The HIV-Tat protein induces chromosome number aberrations by affecting mitosis

Drosophila as a Model for Human Viral Neuroinfections

The study of human neurological infection faces many technical and ethical challenges. While not as common as mammalian models, the use of Drosophila (fruit fly) in the investigation of virus–host dynamics is a powerful research tool. In this review, we focus on the benefits and caveats of using Drosophila as a model for neurological infections and neuroimmunity. Through the examination of in vitro, in vivo and transgenic systems, we highlight select examples to illustrate the use of flies for the study of exogenous and endogenous viruses associated with neurological disease. In each case, phenotypes in Drosophila are compared to those in human conditions. In addition, we discuss antiviral drug screening in flies and how investigating virus–host interactions may lead to novel antiviral drug targets. Together, we highlight standardized and reproducible readouts of fly behaviour, motor function and neurodegeneration that permit an accurate assessment of neurological outcomes for the study of viral infection in fly models. Adoption of Drosophila as a valuable model system for neurological infections has and will continue to guide the discovery of many novel virus–host interactions.

The potential use of Drosophila as an in vivo model organism for COVID-19-related research: a review

The world urgently needs effective antiviral approaches against emerging viruses, as shown by the coronavirus disease 2019 (COVID-19) pandemic, which has become an exponentially growing health crisis. Scientists from diverse backgrounds have directed their efforts towards identifying key features of SARS-CoV-2 and clinical manifestations of COVID-19 infection. Reports of more transmissible variants of SARS-CoV-2 also raise concerns over the possibility of an explosive trajectory of the pandemic, so scientific attention should focus on developing new weapons to help win the fight against coronaviruses that may undergo further mutations in the future. Drosophila melanogaster offers a powerful and potential in vivo model that can significantly increase the efficiency of drug screening for viral and bacterial infections. Thanks to its genes with functional human homologs, Drosophila could play a significant role in such gene-editing studies geared towards designing vaccines and antiviral drugs for COVID-19. It can also help rectify current drawbacks of CRISPR-based therapeutics like off-target effects and delivery issues, representing another momentous step forward in healthcare. Here I present an overview of recent literature and the current state of knowledge, explaining how it can open up new avenues for Drosophila in our battle against infectious diseases.

Drosophila as a Model for Infectious Diseases

The fruit fly, Drosophila melanogaster, has been used to understand fundamental principles of genetics and biology for over a century. Drosophila is now also considered an essential tool to study mechanisms underlying numerous human genetic diseases. In this review, we will discuss how flies can be used to deepen our knowledge of infectious disease mechanisms in vivo. Flies make effective and applicable models for studying host-pathogen interactions thanks to their highly conserved innate immune systems and cellular processes commonly hijacked by pathogens. Drosophila researchers also possess the most powerful, rapid, and versatile tools for genetic manipulation in multicellular organisms. This allows for robust experiments in which specific pathogenic proteins can be expressed either one at a time or in conjunction with each other to dissect the molecular functions of each virulent factor in a cell-type-specific manner. Well documented phenotypes allow large genetic and pharmacological screens to be performed with relative ease using huge collections of mutant and transgenic strains that are publicly available. These factors combine to make Drosophila a powerful tool for dissecting out host-pathogen interactions as well as a tool to better understand how we can treat infectious diseases that pose risks to public health, including COVID-19, caused by SARS-CoV-2.

Effect of HIV-1 Tat on the formation of the mitotic spindle by interaction with ribosomal protein S3

Human immunodeficiency virus type 1 (HIV-1) Tat, an important regulator of viral transcription, interacts with diverse cellular proteins and promotes or inhibits cell proliferation. Here, we show that ribosomal protein S3 (RPS3) plays an important role in mitosis through an interaction with α-tubulin and that Tat binds to and inhibits the localization of RPS3 in the mitotic spindle during mitosis. RPS3 colocalized with α-tubulin around chromosomes in the mitotic spindle. Depletion of RPS3 promoted α-tubulin assembly, while overexpression of RPS3 impaired α-tubulin assembly. Depletion of RPS3 resulted in aberrant mitotic spindle formation, segregation failure, and defective abscission. Moreover, ectopic expression of RPS3 rescued the cell proliferation defect in RPS3-knockdown cells. HIV-1 Tat interacted with RPS3 through its basic domain and increased the level of RPS3 in the nucleus. Expression of Tat caused defects in mitotic spindle formation and chromosome assembly in mitosis. Moreover, the localization of RPS3 in the mitotic spindle was disrupted when HIV-1 Tat was expressed in HeLa and Jurkat cells. These results suggest that Tat inhibits cell proliferation via an interaction with RPS3 and thereby disrupts mitotic spindle formation during HIV-1 infection. These results might provide insight into the mechanism underlying lymphocyte pathogenesis during HIV-1 infection.

Non-canonical function of Tat in regulating host microtubule dynamics: Implications for the pathogenesis of lentiviral infections

Lentiviruses are a class of genetically unique retroviruses that share similar features, despite their wide variety of host species. Transactivator of transcription (Tat) proteins of lentiviruses are critical for the regulation of viral transcription and replication. Recent studies demonstrate that in addition to mediating transactivation, Tat binds to the microtubule cytoskeleton of the host cell and interferes with microtubule dynamics, ultimately triggering apoptosis. This non-canonical function of Tat appears to be critical for the pathogenesis of lentiviral diseases, such as acquired immunodeficiency syndrome. Here, we compare the structure and activity of Tat proteins from three different types of lentiviruses, focusing on the roles of these proteins in the alteration of host microtubule dynamics and induction of apoptosis. We propose that further investigation of the Tat-microtubule interaction will provide important insight into the process of lentiviral pathogenesis and elucidate new avenues for the development of antiviral therapies.

Modulation of Eg5 activity contributes to mitotic spindle checkpoint activation and Tat-mediated apoptosis in CD4-positive T-lymphocytes

Tat, the transactivation factor of human immunodeficiency virus type 1 (HIV-1), represents one of the major players mediating the loss of CD4-positive T-lymphocytes in HIV-1-infected patients, primarily due to the ability of Tat to trigger apoptosis. However, the molecular events underlying this process remain elusive. In this study, we provide evidence that Tat interacts with Eg5, a microtubule-associated motor protein, and allosterically modulates the ATPase activity of Eg5 by affecting ADP release from the enzyme's active centre. This action of Tat impairs the formation of the mitotic spindle and activates the spindle checkpoint, thereby blocking cell cycle progression at mitosis and leading to apoptosis. Further studies reveal that lysine 85 in the carboxyl terminus of Tat is critical for its interaction with Eg5 and hence its effects on Eg5 activity, mitotic progression, and apoptosis. These findings identify Tat as a viral regulator of Eg5 and provide novel insights into the mechanisms of action of Tat in mediating the reduction of CD4-positive T-lymphocytes.

The ins and outs of HIV-1 Tat

HIV-1 encodes for the small basic protein Tat (86-101 residues) that drastically enhances the efficiency of viral transcription. The mechanism enabling Tat nuclear import is not yet clear, but studies using reporter proteins fused to the Tat basic domain indicate that Tat could reach the nucleus by passive diffusion. Tat also uses an unusual transcellular transport pathway. The first step of this pathway involves high-affinity binding of Tat to phosphatidylinositol (4,5) bisphosphate (PI(4,5)P(2)), a phospholipid that is concentrated in the inner leaflet of the plasma membrane and enables Tat recruitment at this level. Tat then crosses the plasma membrane to reach the outside medium. Although unconventional, Tat secretion by infected cells is highly active, and export is the major destination for HIV-1 Tat. Secreted Tat can bind to a variety of cell types using several different receptors. Most of them will allow Tat endocytosis. Upon internalization, low endosomal pH triggers a conformational change in Tat that results in membrane insertion. Later steps of Tat translocation to the target-cell cytosol are assisted by Hsp90, a general cytosolic chaperone. Cytosolic Tat can trigger various cell responses. Indeed, accumulating evidence suggests that extracellular Tat acts as a viral toxin that affects the biological activity of different cell types and has a key role in acquired immune-deficiency syndrome development. This review focuses on some of the recently identified molecular details underlying the unusual transcellular transport pathway used by Tat, such as the role of the single Trp in Tat for its membrane insertion and translocation.

The HIV Tat protein affects processing of ribosomal RNA precursor

Background

Inside the cell, the HIV Tat protein is mainly found in the nucleus and nucleolus. The nucleolus, the site of ribosome biogenesis, is a highly organized, non-membrane-bound sub-compartment where proteins with a high affinity for nucleolar components are found. While it is well known that Tat accumulates in the nucleolus via a specific nucleolar targeting sequence, its function in this compartment it still unknown.

Results

To clarify the significance of the Tat nucleolar localization, we induced the expression of the protein during oogenesis in Drosophila melanogaster strain transgenic for HIV-tat gene. Here we show that Tat localizes in the nucleoli of Drosophila oocyte nurse cells, where it specifically co-localizes with fibrillarin. Tat expression is accompanied by a significant decrease of cytoplasmic ribosomes, which is apparently related to an impairment of ribosomal rRNA precursor processing. Such an event is accounted for by the interaction of Tat with fibrillarin and U3 snoRNA, which are both required for pre-rRNA maturation.

Conclusion

Our data contribute to understanding the function of Tat in the nucleolus, where ribosomal RNA synthesis and cell cycle control take place. The impairment of nucleolar pre-rRNA maturation through the interaction of Tat with fibrillarin-U3snoRNA complex suggests a process by which the virus modulates host response, thus contributing to apoptosis and protein shut-off in HIV-uninfected cells.

The SARS-Coronavirus Membrane protein induces apoptosis through modulating the Akt survival pathway

A number of viral gene products are capable of triggering apoptotic cell death through interfering with cellular signaling cascades, including the Akt kinase pathway. In this study, the pro-apoptotic role of the SARS-CoV Membrane (M) structural protein is described. We found that the SARS-CoV M protein induced apoptosis in both HEK293T cells and transgenic Drosophila. We further showed that M protein-induced apoptosis involved mitochondrial release of cytochrome c protein, and could be suppressed by caspase inhibitors. Over-expression of M caused a dominant rough-eye phenotype in adult Drosophila. By performing a forward genetic modifier screen, we identified phosphoinositide-dependent kinase-1 (PDK-1) as a dominant suppressor of M-induced apoptotic cell death. Both PDK-1 and Akt kinases play essential roles in the cell survival signaling pathway. Altogether, our data show that SARS-CoV M protein induces apoptosis through the modulation of the cellular Akt pro-survival pathway and mitochondrial cytochrome c release.

In vivo functional characterization of the SARS-Coronavirus 3a protein in Drosophila

The Severe Acute Respiratory Syndrome-Coronavirus (SARS-CoV) 3a locus encodes a 274 a.a. novel protein, and its expression has been confirmed in SARS patients. To study functional roles of 3a, we established a transgenic fly model for the SARS-CoV 3a gene. Misexpression of 3a in Drosophila caused a dominant rough eye phenotype. Using a specific monoclonal antibody, we demonstrated that the 3a protein displayed a punctate cytoplasmic localization in Drosophila as in SARS-CoV-infected cells. We provide genetic evidence to support that 3a is functionally related to clathrin-mediated endocytosis. We further found that 3a misexpression induces apoptosis, which could be modulated by cellular cytochrome c levels and caspase activity. From a forward genetic screen, 78 dominant 3a modifying loci were recovered and the identity of these modifiers revealed that the severity of the 3a-induced rough eye phenotype depends on multiple cellular processes including gene transcriptional regulation.