SUN2 Silencing Impairs CD4 T Cell Proliferation and Alters Sensitivity to HIV-1 Infection Independently of Cyclophilin A

Comparative immunological landscape between pre- and early-stage LUAD manifested as ground-glass nodules revealed by scRNA and scTCR integrated analysis

BACKGROUND

Mechanism underlying the malignant progression of precancer to early-stage lung adenocarcinoma (LUAD) as well as their indolence nature remains elusive.

METHODS

Single-cell RNA sequencing (scRNA) with simultaneous T cell receptor (TCR) sequencing on 5 normal lung tissues, 3 precancerous and 4 early-stage LUAD manifested as pulmonary ground-glass nodules (GGNs) were performed.

RESULTS

Through this integrated analysis, we have delineated five key modules that drive the malignant progression of early-stage LUAD in a disease stage-dependent manner. These modules are related to cell proliferation and metabolism, immune response, mitochondria, cilia, and cell adhesion. We also find that the tumor micro-environment (TME) of early-stage LUAD manifested as GGN are featured with regulatory T (Tregs) cells accumulation with three possible origins, and loss-functional state (decreased clonal expansion and cytotoxicity) of CD8 + T cells. Instead of exhaustion, the CD8 + T cells are featured with a shift to memory phenotype, which is significantly different from the late stage LUAD. Furthermore, we have identified monocyte-derived macrophages that undergo a lipid-phenotype transition and may contribute to the suppressive TME. Intense interaction between stromal cells, myeloid cells including lipid associated macrophages and LAMP3 + DCs, and lymphocytes were also characterized.

CONCLUSIONS

Our work provides new insight into the molecular and cellular mechanism underlying malignant progression of LUAD manifested as GGN, and pave way for novel immunotherapies for GGN. Video Abstract.

Tuning immunity through tissue mechanotransduction

Immune responses are governed by signals from the tissue microenvironment, and in addition to biochemical signals, mechanical cues and forces arising from the tissue, its extracellular matrix and its constituent cells shape immune cell function. Indeed, changes in biophysical properties of tissue alter the mechanical signals experienced by cells in many disease conditions, in inflammatory states and in the context of ageing. These mechanical cues are converted into biochemical signals through the process of mechanotransduction, and multiple pathways of mechanotransduction have been identified in immune cells. Such pathways impact important cellular functions including cell activation, cytokine production, metabolism, proliferation and trafficking. Changes in tissue mechanics may also represent a new form of 'danger signal' that alerts the innate and adaptive immune systems to the possibility of injury or infection. Tissue mechanics can change temporally during an infection or inflammatory response, offering a novel layer of dynamic immune regulation. Here, we review the emerging field of mechanoimmunology, focusing on how mechanical cues at the scale of the tissue environment regulate immune cell behaviours to initiate, propagate and resolve the immune response.

The Nuclear Envelope as a Regulator of Immune Cell Function

The traditional view of the nuclear envelope (NE) was that it represented a relatively inert physical barrier within the cell, whose main purpose was to separate the nucleoplasm from the cytoplasm. However, recent research suggests that this is far from the case, with new and important cellular functions being attributed to this organelle. In this review we describe research suggesting an important contribution of the NE and its constituents in regulating the functions of cells of the innate and adaptive immune system. One of the standout properties of immune cells is their ability to migrate around the body, allowing them to carry out their physiological/pathophysiology cellular role at the appropriate location. This together with the physiological role of the tissue, changes in tissue matrix composition due to disease and aging, and the activation status of the immune cell, all result in immune cells being subjected to different mechanical forces. We report research which suggests that the NE may be an important sensor/transducer of these mechanical signals and propose that the NE is an integrator of both mechanical and chemical signals, allowing the cells of the innate immune system to precisely regulate gene transcription and functionality. By presenting this overview we hope to stimulate the interests of researchers into this often-overlooked organelle and propose it should join the ranks of mitochondria and phagosome, which are important organelles contributing to immune cell function.

SUN2 Modulates the Propagation of HSV-1

Herpesviruses assemble new viral particles in the nucleus. These nucleocapsids bud through the inner nuclear membrane to produce enveloped viral particles in the perinuclear space before fusing with the outer nuclear membrane to reach the cytoplasm. This unusual route is necessary since viral capsids are too large to pass through nuclear pores. However, the transient perinuclear nucleocapsids (250 nm in diameter) are also larger than the width of the perinuclear space (30 to 50 nm). Interestingly, linker of the nucleoskeleton and cytoskeleton (LINC) components SUN and KASH connect the inner and outer nuclear membranes and regulate their spacing. Previous work by others on the related pseudorabies virus and human cytomegalovirus showed that they functionally interact with SUN proteins. To clarify the role of SUN proteins, we explored their impact on herpes simplex virus 1 (HSV-1), another herpesvirus. Using dominant negative SUN mutants and RNA interference, we show that HSV-1 propagation is dependent on the LINC complex. In contrast to pseudorabies virus, SUN2 disruption by either approach led to increased HSV-1 extracellular viral yields. This SUN2 dependency may be linked to its greater impact on perinuclear spacing in infected cells compared to SUN1. Finally, the virus itself seems to modulate perinuclear spacing. IMPORTANCE The large size of herpesviruses prevents them from travelling across the nuclear pores, and they instead egress across the two nuclear membranes, generating short-lived enveloped perinuclear virions. This poses a challenge as the perinuclear space is smaller than the virions. This implies the separation (unzipping) of the two nuclear membranes to accommodate the viral particles. The LINC complex bridges the two nuclear membranes and is an important regulator of perinuclear spacing. Work by others hint at its functional implication during pseudorabies virus and cytomegalovirus propagation. The present study probes the importance for HSV-1 of the SUN proteins, the LINC components found in the inner nuclear membrane. Using dominant negative constructs and RNA interference (RNAi), the data reveal that SUN2 exhibits antiviral propriety toward HSV-1, as disrupting the protein leads to increased viral yields. This is in contrast with that reported for pseudorabies and suggests that differences among herpesviruses may, once again, prevail.

LRMP Associates With Immune Infiltrates and Acts as a Prognostic Biomarker in Lung Adenocarcinoma

Background: Lymphoid-restricted membrane protein (LRMP) is an endoplasmic reticulum-associated protein that is expressed in a developmentally regulated manner in both B and T cell lineages. However, the role of LRMP in the growth, prognosis and immune infiltration in lung adenocarcinoma (LUAD) remains unclear.

Method: The expression levels of LRMP mRNA in tumor and normal tissues were analyzed using Tumor Immune Estimation Resource 2.0 (TIMER 2.0) and Gene Expression Profiling Interactive Analysis 2 (GEPIA 2). LRMP protein expression was examined using the Human Protein Atlas. In vitro experiments, including qRT-PCR Western blot and immunohistochemistry staining were also performed to investigate LRMP expression. GEPIA2 and Kaplan-Meier plotter databases were used to analyze the clinical prognostic significance of LRMP. To further confirm the underlying function of LRMP, the data were analyzed using gene set enrichment analysis. Moreover, we also constructed plasmids to overexpress LRMP and explored the effect of LRMP in A549 cell line. Additionally, Tumor Immune single-cell Hub was used to investigate the distribution of LRMP in the LUAD immune microenvironment; TIMER and CIBERSORT were used to investigate the relationships among LRMP, LRMP co-expressed genes, and tumor-infiltrating immune cells; Finally, the correlations between LRMP and immune checkpoints were analyzed using TIMER 2.0.

Results: The expression of LRMP was significantly lower in LUAD tissues and cell lines. High LRMP expression is associated with a better prognosis in patients with LUAD. In vitro experimental studies demonstrated that overexpression of LRMP could decrease the proliferation, migration and invasion in A549 cells, and downregulated multiple oncogenic signaling pathways, including p-STAT3, p-PI3K-p-AKT, p-MEK and EMT pathways. GSEA results showed that immuno-related and cell adhesion pathways were enriched in samples with high LRMP expression. LRMP and its co-expressed genes were positively correlated with various tumor-infiltrating immune cells and their markers. Additionally, LRMP positively correlated with immune checkpoints.

Conclusions: Our data suggest that LRMP may act as a tumor suppressor gene and indicates a better prognosis. Moreover, LRMP is associated with immune infiltrates which may be involved in immunotherapy response in LUAD. Further studies are needed to validate these findings.

Nuclear Import of HIV-1

The delivery of the HIV-1 genome into the nucleus is an indispensable step in retroviral infection of non-dividing cells, but the mechanism of HIV-1 nuclear import has been a longstanding debate due to controversial experimental evidence. It was commonly believed that the HIV-1 capsid would need to disassemble (uncoat) in the cytosol before nuclear import because the capsid is larger than the central channel of nuclear pore complexes (NPCs); however, increasing evidence demonstrates that intact, or nearly intact, HIV-1 capsid passes through the NPC to enter the nucleus. With the protection of the capsid, the HIV-1 core completes reverse transcription in the nucleus and is translocated to the integration site. Uncoating occurs while, or after, the viral genome is released near the integration site. These independent discoveries reveal a compelling new paradigm of this important step of the HIV-1 life cycle. In this review, we summarize the recent studies related to HIV-1 nuclear import, highlighting the spatial-temporal relationship between the nuclear entry of the virus core, reverse transcription, and capsid uncoating.

Inhibition of HIV infection by structural proteins of the inner nuclear membrane is associated with reduced chromatin dynamics

The human immunodeficiency virus (HIV) enters the nucleus to establish infection, but the role of nuclear envelope proteins in this process is incompletely understood. Inner nuclear transmembrane proteins SUN1 and SUN2 connect nuclear lamins to the cytoskeleton and participate in the DNA damage response (DDR). Increased levels of SUN1 or SUN2 potently restrict HIV infection through an unresolved mechanism. Here, we find that the antiviral activities of SUN1 and SUN2 are distinct. HIV-1 and HIV-2 are preferentially inhibited by SUN1 and SUN2, respectively. We identify DNA damage inducers that stimulate HIV-1 infection and show that SUN1, but not SUN2, neutralizes this effect. Finally, we show that chromatin movements and nuclear rotations are associated with the effects of SUN proteins and Lamin A/C on infection. These results reveal an emerging role of chromatin dynamics and the DDR in the control of HIV infection by structural components of the nuclear envelope.

Nuclear restriction of HIV-1 infection by SUN1

Overexpression of the human Sad-1-Unc-84 homology protein 2 (SUN2) blocks HIV-1 infection in a capsid-dependent manner. In agreement, we showed that overexpression of SUN1 (Sad1 and UNC-84a) also blocks HIV-1 infection in a capsid-dependent manner. SUN2 and the related protein SUN1 are transmembrane proteins located in the inner membrane of the nuclear envelope. The N-terminal domains of SUN1/2 localizes to the nucleoplasm while the C-terminal domains are localized in the nuclear lamina. Because the N-terminal domains of SUN1/2 are located in the nucleoplasm, we hypothesized that SUN1/2 might be interacting with the HIV-1 replication complex in the nucleus leading to HIV-1 inhibition. Our results demonstrated that SUN1/2 interacts with the HIV-1 capsid, and in agreement with our hypothesis, the use of N-terminal deletion mutants showed that SUN1/2 proteins bind to the viral capsid by using its N-terminal domain. SUN1/2 deletion mutants correlated restriction of HIV-1 with capsid binding. Interestingly, the ability of SUN1/2 to restrict HIV-1 also correlated with perinuclear localization of these proteins. In agreement with the notion that SUN proteins interact with the HIV-1 capsid in the nucleus, we found that restriction of HIV-1 by overexpression of SUN proteins do not block the entry of the HIV-1 core into the nucleus. Our results showed that HIV-1 restriction is mediated by the interaction of SUN1/2N-terminal domains with the HIV-1 core in the nuclear compartment.

Cyclophilin A: a key player for etiological agent infection

Abstract

Cyclophilin A (CypA), a key member of the immunophilin family, is the most abundantly expressed isozyme of the 18 known human cyclophilins. Besides acting as an intracellular receptor for cyclosporine A, CypA plays a vital role in microorganismal infections, cardiovascular diseases, liver diseases, kidney diseases, neurodegeneration, cancer, rheumatoid arthritis, periodontitis, sepsis, asthma, and aging. This review focuses on the pivotal roles of CypA in the infection of etiological agents, which manifests mainly in promoting or inhibiting viral replication based on the host cell type and viral species. CypA can interact with viral proteins and thus regulate the replication cycle of the virus. CypA is involved in pathogenic bacterial infections by regulating the formation of host actin skeleton or membrane translocation of bacterial toxins, or mediated the adhesion of Mycoplasma genitalium during the infection processes by acting as a cellular receptor of M. genitalium. CypA also plays a critical role in infection or the life cycle of certain parasites or host immune regulation. Moreover, we summarized the current understanding of CypA inhibitors acting as host-targeting antiviral agents, thus opening an avenue for the treatment of multiple viral infections due to their broad antiviral effects and ability to effectively prevent drug resistance. Therefore, the antiviral effect of CypA has the potential to promote CypA inhibitors as host-targeting drugs to CypA-involved etiological agent infections and human diseases.

Key points

• CypA is involved in the replication and infection of several viruses, pathogenic bacteria, mycoplasma, and parasites.

• CypA inhibitors are in a strong position to inhibit the infection of viruses, bacterial, and mycoplasma.

Vpr and Its Cellular Interaction Partners: R We There Yet?

Vpr is a lentiviral accessory protein that is expressed late during the infection cycle and is packaged in significant quantities into virus particles through a specific interaction with the P6 domain of the viral Gag precursor. Characterization of the physiologically relevant function(s) of Vpr has been hampered by the fact that in many cell lines, deletion of Vpr does not significantly affect viral fitness. However, Vpr is critical for virus replication in primary macrophages and for viral pathogenesis in vivo. It is generally accepted that Vpr does not have a specific enzymatic activity but functions as a molecular adapter to modulate viral or cellular processes for the benefit of the virus. Indeed, many Vpr interacting factors have been described by now, and the goal of this review is to summarize our current knowledge of cellular proteins targeted by Vpr.

Enhanced gene delivery in tumor cells using chemical carriers and mechanical loadings

Intracellular delivery of DNA is considered a challenge in biological research and treatment of diseases. The previously reported transfection rate by commercially available transfection reagents in cancer cell lines, such as the mouse lung tumor cell line (TC-1), is very low. The purpose of this study is to introduce and optimize an efficient gene transfection method by mechanical approaches. The combinatory transfection effect of mechanical treatments and conventional chemical carriers is also investigated on a formerly reported hard-to-transfect cell line (TC-1). To study the effect of mechanical loadings on transfection rate, TC-1 tumor cells are subjected to uniaxial cyclic stretch, equiaxial cyclic stretch, and shear stress. The TurboFect transfection reagent is exerted for chemical transfection purposes. The pEGFP-N1 vector encoding the green fluorescent protein (GFP) expression is utilized to determine gene delivery into the cells. The results show a significant DNA delivery rate (by ~30%) in mechanically transfected cells compared to the samples that were transfected with chemical carriers. Moreover, the simultaneous treatment of TC-1 tumor cells with chemical carriers and mechanical loadings significantly increases the gene transfection rate up to ~ 63% after 24 h post-transfection. Our results suggest that the simultaneous use of mechanical loading and chemical reagent can be a promising approach in delivering cargoes into cells with low transfection potentials and lead to efficient cancer treatments.

Cyclosporine H Overcomes Innate Immune Restrictions to Improve Lentiviral Transduction and Gene Editing In Human Hematopoietic Stem Cells

Innate immune factors may restrict hematopoietic stem cell (HSC) genetic engineering and contribute to broad individual variability in gene therapy outcomes. Here, we show that HSCs harbor an early, constitutively active innate immune block to lentiviral transduction that can be efficiently overcome by cyclosporine H (CsH). CsH potently enhances gene transfer and editing in human long-term repopulating HSCs by inhibiting interferon-induced transmembrane protein 3 (IFITM3), which potently restricts VSV glycoprotein-mediated vector entry. Importantly, individual variability in endogenous IFITM3 levels correlated with permissiveness of HSCs to lentiviral transduction, suggesting that CsH treatment will be useful for improving ex vivo gene therapy and standardizing HSC transduction across patients. Overall, our work unravels the involvement of innate pathogen recognition molecules in immune blocks to gene correction in primary human HSCs and highlights how these roadblocks can be overcome to develop innovative cell and gene therapies.

SUN1 Regulates HIV-1 Nuclear Import in a Manner Dependent on the Interaction between the Viral Capsid and Cellular Cyclophilin A

Human immunodeficiency virus type 1 (HIV-1) can infect nondividing cells via passing through the nuclear pore complex. The nuclear membrane-imbedded protein SUN2 was recently reported to be involved in the nuclear import of HIV-1. Whether SUN1, which shares many functional similarities with SUN2, is involved in this process remained to be explored. Here we report that overexpression of SUN1 specifically inhibited infection by HIV-1 but not that by simian immunodeficiency virus (SIV) or murine leukemia virus (MLV). Overexpression of SUN1 did not affect reverse transcription but led to reduced accumulation of the 2-long-terminal-repeat (2-LTR) circular DNA and integrated viral DNA, suggesting a block in the process of nuclear import. HIV-1 CA was mapped as a determinant for viral sensitivity to SUN1. Treatment of SUN1-expressing cells with cyclosporine (CsA) significantly reduced the sensitivity of the virus to SUN1, and an HIV-1 mutant containing CA-G89A, which does not interact with cyclophilin A (CypA), was resistant to SUN1 overexpression. Downregulation of endogenous SUN1 inhibited the nuclear entry of the wild-type virus but not that of the G89A mutant. These results indicate that SUN1 participates in the HIV-1 nuclear entry process in a manner dependent on the interaction of CA with CypA.IMPORTANCE HIV-1 infects both dividing and nondividing cells. The viral preintegration complex (PIC) can enter the nucleus through the nuclear pore complex. It has been well known that the viral protein CA plays an important role in determining the pathways by which the PIC enters the nucleus. In addition, the interaction between CA and the cellular protein CypA has been reported to be important in the selection of nuclear entry pathways, though the underlying mechanisms are not very clear. Here we show that both SUN1 overexpression and downregulation inhibited HIV-1 nuclear entry. CA played an important role in determining the sensitivity of the virus to SUN1: the regulatory activity of SUN1 toward HIV-1 relied on the interaction between CA and CypA. These results help to explain how SUN1 is involved in the HIV-1 nuclear entry process.

SUN2 Modulates HIV-1 Infection and Latency through Association with Lamin A/C To Maintain the Repressive Chromatin

The postintegrational latency of HIV-1 is characterized by reversible silencing of long terminal repeat (LTR)-driven transcription of the HIV genome. It is known that the formation of repressive chromatin at the 5′-LTR of HIV-1 proviral DNA impedes viral transcription by blocking the recruitment of positive transcription factors. How the repressive chromatin is formed and modulated during HIV-1 infection remains elusive. Elucidation of which chromatin reassembly factor mediates the reorganization of chromatin is likely to facilitate the understanding of the host’s modulation of HIV-1 transcription and latency. Here we revealed that “Sad1 and UNC84 domain containing 2” (SUN2), an inner nuclear membrane protein, maintained the repressive chromatin and inhibited HIV LTR-driven transcription of proviral DNA through an association with lamin A/C. Specifically, lamin A/C tethered SUN2 to the nucleosomes 1 and 2 of the HIV-1 5′-LTR to block the initiation and elongation of HIV-1 transcription. SUN2 knockdown converted chromatin to an active form and thus enhanced the phosphorylation of RNA polymerase II and its recruitment to the 5′-LTR HIV-1 proviral DNA, leading to reactivation of HIV-1 from latency. Conversely, the exogenous factors such as tumor necrosis factor alpha (TNF-α) induced reactivation, and the replication of HIV-1 led to the disassociation between SUN2 and lamin A/C, suggesting that disruption of the association between SUN2 and lamin A/C to convert the repressive chromatin to the active form might be a prerequisite for the initiation of HIV-1 transcription and replication. Together, our findings indicate that SUN2 is a novel chromatin reassembly factor that helps to maintain chromatin in a repressive state and consequently inhibits HIV-1 transcription.

Despite the successful use of scores of antiretroviral drugs, HIV latency poses a major impediment to virus eradication. Elucidation of the mechanism of latency facilitates the discovery of new therapeutic strategies. It has been known that the formation of repressive chromatin at the 5′-LTR of HIV-1 proviral DNA impedes viral transcription and maintains viral latency, but how the repressive chromatin is formed and modulated during HIV-1 infection remains elusive. In this study, we performed in-depth virological and cell biological studies and discovered that an inner nuclear membrane protein, SUN2, is a novel chromatin reassembly factor that maintains repressive chromatin and thus modulates HIV-1 transcription and latency: therefore, targeting SUN2 may lead to new strategies for HIV cure.

Let me in: Control of HIV nuclear entry at the nuclear envelope

The nuclear envelope is a physical barrier that isolates the cellular DNA from the rest of the cell, thereby limiting pathogen invasion. The Human Immunodeficiency Virus (HIV) has a remarkable ability to enter the nucleus of non-dividing target cells such as lymphocytes, macrophages and dendritic cells. While this step is critical for replication of the virus, it remains one of the less understood aspects of HIV infection. Here, we review the viral and host factors that favor or inhibit HIV entry into the nucleus, including the viral capsid, integrase, the central viral DNA flap, and the host proteins CPSF6, TNPO3, Nucleoporins, SUN1, SUN2, Cyclophilin A and MX2. We review recent perspectives on the mechanism of action of these factors, and formulate fundamental questions that remain. Overall, these findings deepen our understanding of HIV nuclear import and strengthen the favorable position of nuclear HIV entry for antiviral targeting.

Are Evolution and the Intracellular Innate Immune System Key Determinants in HIV Transmission?

HIV-1 is the single most important sexually transmitted disease in humans from a global health perspective. Among human lentiviruses, HIV-1 M group has uniquely achieved pandemic levels of human-to-human transmission. The requirement to transmit between hosts likely provides the strongest selective forces on a virus, as without transmission, there can be no new infections within a host population. Our perspective is that evolution of all of the virus-host interactions, which are inherited and perpetuated from host-to-host, must be consistent with transmission. For example, CXCR4 use, which often evolves late in infection, does not favor transmission and is therefore lost when a virus transmits to a new host. Thus, transmission inevitably influences all aspects of virus biology, including interactions with the innate immune system, and dictates the biological niche in which the virus exists in the host. A viable viral niche typically does not select features that disfavor transmission. The innate immune response represents a significant selective pressure during the transmission process. In fact, all viruses must antagonize and/or evade the mechanisms of the host innate and adaptive immune systems that they encounter. We believe that viewing host-virus interactions from a transmission perspective helps us understand the mechanistic details of antiviral immunity and viral escape. This is particularly true for the innate immune system, which typically acts from the very earliest stages of the host-virus interaction, and must be bypassed to achieve successful infection. With this in mind, here we review the innate sensing of HIV, the consequent downstream signaling cascades and the viral restriction that results. The centrality of these mechanisms to host defense is illustrated by the array of countermeasures that HIV deploys to escape them, despite the coding constraint of a 10 kb genome. We consider evasion strategies in detail, in particular the role of the HIV capsid and the viral accessory proteins highlighting important unanswered questions and discussing future perspectives.

Differential Recognition of Mycobacterium tuberculosis-Specific Epitopes as a Function of Tuberculosis Disease History

RATIONALE

Individuals with a history of tuberculosis (TB) disease are at elevated risk of disease recurrence. The underlying cause is not known, but one explanation is that previous disease results in less-effective immunity against Mycobacterium tuberculosis (Mtb).

OBJECTIVES

We hypothesized that the repertoire of Mtb-derived epitopes recognized by T cells from individuals with latent Mtb infection differs as a function of previous diagnosis of active TB disease.

METHODS

T-cell responses to peptide pools in samples collected from an adult screening and an adolescent validation cohort were measured by IFN-γ enzyme-linked immunospot assay or intracellular cytokine staining.

MEASUREMENTS AND MAIN RESULTS

We identified a set of "type 2" T-cell epitopes that were recognized at 10-fold-lower levels in Mtb-infected individuals with a history of TB disease less than 6 years ago than in those without previous TB. By contrast, "type 1" epitopes were recognized equally well in individuals with or without previous TB. The differential epitope recognition was not due to differences in HLA class II binding, memory phenotypes, or gene expression in the responding T cells. Instead, "TB disease history-sensitive" type 2 epitopes were significantly (P < 0.0001) more homologous to sequences from bacteria found in the human microbiome than type 1 epitopes.

CONCLUSIONS

Preferential loss of T-cell reactivity to Mtb epitopes that are homologous to bacteria in the microbiome in persons with previous TB disease may reflect long-term effects of antibiotic TB treatment on the microbiome.

Effects of Inner Nuclear Membrane Proteins SUN1/UNC-84A and SUN2/UNC-84B on the Early Steps of HIV-1 Infection

Human immunodeficiency virus type 1 (HIV-1) infection of dividing and nondividing cells involves regulatory interactions with the nuclear pore complex (NPC), followed by translocation to the nucleus and preferential integration into genomic areas in proximity to the inner nuclear membrane (INM). To identify host proteins that may contribute to these processes, we performed an overexpression screen of known membrane-associated NE proteins. We found that the integral transmembrane proteins SUN1/UNC84A and SUN2/UNC84B are potent or modest inhibitors of HIV-1 infection, respectively, and that suppression corresponds to defects in the accumulation of viral cDNA in the nucleus. While laboratory strains (HIV-1_NL4.3 and HIV-1_IIIB) are sensitive to SUN1-mediated inhibition, the transmitted founder viruses RHPA and ZM247 are largely resistant. Using chimeric viruses, we identified the HIV-1 capsid (CA) protein as a major determinant of sensitivity to SUN1, and in vitro-assembled capsid-nucleocapsid (CANC) nanotubes captured SUN1 and SUN2 from cell lysates. Finally, we generated SUN1^−/− and SUN2^−/− cells by using CRISPR/Cas9 and found that the loss of SUN1 had no effect on HIV-1 infectivity, whereas the loss of SUN2 had a modest suppressive effect. Taken together, these observations suggest that SUN1 and SUN2 may function redundantly to modulate postentry, nuclear-associated steps of HIV-1 infection.

IMPORTANCE HIV-1 causes more than 1 million deaths per year. The life cycle of HIV-1 has been studied extensively, yet important steps that occur between viral capsid release into the cytoplasm and the expression of viral genes remain elusive. We propose here that the INM components SUN1 and SUN2, two members of the linker of nucleoskeleton and cytoskeleton (LINC) complex, may interact with incoming HIV-1 replication complexes and affect key steps of infection. While overexpression of these proteins reduces HIV-1 infection, disruption of the individual SUN2 and SUN1 genes leads to a mild reduction or no effect on infectivity, respectively. We speculate that SUN1/SUN2 may function redundantly in early HIV-1 infection steps and therefore influence HIV-1 replication and pathogenesis.

Dynamics and regulation of nuclear import and nuclear movements of HIV-1 complexes

The dynamics and regulation of HIV-1 nuclear import and its intranuclear movements after import have not been studied. To elucidate these essential HIV-1 post-entry events, we labeled viral complexes with two fluorescently tagged virion-incorporated proteins (APOBEC3F or integrase), and analyzed the HIV-1 dynamics of nuclear envelope (NE) docking, nuclear import, and intranuclear movements in living cells. We observed that HIV-1 complexes exhibit unusually long NE residence times (1.5±1.6 hrs) compared to most cellular cargos, which are imported into the nuclei within milliseconds. Furthermore, nuclear import requires HIV-1 capsid (CA) and nuclear pore protein Nup358, and results in significant loss of CA, indicating that one of the viral core uncoating steps occurs during nuclear import. Our results showed that the CA-Cyclophilin A interaction regulates the dynamics of nuclear import by delaying the time of NE docking as well as transport through the nuclear pore, but blocking reverse transcription has no effect on the kinetics of nuclear import. We also visualized the translocation of viral complexes docked at the NE into the nucleus and analyzed their nuclear movements and determined that viral complexes exhibited a brief fast phase (<9 min), followed by a long slow phase lasting several hours. A comparison of the movement of viral complexes to those of proviral transcription sites supports the hypothesis that HIV-1 complexes quickly tether to chromatin at or near their sites of integration in both wild-type cells and cells in which LEDGF/p75 was deleted using CRISPR/cas9, indicating that the tethering interactions do not require LEDGF/p75. These studies provide novel insights into the dynamics of viral complex-NE association, regulation of nuclear import, viral core uncoating, and intranuclear movements that precede integration site selection.

Although nuclear import of HIV-1 is essential for viral replication, many aspects of this process are currently unknown. Here, we defined the dynamics of HIV-1 nuclear envelope (NE) docking, nuclear import and its relationship to viral core uncoating, and intranuclear movements. We observed that HIV-1 complexes exhibit an unusually long residence time at the NE (∼1.5 hrs) compared to other cellular and viral cargos, and that HIV-1 capsid (CA) and host nuclear pore protein Nup358 are required for NE docking and nuclear import. Soon after import, the viral complexes exhibit a brief fast phase of movement, followed by a long slow phase, during which their movement is similar to that of integrated proviruses, suggesting that they rapidly become tethered to chromatin through interactions that do not require LEDGF/p75. Importantly, we found that NE association and nuclear import is regulated by the CA-cyclophilin A interaction, but not reverse transcription, and that one of the viral core uncoating steps, characterized by substantial loss of CA, occurs concurrently with nuclear import.

Capsid-Dependent Host Factors in HIV-1 Infection

After invasion of a susceptible target cell, HIV-1 completes the early phase of its life cycle upon integration of reverse-transcribed viral DNA into host chromatin. The viral capsid, a conical shell encasing the viral ribonucleoprotein complex, along with its constitutive capsid protein, plays essential roles at virtually every step in the early phase of the viral life cycle. Recent work has begun to reveal how the viral capsid interacts with specific cellular proteins to promote these processes. At the same time, cellular restriction factors target the viral capsid to thwart infection. Comprehensive understanding of capsid-host interactions that promote or impede HIV-1 infection may provide unique insight to exploit for novel therapeutic interventions.