytic function and infiltration through the blood brain barrier. Further, METH triggers the dopamine reward pathway and leads to impaired neuronal activity and direct toxicity. Concurrently, METH and HIV-1 alter the neuroimmune balance and induce neuroinflammation, which modulates a wide range of brain functions including neuronal signaling and activity, glial activation, viral infection, oxidative stress, and excitotoxicity. Pathologically, reactive gliosis is a hallmark of both HIV-1- and METH-associated neuroinflammation. Significant commonality exists in the neurotoxic mechanisms for both METH and HAND; however, the pathways dysregulated in astroglia during METH exposure are less clear. Thus, this review highlights alterations in astrocyte intracellular signaling pathways, gene expression and function during METH and HIV-1 comorbidity, with special emphasis on HAND-associated neuroinflammation. Importantly, this review carefully evaluates interventions targeting astrocytes in HAND and METH as potential novel therapeutic approaches. This comprehensive overview indicates, without a doubt, that during HIV-1 infection and METH abuse, a complex dialog between all neural cells is orchestrated through astrocyte regulated neuroinflammation.

Neuropathogenesis of HIV: from initial neuroinvasion to HIV-associated neurocognitive disorder (HAND)

Early in the HIV epidemic, the central nervous system (CNS) was recognized as a target of infection and injury in the advanced stages of disease. Though the most severe forms of HIV-associated neurocognitive disorder (HAND) related to severe immunosuppression are rare in the current era of widespread combination antiretroviral therapy (cART), evidence now supports pathological involvement of the CNS throughout the course of infection. Recent work suggests that the stage for HIV neuropathogenesis may be set with initial viral entry into the CNS, followed by initiation of pathogenetic processes including neuroinflammation and neurotoxicity, and establishment of local, compartmentalized HIV replication that may reflect a tissue reservoir for HIV. Key questions still exist as to when HIV establishes local infection in the CNS, which CNS cells are the primary targets of HIV, and what mechanistic processes underlie the injury to neurons that produce clinical symptoms of HAND. Advances in these areas will provide opportunities for improved treatment of patients with established HAND, prevention of neurological disease in those with early stage infection, and understanding of HIV tissue reservoirs that will aid efforts at HIV eradication.

Endocytosis of human immunodeficiency virus 1 (HIV-1) in astrocytes: a fiery path to its destination

Despite successful suppression of peripheral HIV-1 infection by combination antiretroviral therapy, immune activation by residual virus in the brain leads to HIV-associated neurocognitive disorders (HAND). In the brain, several types of cells, including microglia, perivascular macrophage, and astrocytes have been reported to be infected by HIV-1. Astrocytes, the most abundant cells in the brain, maintain homeostasis. The general consensus on HIV-1 infection in astrocytes is that it produces unproductive viral infection. HIV-1 enters astrocytes by pH-dependent endocytosis, leading to degradation of the virus in endosomes, but barely succeeds in infection. Here, we have discussed endocytosis-mediated HIV-1 entry and viral programming in astrocytes.