Defining the molecular mechanisms of HIV-1 Tat secretion: PtdIns(4,5)P2 at the epicenter

A novel high-throughput microwell outgrowth assay for HIV-infected cells

Although antiretroviral therapy (ART) is effective at suppressing HIV replication, a viral reservoir persists that can reseed infection if ART is interrupted. Curing HIV will require elimination or containment of this reservoir, but the size of the HIV reservoir is highly variable between individuals. To evaluate the size of the HIV reservoir, several assays have been developed, including PCR-based assays for viral DNA, the intact proviral DNA assay, and the quantitative viral outgrowth assay (QVOA). QVOA is the gold standard assay for measuring inducible replication-competent proviruses, but this assay is technically challenging and time-consuming. To begin progress toward a more rapid and less laborious tool for quantifying cells infected with replication-competent HIV, we developed the Microwell Outgrowth Assay, in which infected CD4 T cells are co-cultured with an HIV-detecting reporter cell line in a polydimethylsiloxane (PDMS)/polystyrene array of nanoliter-sized wells. Transmission of HIV from infected cells to the reporter cell line induces fluorescent reporter protein expression that is detected by automated scanning across the array. Using this approach, we were able to detect HIV-infected cells from ART-naïve people with HIV (PWH) and from PWH on ART with large reservoirs. Furthermore, we demonstrate that infected cells can be recovered from individual rafts and used to analyze the diversity of viral sequences. Although additional development and optimization will be required for quantifying the reservoir in PWH with small latent reservoirs, this assay may be a useful prototype for microwell assays of infected cells.IMPORTANCEMeasuring the size of the HIV reservoir in people with HIV (PWH) will be important for determining the impact of HIV cure strategies. However, measuring this reservoir is challenging. We report a new method for quantifying HIV-infected cells that involves culturing cells from PWH in an array of microwells with a cell line that detects HIV infection. We show that this approach can detect rare HIV-infected cells and derive detailed virus sequence information for each infected cell.

Role of HIV-1 Tat Protein Interactions with Host Receptors in HIV Infection and Pathogenesis

Each time the virus starts a new round of expression/replication, even under effective antiretroviral therapy (ART), the transactivator of viral transcription Tat is one of the first HIV-1 protein to be produced, as it is strictly required for HIV replication and spreading. At this stage, most of the Tat protein exits infected cells, accumulates in the extracellular matrix and exerts profound effects on both the virus and neighbor cells, mostly of the innate and adaptive immune systems. Through these effects, extracellular Tat contributes to the acquisition of infection, spreading and progression to AIDS in untreated patients, or to non-AIDS co-morbidities in ART-treated individuals, who experience inflammation and immune activation despite virus suppression. Here, we review the role of extracellular Tat in both the virus life cycle and on cells of the innate and adaptive immune system, and we provide epidemiological and experimental evidence of the importance of targeting Tat to block residual HIV expression and replication. Finally, we briefly review vaccine studies showing that a therapeutic Tat vaccine intensifies ART, while its inclusion in a preventative vaccine may blunt escape from neutralizing antibodies and block early events in HIV acquisition.

HIV-1 Tat Induces Dysregulation of PGC1-Alpha and Sirtuin 3 Expression in Neurons: The Role of Mitochondrial Biogenesis in HIV-Associated Neurocognitive Disorder (HAND)

During the antiretroviral era, individuals living with HIV continue to experience milder forms of HIV-associated neurocognitive disorder (HAND). Viral proteins, including Tat, play a pivotal role in the observed alterations within the central nervous system (CNS), with mitochondrial dysfunction emerging as a prominent hallmark. As a result, our objective was to examine the expression of genes associated with mitophagy and mitochondrial biogenesis in the brain exposed to the HIV-1 Tat protein. We achieved this by performing bilateral stereotaxic injections of 100 ng of HIV-1 Tat into the hippocampus of Sprague-Dawley rats, followed by immunoneuromagnetic cell isolation. Subsequently, we assessed the gene expression of Ppargc1a, Pink1, and Sirt1-3 in neurons using RT-qPCR. Additionally, to understand the role of Tert in telomeric dysfunction, we quantified the activity and expression of Tert. Our results revealed that only Ppargc1a, Pink1, and mitochondrial Sirt3 were downregulated in response to the presence of HIV-1 Tat in hippocampal neurons. Interestingly, we observed a reduction in the activity of Tert in the experimental group, while mRNA levels remained relatively stable. These findings support the compelling evidence of dysregulation in both mitophagy and mitochondrial biogenesis in neurons exposed to HIV-1 Tat, which in turn induces telomeric dysfunction.

Structural Insights into the Mechanism of HIV-1 Tat Secretion from the Plasma Membrane

Human immunodeficiency virus type 1 (HIV-1) trans-activator of transcription (Tat) is a small, intrinsically disordered basic protein that plays diverse roles in the HIV-1 replication cycle, including promotion of efficient viral RNA transcription. Tat is released by infected cells and subsequently absorbed by healthy cells, thereby contributing to HIV-1 pathogenesis including HIV-associated neurocognitive disorder. It has been shown that, in HIV-1-infected primary CD4 T-cells, Tat accumulates at the plasma membrane (PM) for secretion, a mechanism mediated by phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). However, the structural basis for Tat interaction with the PM and thereby secretion is lacking. Herein, we employed NMR and biophysical methods to characterize Tat86 (86 amino acids) interactions with PI(4,5)P2 and lipid nanodiscs (NDs). Our data revealed that Arg49, Lys50 and Lys51 (RKK motif) constitute the PI(4,5)P2 binding site, that Tat86 interaction with lipid NDs is dependent on PI(4,5)P2 and phosphatidylserine (PS), and that the arginine-rich motif (RRQRRR) preferentially interacts with PS. Furthermore, we show that Trp11, previously implicated in Tat secretion, penetrates deeply in the membrane; substitution of Trp11 severely reduced Tat86 interaction with membranes. Deletion of the entire highly basic region and Trp11 completely abolished Tat86 binding to lipid NDs. Our data support a mechanism by which HIV-1 Tat secretion from the PM is mediated by a tripartite signal consisting of binding of the RKK motif to PI(4,5)P2, arginine-rich motif to PS, and penetration of Trp11 in the membrane. Altogether, these findings provide new insights into the molecular requirements for Tat binding to membranes during secretion.

HIV-1 Tat drives the Fabp4/NF-κB feedback loop in microglia to mediate inflammatory response and neuronal apoptosis

Fatty acid-binding proteins (FABPs) are relevant to multiple neurodegenerative diseases. However, the roles and mechanisms of FABPs in HIV-associated neurocognitive disorder (HAND) remain yet unclear. In this study, cultured BV-2 microglial cells and HT-22 neuronal cells were used for in vitro experiments and HAND mouse models were constructed through intracerebroventricular injection of lentiviral vectors for in vivo experiments. FABP expression was determined using quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Western blot. The interrelationship between Fabp4 and NF-κB signaling was investigated using chromatin immunoprecipitation, qRT-PCR, and Western blot. The role of Fabp4 in regulating inflammatory response was determined using qRT-PCR, enzyme-linked immunosorbent assay, Western blot, and immunofluorescence staining. Cell viability and apoptosis were analyzed using cell counting kit-8 assay and flow cytometry assay, respectively. Our results suggested an upregulation of Fabp4 expression in the presence of Tat. Tat-induced Fabp4 expression was directly regulated by NF-κB p65, followed by, Fabp4 facilitating Tat-activated NF-κB signaling pathway. We also observed that Fabp4 knockdown in microglial cells significantly suppressed inflammatory response and neuronal apoptosis both in vitro and in vivo. In conclusion, the presence of Tat in microglial cells results in Fabp4 and NF-κB to form a positive feedback loop leading to exacerbate inflammatory response and neuronal apoptosis.

The Effect of Human Immunodeficiency Virus and Human Papillomavirus Strain Diversity on the Progression of Anal Squamous Intraepithelial Lesions

Anal squamous cell carcinoma (SCC) is a human papillomavirus (HPV)-mediated malignancy with increasing incidence. HIV infection is a significant risk factor for anal SCC, however it is unknown if HIV infection alters anal lesion progression and HPV strain profile. This study aims to determine whether HIV co-infection is associated with progression of HPV-mediated anal lesions and on their HPV strain diversity. This is a retrospective cohort study of adults with anal squamous intraepithelial lesion (SIL) who presented for anorectal sampling between 2010-2019. Using the full cohort, we performed clinicopathologic epidemiologic analysis of HIV co-infection on lesion progression. Using a subset of patients, we conducted molecular analysis of HPV strain diversity as related to HIV status and progression. Our cohort included 2203 individuals, of which 940 (43%) were HIV⁺. HIV⁺ status was associated with faster progression at all levels of dysplasia. Our molecular cohort included 329 adults, of which 190 (57.8%) were HIV+. HIV+ status was associated with higher HPV strain diversity (median: 7 [5-9] vs. median: 4 [4-6], P<0.001). Latent class analysis identified specific HPV strain signatures associated with progression. We demonstrate that HIV⁺ individuals had faster rates of anal SIL progression and that almost all HPV strains were more prevalent in anal samples from HIV+ adults. Our results imply that HIV⁺ adults with anal SIL should undergo more frequent screening and obtain HPV genotyping at initial presentation, as it shows value as a biomarker of lesion progression.

The Role of Inflammation in Anal Cancer

The aim of this article was to present a summary of the current resources available in the literature regarding the role of inflammation in anal cancer development. Anal cancer is relatively uncommon, accounting for about 2.7% of all reported gastrointestinal cancers in the United States. However, the importance of understanding the pathogenesis and risk factors for anal cancer has been recognized over the last several decades due to a noticed increase in incidence worldwide. Infections, autoimmune diseases, and inflammatory diseases of unknown etiology cause chronic inflammation that promotes tumorigenesis. The association between chronic inflammation and cancer development is widely accepted. It is based on different pathophysiological mechanisms that lead to cellular transformation and changes in immunological response, allowing tumor cells to avoid apoptosis and immune surveillance. However, there are still many molecular and cellular mechanisms that remain largely unexplored. Further studies on this topic could be of tremendous significance in elucidating anal cancer pathogenesis and developing immunotherapeutic approaches for its treatment.

Unconventional Pathways of Protein Secretion: Mammals vs. Plants

In eukaryotes, many proteins contain an N-terminal signal peptide that allows their translocation into the endoplasmic reticulum followed by secretion outside the cell according to the classical secretory system. However, an increasing number of secreted proteins lacking the signal peptide sequence are emerging. These proteins, secreted in several alternative ways collectively known as unconventional protein secretion (UPS) pathways, exert extracellular functions including cell signaling, immune modulation, as well as moonlighting activities different from their well-described intracellular functions. Pathways for UPS include direct transfer across the plasma membrane, secretion from endosomal/multivesicular body-related components, release within plasma membrane-derived microvesicles, or use of elements of autophagy. In this review we describe the mammals and plants UPS pathways identified so far highlighting commonalities and differences.

Sirtuins Modulation: A Promising Strategy for HIV-Associated Neurocognitive Impairments

HIV-Associated neurocognitive disorder (HAND) is one of the major concerns since it persists in 40% of this population. Nowadays, HAND neuropathogenesis is considered to be caused by the infected cells that cross the brain-blood barrier and produce viral proteins that can be secreted and internalized into neurons leading to disruption of cellular processes. The evidence points to viral proteins such as Tat as the causal agent for neuronal alteration and thus HAND. The hallmarks in Tat-induced neurodegeneration are endoplasmic reticulum stress and mitochondrial dysfunction. Sirtuins (SIRTs) are NAD+-dependent deacetylases involved in mitochondria biogenesis, unfolded protein response, and intrinsic apoptosis pathway. Tat interaction with these deacetylases causes inhibition of SIRT1 and SIRT3. Studies revealed that SIRTs activation promotes neuroprotection in neurodegenerative diseases such Alzheimer's and Parkinson's disease. Therefore, this review focuses on Tat-induced neurotoxicity mechanisms that involve SIRTs as key regulators and their modulation as a therapeutic strategy for tackling HAND and thereby improving the quality of life of people living with HIV.

Through the back door: Unconventional protein secretion

Proteins are secreted from eukaryotic cells by several mechanisms besides the well-characterized classical secretory system. Proteins destined to enter the classical secretory system contain a signal peptide for translocation into the endoplasmic reticulum. However, many proteins lacking a signal peptide are secreted nonetheless. Contrary to conventional belief, these proteins are not just released as a result of membrane damage leading to cell leakage, but are actively packaged for secretion in alternative pathways. They are called unconventionally secreted proteins, and the best-characterized are from fungi and mammals. These proteins have extracellular functions including cell signaling, immune modulation, as well as moonlighting activities different from their well-described intracellular functions. Among the pathways for unconventional secretion are direct transfer across the plasma membrane, release within plasma membrane-derived microvesicles, use of elements of autophagy, or secretion from endosomal/multivesicular body-related components. We review the fungal and metazoan unconventional secretory pathways and their regulation, and propose experimental criteria to identify their mode of secretion.

Extracellular vesicles in virus infection and pathogenesis

Viruses are obligate intracellular parasites that usurp cellular signaling networks to promote pathogen spread and disease progression. Signaling through extracellular vesicles (EVs) is an emerging field of study in the virus-host interaction network. EVs relay information both locally and distally through incorporated contents, typically without tripping innate immune sensors. Therefore, this extracellular signaling axis presents itself as a tantalizing target for promoting a favorable niche for the pathogen(s) takeover of the host, particularly for chronic infections. From the incorporation of virus-encoded molecules such as micro RNAs and proteins/enzymes to the envelopment of entire infectious particles, evolutionary distinct viruses have shown a remarkable ability to converge on this means of communication. In this review, we will cover the recent advances in this field and explore how EV can be used as potential biomarkers for chronic, persistent, or latent virus infections.

Extracellular HIV-1 Tat Mediates Increased Glutamate in the CNS Leading to Onset of Senescence and Progression of HAND

Human immunodeficiency virus type 1 (HIV-1)- associated neurocognitive disorders (HAND) is a disease of neurologic impairment that involves mechanisms of damage similar to other degenerative neurologic diseases such as Alzheimer’s disease (AD). In the current era of antiretroviral therapy (ART), HIV-1 replication is well-suppressed, and yet, HIV-1-infected patients still have high levels of chronic inflammation, indicating that factors other than viral replication are contributing to the development of neurocognitive impairment in these patients. The underlying mechanisms of HAND are still unknown, but the HIV-1 protein, Tat, has been highlighted as a potential viral product that contributes to the development of cognitive impairment. In AD, the presence of senescent cells in the CNS has been discussed as a contributing factor to the progression of cognitive decline and may be a mechanism also involved in the development of HAND. This mini-review discusses the viral protein HIV-1 Tat, and its potential to induce senescence in the CNS, contributing to the development of HAND.

Tat-Based Therapies as an Adjuvant for an HIV-1 Functional Cure

The human immunodeficiency virus type 1 (HIV) establishes a chronic infection that can be well controlled, but not cured, by combined antiretroviral therapy (cART). Interventions have been explored to accomplish a functional cure, meaning that a patient remains infected but HIV is undetectable in the blood, with the aim of allowing patients to live without cART. Tat, the viral transactivator of transcription protein, plays a critical role in controlling HIV transcription, latency, and viral rebound following the interruption of cART treatment. Therefore, a logical approach for controlling HIV would be to block Tat. Tackling Tat with inhibitors has been a difficult task, but some recent discoveries hold promise. Two anti-HIV proteins, Nullbasic (a mutant of Tat) and HT1 (a fusion of HEXIM1 and Tat functional domains) inhibit viral transcription by interfering with the interaction of Tat and cellular factors. Two small molecules, didehydro-cortistatin A (dCA) and triptolide, inhibit Tat by different mechanisms: dCA through direct binding and triptolide through enhanced proteasomal degradation. Finally, two Tat-based vaccines under development elicit Tat-neutralizing antibodies. These vaccines have increased the levels of CD4⁺ cells and reduced viral loads in HIV-infected people, suggesting that the new vaccines are therapeutic. This review summarizes recent developments of anti-Tat agents and how they could contribute to a functional cure for HIV.

Morphine exposure exacerbates HIV-1 Tat driven changes to neuroinflammatory factors in cultured astrocytes

Despite antiretroviral therapy human immunodeficiency virus type-1 (HIV-1) infection results in neuroinflammation of the central nervous system that can cause HIV-associated neurocognitive disorders (HAND). The molecular mechanisms involved in the development of HAND are unclear, however, they are likely due to both direct and indirect consequences of HIV-1 infection and inflammation of the central nervous system. Additionally, opioid abuse in infected individuals has the potential to exacerbate HIV-comorbidities, such as HAND. Although restricted for productive HIV replication, astrocytes (comprising 40-70% of all brain cells) likely play a significant role in neuropathogenesis in infected individuals due to the production and response of viral proteins. The HIV-1 protein Tat is critical for viral transcription, causes neuroinflammation, and can be secreted from infected cells to affect uninfected bystander cells. The Wnt/β-catenin signaling cascade plays an integral role in restricting HIV-1 infection in part by negatively regulating HIV-1 Tat function. Conversely, Tat can overcome this negative regulation and inhibit β-catenin signaling by sequestering the critical transcription factor TCF-4 from binding to β-catenin. Here, we aimed to explore how opiate exposure affects Tat-mediated suppression of β-catenin in astrocytes and the downstream modulation of neuroinflammatory genes. We observed that morphine can potentiate Tat suppression of β-catenin activity in human astrocytes. In contrast, Tat mutants deficient in secretion, and lacking neurotoxic effects, do not affect β-catenin activity in the presence or absence of morphine. Finally, morphine treatment of astrocytes was sufficient to reduce the expression of genes involved in neuroinflammation. Examining the molecular mechanisms of how HIV-1 infection and opiate exposure exacerbate neuroinflammation may help us inform or predict disease progression prior to HAND development.

Anti-Tat Immunity in HIV-1 Infection: Effects of Naturally Occurring and Vaccine-Induced Antibodies Against Tat on the Course of the Disease

HIV-1 Tat is an essential protein in the virus life cycle, which is required for virus gene expression and replication. Most Tat that is produced during infection is released extracellularly and it plays a key role in HIV pathogenesis, including residual disease upon combination antiretroviral therapy (cART). Here, we review epidemiological and experimental evidence showing that antibodies against HIV-1 Tat, infrequently occurring in natural infection, play a protective role against disease progression, and that vaccine targeting Tat can intensify cART. In fact, Tat vaccination of subjects on suppressive cART in Italy and South Africa promoted immune restoration, including CD4+ T-cell increase in low immunological responders, and a reduction of proviral DNA even after six years of cART, when both CD4+ T-cell gain and DNA decay have reached a plateau. Of note, DNA decay was predicted by the neutralization of Tat-mediated entry of Env into dendritic cells by anti-Tat antibodies, which were cross-clade binding and neutralizing. Anti-Tat cellular immunity also contributed to the DNA decay. Based on these data, we propose the Tat therapeutic vaccine as a pathogenesis-driven intervention that effectively intensifies cART and it may lead to a functional cure, providing new perspectives and opportunities also for prevention and virus eradication strategies.

Investigating the distribution of HIV-1 Tat lengths present in the Drexel Medicine CARES cohort

Human immunodeficiency virus type 1 (HIV-1) encodes for Tat, a multi-functional regulatory protein involved in transcriptional enhancement and in causing neurotoxicity/central nervous system (CNS) dysfunction. This study examines Sanger sequencing of HIV-1 subtype B Tat from 2006 to 2014 within the Drexel University College of Medicine CNS AIDS Research and Eradication Study (CARES) Cohort to investigate Tat length in patients. The Los Alamos National Laboratory (LANL) database was used as a comparator. Miscoded stop codons were present in the CARES Cohort and LANL and protein variability was highly similar. Tat proteins in CARES and LANL were predominantly 101 residues. There was no observed correlation between Tat length and clinical parameters within the CARES Cohort. Unique Tat lengths found in the CARES Cohort and not in LANL were 31, 36, and 39 residues. When CARES patients were longitudinally examined, sequence lengths of 101 had a low probability of reducing to below 48, and sequences had a high probability of increasing to above 86 residues during their next visit, when below 48 residues in length. This suggests that Tat length is conserved to retain the majority of the proteins function highlighting its importance in viral replication.

Genetic variation and function of the HIV-1 Tat protein

Human immunodeficiency virus type 1 (HIV-1) encodes a transactivator of transcription (Tat) protein, which has several functions that promote viral replication, pathogenesis, and disease. Amino acid variation within Tat has been observed to alter the functional properties of Tat and, depending on the HIV-1 subtype, may produce Tat phenotypes differing from viruses' representative of each subtype and commonly used in in vivo and in vitro experimentation. The molecular properties of Tat allow for distinctive functional activities to be determined such as the subcellular localization and other intracellular and extracellular functional aspects of this important viral protein influenced by variation within the Tat sequence. Once Tat has been transported into the nucleus and becomes engaged in transactivation of the long terminal repeat (LTR), various Tat variants may differ in their capacity to activate viral transcription. Post-translational modification patterns based on these amino acid variations may alter interactions between Tat and host factors, which may positively or negatively affect this process. In addition, the ability of HIV-1 to utilize or not utilize the transactivation response (TAR) element within the LTR, based on genetic variation and cellular phenotype, adds a layer of complexity to the processes that govern Tat-mediated proviral DNA-driven transcription and replication. In contrast, cytoplasmic or extracellular localization of Tat may cause pathogenic effects in the form of altered cell activation, apoptosis, or neurotoxicity. Tat variants have been shown to differentially induce these processes, which may have implications for long-term HIV-1-infected patient care in the antiretroviral therapy era. Future studies concerning genetic variation of Tat with respect to function should focus on variants derived from HIV-1-infected individuals to efficiently guide Tat-targeted therapies and elucidate mechanisms of pathogenesis within the global patient population.

Tat basic domain: A "Swiss army knife" of HIV-1 Tat?

Tat (transactivator of transcription) regulates transcription from the HIV provirus. It plays a crucial role in disease progression, supporting efficient replication of the viral genome. Tat also modulates many functions in the host genome via its interaction with chromatin and proteins. Many of the functions of Tat are associated with its basic domain rich in arginine and lysine residues. It is still unknown why the basic domain exhibits so many diverse functions. However, the highly charged basic domain, coupled with the overall structural flexibility of Tat protein itself, makes the basic domain a key player in binding to or associating with cellular and viral components. In addition, the basic domain undergoes diverse posttranslational modifications, which further expand and modulate its functions. Here, we review the current knowledge of Tat basic domain and its versatile role in the interaction between the virus and the host cell.

The role of exosomal transport of viral agents in persistent HIV pathogenesis

Human immunodeficiency virus (HIV) infection, despite great advances in antiretroviral therapy, remains a lifelong affliction. Though current treatment regimens can effectively suppress viral load to undetectable levels and preserve healthy immune function, they cannot fully alleviate all symptoms caused by the presence of the virus, such as HIV-associated neurocognitive disorders. Exosomes are small vesicles that transport cellular proteins, RNA, and small molecules between cells as a mechanism of intercellular communication. Recent research has shown that HIV proteins and RNA can be packaged into exosomes and transported between cells, to pathogenic effect. This review summarizes the current knowledge on the diverse mechanisms involved in the sorting of viral elements into exosomes and the damage those exosomal agents can inflict. In addition, potential therapeutic options to counteract exosome-mediated HIV pathogenesis are reviewed and considered.