Cell-Penetrating Peptide Mediates Intracellular Membrane Passage of Human Papillomavirus L2 Protein to Trigger Retrograde Trafficking.

Cell-penetrating peptides (CPPs) are short protein segments that can transport cargos into cells. Although CPPs are widely studied as potential drug delivery tools, their role in normal cell physiology is poorly understood. Early during infection, the L2 capsid protein of human papillomaviruses binds retromer, a cytoplasmic trafficking factor required for delivery of the incoming non-enveloped virus into the retrograde transport pathway. Here, we show that the C terminus of HPV L2 proteins contains a conserved cationic CPP that drives passage of a segment of the L2 protein through the endosomal membrane into the cytoplasm, where it binds retromer, thereby sorting the virus into the retrograde pathway for transport to the trans-Golgi network. These experiments define the cell-autonomous biological role of a CPP in its natural context and reveal how a luminal viral protein engages an essential cytoplasmic entry factor.

Functional Segregation of Overlapping Genes in HIV.

Overlapping genes pose an evolutionary dilemma as one DNA sequence evolves under the selection pressures of multiple proteins. Here, we perform systematic statistical and mutational analyses of the overlapping HIV-1 genes tat and rev and engineer exhaustive libraries of non-overlapped viruses to perform deep mutational scanning of each gene independently. We find a "segregated" organization in which overlapped sites encode functional residues of one gene or the other, but never both. Furthermore, this organization eliminates unfit genotypes, providing a fitness advantage to the population. Our comprehensive analysis reveals the extraordinary manner in which HIV minimizes the constraint of overlapping genes and repurposes that constraint to its own advantage. Thus, overlaps are not just consequences of evolutionary constraints, but rather can provide population fitness advantages.

Structural rearrangements in the nucleus localize latent HIV proviruses to a perinucleolar compartment supportive of reactivation.

Using an immunofluorescence assay based on CRISPR-dCas9-gRNA complexes that selectively bind to the HIV LTR (HIV Cas-FISH), we traced changes in HIV DNA localization in primary effector T cells from early infection until the cells become quiescent as they transition to memory cells. Unintegrated HIV DNA colocalized with CPSF6 and HIV capsid (CA, p24) was found in the cytoplasm and nuclear periphery at days 1 and 3 post infection. From days 3 to 7, most HIV DNA was distributed primarily in the nuclear intermediate euchromatic compartment and was transcribed. By day 21, the cells had entered quiescence, and HIV DNA accumulated in the perinucleolar compartment (PNC). The localization of proviruses to the PNC was blocked by integrase inhibitor Raltegravir, suggesting it was due to chromosomal rearrangements. During the reactivation of latently infected cells through the T cell receptor (TCR), nascent viral mRNA transcripts associated with HIV DNA in the PNC were detected. The viral trans-activator Tat and its regulatory partners, P-TEFb and 7SK snRNA, assembled in large interchromatin granule clusters near the provirus within 2 h of TCR activation. As T cell activation progressed, the HIV DNA shifted away from the PNC. HIV DNA in latently infected memory T cells from patients also accumulated in the PNC and showed identical patterns of nuclear rearrangements after cellular reactivation. Thus, in contrast to transformed cells where proviruses are found primarily at the nuclear periphery, in primary memory T cells, the nuclear architecture undergoes rearrangements that shape the transcriptional silencing and reactivation of proviral HIV.

New insights into transcription elongation control of HIV-1 latency and rebound.

Antiretroviral therapy reduces circulating HIV-1 to undetectable amounts but does not eliminate the virus due to the persistence of a stable reservoir of latently infected cells. The reservoir is maintained both by proliferation of latently infected cells and by reseeding from reactivated cells. A major challenge for the field is to find safe and effective methods to eliminate this source of rebounding HIV-1. Studies on the molecular mechanisms leading to HIV-1 latency and reactivation are being transformed using latency models in primary and patient CD4+ T cells. These studies have revealed the central role played by the biogenesis of the transcription elongation factor P-TEFb (Positive Transcription Elongation Factor b) and its recruitment to proviral HIV-1, for the maintenance of viral latency and the control of viral reactivation.

The chaperone protein p32 stabilizes HIV-1 Tat and strengthens the p-TEFb/RNAPII/TAR complex promoting HIV transcription elongation.

HIV gene expression is modulated by the combinatorial activity of the HIV transcriptional activator, Tat, host transcription factors, and chromatin remodeling complexes. To identify host factors regulating HIV transcription, we used specific single-guide RNAs and endonuclease-deficient Cas9 to perform chromatin affinity purification of the integrated HIV promoter followed by mass spectrometry. The scaffold protein, p32, also called ASF/SF2 splicing factor-associated protein, was identified among the top enriched factors present in actively transcribing HIV promoters but absent in silenced ones. Chromatin immunoprecipitation analysis confirmed the presence of p32 on active HIV promoters and its enhanced recruitment by Tat. HIV uses Tat to efficiently recruit positive transcription elongation factor b (p-TEFb) (CDK9/CCNT1) to TAR, an RNA secondary structure that forms from the first 59 bp of HIV transcripts, to enhance RNAPII transcriptional elongation. The RNA interference of p32 significantly reduced HIV transcription in primary CD4+T cells and in HIV chronically infected cells, independently of either HIV splicing or p32 anti-splicing activity. Conversely, overexpression of p32 specifically increased Tat-dependent HIV transcription. p32 was found to directly interact with Tat's basic domain enhancing Tat stability and half-life. Conversely, p32 associates with Tat via N- and C-terminal domains. Likely due its scaffold properties, p32 also promoted Tat association with TAR, p-TEFb, and RNAPII enhancing Tat-dependent HIV transcription. In sum, we identified p32 as a host factor that interacts with and stabilizes Tat protein, promotes Tat-dependent transcriptional regulation, and may be explored for HIV-targeted transcriptional inhibition.

Transcriptional regulation of the HIV-1 inhibitory factor human mannose receptor 1 by the myeloid-specific transcription factor PU.1.

HIV-1 infection of human macrophages leads to the downmodulation of human mannose receptor 1 (hMRC1), a cell-surface glycoprotein that is involved in the host innate immune response. We previously reported that downmodulation of hMRC1 involves the transactivator of transcription (Tat)-dependent transcriptional silencing of the hMRC1 promoter. However, the inhibitory effect of Tat on hMRC1 transcription was indirect and involved inhibition of the transcriptional activator PU.1, which normally upregulates hMRC1 expression in macrophages and other myeloid cells. We cloned a 284-bp fragment of the hMRC1 promoter, and within it, we identified four PU.1 box elements. We assessed the relative contribution of each of the four PU.1 boxes to PU.1-dependent transcriptional regulation and, surprisingly, found that only one of the four PU.1 boxes [PU.1(b)] was critically required for PU.1-mediated upregulation of luciferase expression. Transfer of this PU.1 box to a heterologous promoter conferred PU.1 responsiveness to an otherwise PU.1 insensitive promoter. Electrophoretic mobility shift assays identified this PU.1 box as a direct binding site for PU.1 both in the context of the hMRC1 promoter and the heterologous promoter. Furthermore, mutational analysis of the PU.1 protein identified the C-terminal DNA-binding domain in PU.1 as the region responsible for interaction with the PU.1 box. Recombinant HIV-1 Tat protein did not bind to the hMRC1 promoter element but efficiently interfered with the binding of PU.1 protein to the hMRC1 promoter. Thus, Tat is likely to inhibit the formation of active PU.1 transcription complexes, presumably by binding to and depleting common transcriptional cofactors.IMPORTANCEHIV-1 infection of cells results in the modulation of cellular gene expression by virus-encoded proteins in a manner that benefits the virus. We reported that HIV-1 transactivator of transcription (Tat) dysregulates the expression of the human mannose receptor 1 (hMRC1). hMRC1 is involved in the innate immune response of macrophages to foreign pathogens. Tat does not act directly on the hMRC1 promoter but instead inhibits PU.1, a cellular transcription factor regulating hMRC1 gene expression. Here, we characterize the PU.1-dependent regulation of hMRC1 expression. We identified four potential PU.1 binding sites in the hMRC1 promoter region but found that only one, PU.1(b), functioned as a true binding site for PU.1. Transfer of the PU.1(b) box to a heterologous promoter did not activate this promoter per se but rendered it responsive to PU.1. Our results support the view that PU.1 acts as a transcriptional co-factor whose activity can be regulated by HIV-1 Tat.

TAT-beclin1 treatment accelerates the development of atherosclerotic lesions in ApoE-deficient mice.

The importance of autophagy in atherosclerosis has garnered significant attention regarding the potential applications of autophagy inducers. However, the impact of TAT-Beclin1, a peptide inducer of autophagy, on the development of atherosclerotic plaques remains unclear. Single-cell omics analysis indicates a notable reduction in GAPR1 levels within fibroblasts, stromal cells, and macrophages during atherosclerosis. Tat-beclin1 (T-B), an autophagy-inducing peptide derived from Beclin1, could selectively bind to GAPR1, relieving its inhibition on Beclin1 and thereby augmenting autophagosome formation. To investigate its impact on atherosclerosic plaque progression, we established the ApoE-/- mouse model of carotid atherosclerotic plaques. Surprisingly, intravenous administration of Tat-beclin1 dramatically accelerated the development of carotid artery plaques. Immunofluorescence analysis suggested that macrophage aggregation and autophagosome formation within atherosclerotic plaques were significantly increased upon T-B treatment. However, immunofluorescence and transmission electron microscopy (TEM) analysis revealed a reduction in autophagy flux through lysosomes. In vitro, the interaction between T-B and GAPR1 was confirmed in RAW264.7 cells, resulting in the increased accumulation of p62/SQSTM1 and LC3-II in the presence of ox-LDL. Additionally, T-B treatment elevated the protein levels of p62/SQSTM1, LC3-II, and cleaved caspase 1, along with the secretion of IL-1ß in response to ox-LDL exposure. In summary, our study underscores that T-B treatment amplifies abnormal autophagy and inflammation, consequently exacerbating atherosclerotic plaque development in ApoE-/- mice.

A small TAT-TrkB peptide prevents BDNF receptor cleavage and restores synaptic physiology in Alzheimer's disease.

In Alzheimer's disease (AD), amyloid ß (Aß)-triggered cleavage of TrkB-FL impairs brain-derived neurotrophic factor (BDNF) signaling, thereby compromising neuronal survival, differentiation, and synaptic transmission and plasticity. Using cerebrospinal fluid and postmortem human brain samples, we show that TrkB-FL cleavage occurs from the early stages of the disease and increases as a function of pathology severity. To explore the therapeutic potential of this disease mechanism, we designed small TAT-fused peptides and screened their ability to prevent TrkB-FL receptor cleavage. Among these, a TAT-TrkB peptide with a lysine-lysine linker prevented TrkB-FL cleavage both in vitro and in vivo and rescued synaptic deficits induced by oligomeric Aß in hippocampal slices. Furthermore, this TAT-TrkB peptide improved the cognitive performance, ameliorated synaptic plasticity deficits and prevented Tau pathology progression in vivo in the 5XFAD mouse model of AD. No evidence of liver or kidney toxicity was found. We provide proof-of-concept evidence for the efficacy and safety of this therapeutic strategy and anticipate that this TAT-TrkB peptide has the potential to be a disease-modifying drug that can prevent and/or reverse cognitive deficits in patients with AD.

Liposome nanoparticle conjugation and cell penetrating peptide sequences (CPPs) enhance the cellular delivery of the tau aggregation inhibitor RI-AG03.

Given the pathological role of Tau aggregation in Alzheimer's disease (AD), our laboratory previously developed the novel Tau aggregation inhibitor peptide, RI-AG03. As Tau aggregates accumulate intracellularly, it is essential that the peptide can traverse the cell membrane. Here we examine the cellular uptake and intracellular trafficking of RI-AG03, in both a free and liposome-conjugated form. We also characterize the impact of adding the cell-penetrating peptide (CPP) sequences, polyarginine (polyR) or transactivator of transcription (TAT), to RI-AG03. Our data show that liposome conjugation of CPP containing RI-AG03 peptides, with either the polyR or TAT sequence, increased cellular liposome association three-fold. Inhibition of macropinocytosis modestly reduced the uptake of unconjugated and RI-AG03-polyR-linked liposomes, while having no effect on RI-AG03-TAT-conjugated liposome uptake. Further supporting macropinocytosis-mediated internalization, a 'fair' co-localisation of the free and liposome-conjugated RI-AG03-polyR peptide with macropinosomes and lysosomes was observed. Interestingly, we also demonstrate that RI-AG03-polyR detaches from liposomes following cellular uptake, thereby largely evading organellar entrapment. Collectively, our data indicate that direct membrane penetration and macropinocytosis are key routes for the internalization of liposomes conjugated with CPP containing RI-AG03. Our study also demonstrates that peptide-liposomes are suitable nanocarriers for the cellular delivery of RI-AG03, furthering their potential use in targeting Tau pathology in AD.

Compensatory reactions of B cells in response to chronic HIV-1 Tat exposure.

Patients infected with human immunodeficiency virus-1 (HIV-1) have an increased incidence of B-cell lymphoma, even though HIV-1 does not infect B cells. The development of B-cell lymphomas appears to be related to the action of the HIV-1 transactivator protein (Tat), which is released from HIV-infected cells and penetrates uninfected B cells, affecting host cell gene expression. Upon chronic HIV-1 infection, Tat acts on the cells for a long time, probably allowing the cells to adapt to the presence of the viral protein. The aim of this work was to identify and study the mechanism of adaptation of cells to prolonged (chronic) exposure to HIV-1 Tat. We performed a comparative analysis of cells expressing Tat under the action of either an inducible promoter or a constitutive promoter, allowing us to model acute and chronic Tat effects, respectively. We found that the acute action of Tat leads to the suppression of cell proliferation, probably due to the downregulation of genes associated with replication and protein synthesis. In the case of chronic action of Tat, cell proliferation was restored and the expression of genes associated with the implementation of protective (antiviral) functions of the cell was increased. Analysis using proteasome inhibitors showed that in the case of chronic action, intense Tat proteolysis occurred, which could be the main mechanism of B-cell adaptation. Thus, B cells have a powerful mechanism to adapt to the entry of HIV-1 Tat, the efficiency of which may determine the frequency of lymphomagenesis in HIV-1-infected patients.

ROS Chronicles in HIV Infection: Genesis of Oxidative Stress, Associated Pathologies, and Therapeutic Strategies.

Reactive oxygen species (ROS) are widely regarded as signaling molecules and play essential roles in various cellular processes, but when present in excess, they can lead to oxidative stress (OS). Growing evidence suggests that the OS plays a critical role in the pathogenesis of HIV infection and is associated with several comorbidities in HIV-infected individuals. ROS, generated both naturally during mitochondrial oxidative metabolism and as a response to various cellular processes, can trigger host antiviral responses but can also promote viral replication. While the multifaceted roles of ROS in HIV pathophysiology clearly need more investigation, this review paper unravels the mechanisms of OS generation in the context of HIV infections, offering insights into HIV viral protein-mediated and antiretroviral therapy-generated OS. Though the viral protein Tat is significantly attributed to the endogenous cellular increase in ROS post HIV infection, this paper sums up the contribution of other viral proteins in HIV-mediated elicitation of ROS. Given the investigations recognizing the significant role of ROS in the onset and progression of diverse pathologies, the paper also explores the critical function of ROS in the mediation of an of array of pathologies associated with HIV infection and retroviral therapy. HIV patients are observed with disruption to the antioxidant defense system, the antioxidant therapy is gaining focus as a potential therapeutic intervention and is well discussed. While ROS play a significant role in the HIV scenario, further exploratory studies are imperative to identifying alternative therapeutic strategies that could mitigate the toxicities and pathologies associated with ART-induced OS.

The cell biology of HIV-1 latency and rebound.

Transcriptionally latent forms of replication-competent proviruses, present primarily in a small subset of memory CD4+ T cells, pose the primary barrier to a cure for HIV-1 infection because they are the source of the viral rebound that almost inevitably follows the interruption of antiretroviral therapy. Over the last 30 years, many of the factors essential for initiating HIV-1 transcription have been identified in studies performed using transformed cell lines, such as the Jurkat T-cell model. However, as highlighted in this review, several poorly understood mechanisms still need to be elucidated, including the molecular basis for promoter-proximal pausing of the transcribing complex and the detailed mechanism of the delivery of P-TEFb from 7SK snRNP. Furthermore, the central paradox of HIV-1 transcription remains unsolved: how are the initial rounds of transcription achieved in the absence of Tat? A critical limitation of the transformed cell models is that they do not recapitulate the transitions between active effector cells and quiescent memory T cells. Therefore, investigation of the molecular mechanisms of HIV-1 latency reversal and LRA efficacy in a proper physiological context requires the utilization of primary cell models. Recent mechanistic studies of HIV-1 transcription using latently infected cells recovered from donors and ex vivo cellular models of viral latency have demonstrated that the primary blocks to HIV-1 transcription in memory CD4+ T cells are restrictive epigenetic features at the proviral promoter, the cytoplasmic sequestration of key transcription initiation factors such as NFAT and NF-κB, and the vanishingly low expression of the cellular transcription elongation factor P-TEFb. One of the foremost schemes to eliminate the residual reservoir is to deliberately reactivate latent HIV-1 proviruses to enable clearance of persisting latently infected cells-the "Shock and Kill" strategy. For "Shock and Kill" to become efficient, effective, non-toxic latency-reversing agents (LRAs) must be discovered. Since multiple restrictions limit viral reactivation in primary cells, understanding the T-cell signaling mechanisms that are essential for stimulating P-TEFb biogenesis, initiation factor activation, and reversing the proviral epigenetic restrictions have become a prerequisite for the development of more effective LRAs.

Nanoparticle delivery of Tat synergizes with classical latency reversal agents to express HIV antigen targets.

Limited cellular levels of the HIV transcriptional activator Tat are one contributor to proviral latency that might be targeted in HIV cure strategies. We recently demonstrated that lipid nanoparticles containing HIV tat mRNA induce HIV expression in primary CD4 T cells. Here, we sought to further characterize tat mRNA in the context of several benchmark latency reversal agents (LRAs), including inhibitor of apoptosis protein antagonists (IAPi), bromodomain and extra-Terminal motif inhibitors (BETi), and histone deacetylase inhibitors (HDACi). tat mRNA reversed latency across several different cell line models of HIV latency, an effect dependent on the TAR hairpin loop. Synergistic enhancement of tat mRNA activity was observed with IAPi, HDACi, and BETi, albeit to variable degrees. In primary CD4 T cells from durably suppressed people with HIV, tat mRNA profoundly increased the frequencies of elongated, multiply-spliced, and polyadenylated HIV transcripts, while having a lesser impact on TAR transcript frequencies. tat mRNAs alone resulted in variable HIV p24 protein induction across donors. However, tat mRNA in combination with IAPi, BETi, or HDACi markedly enhanced HIV RNA and protein expression without overt cytotoxicity or cellular activation. Notably, combination regimens approached or in some cases exceeded the latency reversal activity of maximal mitogenic T cell stimulation. Higher levels of tat mRNA-driven HIV p24 induction were observed in donors with larger mitogen-inducible HIV reservoirs, and expression increased with prolonged exposure time. Combination LRA strategies employing both small molecule inhibitors and Tat delivered to CD4 T cells are a promising approach to effectively target the HIV reservoir.

Discovery of new acetamide derivatives of 5-indole-1,3,4-oxadiazol-2-thiol as inhibitors of HIV-1 Tat-mediated viral transcription.

Human immunodeficiency virus-1 (HIV-1) encodes a transcriptional factor called Tat, which is critical for viral transcription. Tat-mediated transcription is highly ordered apart from the cellular manner; therefore, it is considered a target for developing anti-HIV-1 drugs. However, drugs targeting Tat-mediated viral transcription are not yet available. Our high-throughput screen of a compound library employing a dual-reporter assay identified a 1,3,4-oxadiazole scaffold against Tat and HIV-1 infection. Furthermore, a serial structure-activity relation (SAR) study performed with biological assays found 1,3,4-oxadiazole derivatives (9 and 13) containing indole and acetamide that exhibited potent inhibitory effects on HIV-1 infectivity, with half-maximal effective concentrations (EC50) of 0.17 (9) and 0.24 µM (13), respectively. The prominent derivatives specifically interfered with the viral transcriptional step without targeting other infection step(s) and efficiently inhibited the HIV-1 replication cycle in the T cell lines and peripheral blood mononuclear cells infected with HIV-1. Additionally, compared to the wild type, the compounds exhibited similar potency against anti-retroviral drug-resistant HIV-1 strains. In a series of mode-of-action studies, the compounds inhibited the ejection of histone H3 for facilitating viral transcription on the long-terminal repeat (LTR) promoter. Furthermore, SAHA (a histone deacetylase inhibitor) treatment abolished the compound potency, revealing that the compounds can possibly target Tat-regulated epigenetic modulation of LTR to inhibit viral transcription. Overall, our screening identified novel 1,3,4-oxadiazole compounds that inhibited HIV-1 Tat, and subsequent SAR-based optimization led to the derivatives 9 and 13 development that could be a promising scaffold for developing a new class of therapeutic agents for HIV-1 infection.

Identification of novel tail-anchored membrane proteins integrated by the bacterial twin-arginine translocase.

The twin-arginine protein transport (Tat) system exports folded proteins across the cytoplasmic membranes of prokaryotes and the energy transducing-membranes of plant thylakoids and mitochondria. Proteins are targeted to the Tat machinery by N-terminal signal peptides with a conserved twin-arginine motif, and some substrates are exported as heterodimers where the signal peptide is present on one of the partner proteins. A subset of Tat substrates is found in the membrane. Tat-dependent membrane proteins usually have large globular domains and a single transmembrane helix present at the N- or C-terminus. Five Tat substrates that have C-terminal transmembrane helices have previously been characterized in the model bacterium Escherichia coli. Each of these is an iron-sulfur cluster-containing protein involved in electron transfer from hydrogen or formate. Here we have undertaken a bioinformatic search to identify further tail-anchored Tat substrates encoded in bacterial genomes. Our analysis has revealed additional tail-anchored iron-sulfur proteins associated in modules with either a b-type cytochrome or a quinol oxidase. We also identified further candidate tail-anchored Tat substrates, particularly among members of the actinobacterial phylum, that are not predicted to contain cofactors. Using reporter assays, we show experimentally that six of these have both N-terminal Tat signal peptides and C-terminal transmembrane helices. The newly identified proteins include a carboxypeptidase and a predicted protease, and four sortase substrates for which membrane integration is a prerequisite for covalent attachment to the cell wall.

Integrin Facilitates the Internalization of TAT Peptide Conjugated to RGD Motif in Model Lipid Membranes.

In recent years, targeted drug delivery has attracted a great interest for enhanced therapeutic efficiency, with diminished side effects, especially in cancer therapy. Cell penetrating peptides (CPPs) like HIV1-TAT peptides, appear to be the perfect vectors for translocating drugs or other cargoes across the plasma membrane, but their application is limited mostly due to insufficient specificity for intended targets. Although these molecules were successfully used, the mechanism by which the peptides enter the cell interior still needs to be clarified. The tripeptide motif RGD (arginine-glycine-aspartate), found in extracellular matrix proteins has high affinity for integrin receptors overexpressed in cancer and it is involved in different phases of disease progression, including proliferation, invasion and migration. Discovery of new peptides with high binding affinity for disease receptors and permeability of plasma membranes is desirable for both, development of targeted drug delivery systems and early detection and diagnosis. To complement the TAT peptide with specific targeting ability, we conjugated it with an integrin-binding RGD motif. Although the idea of RGD-CPPs conjugates is not entirely new,[1] here we describe the permeability abilities and specificity of integrin receptors of RGD-TAT peptides in model membranes. Our findings reveal that this novel RGD sequence based on TAT peptide maintains its ability to permeate lipid membranes and exhibits specificity for integrin receptors embedded in giant unilamellar vesicles. This promising outcome suggests that the RGD-TAT peptide has significant potential for applications in the field of targeted drug delivery systems.

A Tripartite Complex HIV-1 Tat-Cyclophilin A-Capsid Protein Enables Tat Encapsidation That Is Required for HIV-1 Infectivity.

HIV-1 Tat is a key viral protein that stimulates several steps of viral gene expression. Tat is especially required for the transcription of viral genes. Nevertheless, it is still not clear if and how Tat is incorporated into HIV-1 virions. Cyclophilin A (CypA) is a prolyl isomerase that binds to HIV-1 capsid protein (CA) and is thereby encapsidated at the level of 200 to 250 copies of CypA/virion. Here, we found that a Tat-CypA-CA tripartite complex assembles in HIV-1-infected cells and allows Tat encapsidation into HIV virions (1 Tat/1 CypA). Biochemical and biophysical studies showed that high-affinity interactions drive the assembly of the Tat-CypA-CA complex that could be purified by size exclusion chromatography. We prepared different types of viruses devoid of transcriptionally active Tat. They showed a 5- to 10 fold decrease in HIV infectivity, and conversely, encapsidating Tat into ΔTat viruses greatly enhanced infectivity. The absence of encapsidated Tat decreased the efficiency of reverse transcription by ~50% and transcription by more than 90%. We thus identified a Tat-CypA-CA complex that enables Tat encapsidation and showed that encapsidated Tat is required to initiate robust viral transcription and thus viral production at the beginning of cell infection, before neosynthesized Tat becomes available. IMPORTANCE The viral transactivating protein Tat has been shown to stimulate several steps of HIV gene expression. It was found to facilitate reverse transcription. Moreover, Tat is strictly required for the transcription of viral genes. Although the presence of Tat within HIV virions would undoubtedly favor these steps and therefore enable the incoming virus to boost initial viral production, whether and how Tat is present within virions has been a matter a debate. We here described and characterized a tripartite complex between Tat, HIV capsid protein, and the cellular chaperone cyclophilin A that enables efficient and specific Tat encapsidation within HIV virions. We further showed that Tat encapsidation is required for the virus to efficiently initiate infection and viral production. This effect is mainly due to the transcriptional activity of Tat.

HIV-1 Tat commandeers nuclear export of Rev-viral RNA complex by controlling hnRNPA2-mediated splicing.

IMPORTANCE: HIV-infected host cells impose varied degrees of regulation on viral replication, from very high to abortive. Proliferation of HIV in astrocytes is limited when compared to immune cells, such as CD4+ T lymphocytes. Understanding such differential regulation is one of the key questions in the field as these cells permit HIV persistence and rebound viremia, challenging HIV treatment and clinical cure. This study focuses on understanding the molecular mechanism behind such cell-specific disparities. We show that one of the key mechanisms is the regulation of heterogenous nuclear ribonucleoprotein A2, a host factor involved in alternative splicing and RNA processing, by HIV-1 Tat in CD4+ T lymphocytes, not observed in astrocytes. This regulation causes an increase in the levels of unspliced/partially spliced viral RNA and nuclear export of Rev-RNA complexes which results in high viral propagation in CD4+ T lymphocytes. The study reveals a new mechanism imposed by HIV on host cells that determines the fate of infection.

People living with HIV co-infected with the Kaposi Sarcoma-associated Herpes Virus have a distinct HIV Tat profile and higher rates of antiretroviral virologic failure, more evident among those with Kaposi's sarcoma.

Kaposi sarcoma (KS) is a neoplasm of vascular origin that promotes angiogenesis and the growth of endothelial cells triggered by the Kaposi Sarcoma-associated Herpes Virus (KSHV). When associated with HIV, KSHV becomes more aggressive and rapidly evolves. The HIV-1 TAT protein can be essential in developing AIDS-associated KS by promoting angiogenesis and increasing KSHV replication. Therefore, we evaluated the genetic profile of the first exon of tat gene among groups of people living with HIV (PLHIV) with (case group, n = 36) or without KS, this later with (positive control group, n = 46) and without KSHV infection (negative control group, n = 24); all individuals under antiretroviral therapy. The genetic diversity, the DN/DS ratio, and the genetic entropy of the first exon of tat were higher in the case group, followed by the positive control group, which was higher than the negative control group. The number of tat codons under positive selection was seven in the case group, six in the positive control group, and one in the negative control group. The prevalence of HIV viral loads below the detection limit was equal in the case and positive control groups, which were lower than in the negative control group. The mean CD4+ T cell counts were higher in the negative control group, followed by the positive control group, and followed by the case group. These results emphasize the negative influence of KSHV in antiretroviral treatment, as well as the HIV-specific TAT profile among PLHIV who developed KS.

Chronic HIV-1 Tat action induces HLA-DR downregulation in B cells: A mechanism for lymphoma immune escape in people living with HIV.

Despite the success of combination antiretroviral therapy, people living with human immunodeficiency virus (HIV) still have an increased risk of Epstein-Barr virus (EBV)-associated B cell malignancies. In the HIV setting, B cell physiology is altered by coexistence with HIV-infected cells and the chronic action of secreted viral proteins, for example, HIV-1 Tat that, once released, efficiently penetrates noninfected cells. We modeled the chronic action of HIV-1 Tat on B cells by ectopically expressing Tat or TatC22G mutant in two lymphoblastoid B cell lines. The RNA-sequencing analysis revealed that Tat deregulated the expression of hundreds of genes in B cells, including the downregulation of a subset of major histocompatibility complex (MHC) class II-related genes. Tat-induced downregulation of HLA-DRB1 and HLA-DRB5 genes led to a decrease in HLA-DR surface expression; this effect was reproduced by coculturing B cells with Tat-expressing T cells. Chronic Tat presence decreased the NF-ᴋB pathway activity in B cells; this downregulated NF-ᴋB-dependent transcriptional targets, including MHC class II genes. Notably, HLA-DRB1 and surface HLA-DR expression was also decreased in B cells from people with HIV. Tat-induced HLA-DR downregulation in B cells impaired EBV-specific CD4+ T cell response, which contributed to the escape from immune surveillance and could eventually promote B cell lymphomagenesis in people with HIV.