Human immunodeficiency virus type 1 Vpr induces cell cycle G2 arrest through Srk1/MK2-mediated phosphorylation of Cdc25

Understanding the molecular mechanisms of human diseases: the benefits of fission yeasts

The role of model organisms such as yeasts in life science research is crucial. Although the baker's yeast (Saccharomyces cerevisiae) is the most popular model among yeasts, the contribution of the fission yeasts (Schizosaccharomyces) to life science is also indisputable. Since both types of yeasts share several thousands of common orthologous genes with humans, they provide a simple research platform to investigate many fundamental molecular mechanisms and functions, thereby contributing to the understanding of the background of human diseases. In this review, we would like to highlight the many advantages of fission yeasts over budding yeasts. The usefulness of fission yeasts in virus research is shown as an example, presenting the most important research results related to the Human Immunodeficiency Virus Type 1 (HIV-1) Vpr protein. Besides, the potential role of fission yeasts in the study of prion biology is also discussed. Furthermore, we are keen to promote the uprising model yeast Schizosaccharomyces japonicus, which is a dimorphic species in the fission yeast genus. We propose the hyphal growth of S. japonicus as an unusual opportunity as a model to study the invadopodia of human cancer cells since the two seemingly different cell types can be compared along fundamental features. Here we also collect the latest laboratory protocols and bioinformatics tools for the fission yeasts to highlight the many possibilities available to the research community. In addition, we present several limiting factors that everyone should be aware of when working with yeast models.

Mechanism of cell cycle regulation and cell proliferation during human viral infection

Over the history of the coevolution of Host viral interaction, viruses have customized the host cellular machinery into their use for viral genome replication, causing effective infection and ultimately aiming for survival. They do so by inducing subversions to the host cellular pathways like cell cycle via dysregulation of important cell cycle checkpoints by viral encoded proteins, arresting the cell cycle machinery, blocking cytokinesis as well as targeting subnuclear bodies, thus ultimately disorienting the cell proliferation. Both DNA and RNA viruses have been active participants in such manipulation resulting in serious outcomes of cancer. They achieve this by employing different mechanisms-Protein-protein interaction, protein-phosphorylation, degradation, redistribution, viral homolog, and viral regulation of APC at different stages of cell cycle events. Several DNA viruses cause the quiescent staged cells to undergo cell cycle which increases nucleotide pools logistically significantly persuading viral replication whereas few other viruses arrest a particular stage of cell cycle. This allows the latter group to sustain the infection which allows them to escape host immune response and support viral multiplication. Mechanical study of signaling such viral mediated pathways could give insight into understanding the etiology of tumorigenesis and progression. Overall this chapter highlights the possible strategies employed by DNA/RNA viral families which impact the normal cell cycle but facilitate viral infected cell replication. Such information could contribute to comprehending viral infection-associated disorders to further depth.

Contribution of yeast models to virus research

Abstract

Time and again, yeast has proven to be a vital model system to understand various crucial basic biology questions. Studies related to viruses are no exception to this. This simple eukaryotic organism is an invaluable model for studying fundamental cellular processes altered in the host cell due to viral infection or expression of viral proteins. Mechanisms of infection of several RNA and relatively few DNA viruses have been studied in yeast to date. Yeast is used for studying several aspects related to the replication of a virus, such as localization of viral proteins, interaction with host proteins, cellular effects on the host, etc. The development of novel techniques based on high-throughput analysis of libraries, availability of toolboxes for genetic manipulation, and a compact genome makes yeast a good choice for such studies. In this review, we provide an overview of the studies that have used yeast as a model system and have advanced our understanding of several important viruses.

Key points

• Yeast, a simple eukaryote, is an important model organism for studies related to viruses.

• Several aspects of both DNA and RNA viruses of plants and animals are investigated using the yeast model.

• Apart from the insights obtained on virus biology, yeast is also extensively used for antiviral development.

Vpr Is a VIP: HIV Vpr and Infected Macrophages Promote Viral Pathogenesis

HIV infects several cell types in the body, including CD4+ T cells and macrophages. Here we review the role of macrophages in HIV infection and describe complex interactions between viral proteins and host defenses in these cells. Macrophages exist in many forms throughout the body, where they play numerous roles in healthy and diseased states. They express pattern-recognition receptors (PRRs) that bind viral, bacterial, fungal, and parasitic pathogens, making them both a key player in innate immunity and a potential target of infection by pathogens, including HIV. Among these PRRs is mannose receptor, a macrophage-specific protein that binds oligosaccharides, restricts HIV replication, and is downregulated by the HIV accessory protein Vpr. Vpr significantly enhances infection in vivo, but the mechanism by which this occurs is controversial. It is well established that Vpr alters the expression of numerous host proteins by using its co-factor DCAF1, a component of the DCAF1-DDB1-CUL4 ubiquitin ligase complex. The host proteins targeted by Vpr and their role in viral replication are described in detail. We also discuss the structure and function of the viral protein Env, which is stabilized by Vpr in macrophages. Overall, this literature review provides an updated understanding of the contributions of macrophages and Vpr to HIV pathogenesis.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

FPHPB inhibits gastric tumor cell proliferation by inducing G2-M cell cycle arrest

Gastric cancer is a common cancer in the world with high morbidity and mortality. Here, we report that FPHPB (4-(4-(2-fluoropyridin-3-yl)phenyl)-N-(4-hydroxyphenyl)), a derivative of CMPD-1/MK2a Inhibitor, had anti-tumor activities by inhibiting gastric tumor SNU-16 and SGC7901 cells. FPHPB dose-dependently inhibited cell proliferation, induced cell apoptosis and arrested SNU-16 and SGC7901 cells in G2-M cell cycle checkpoint. Upon treatment with FPHPB, apoptotic proteins cleaved PARP and cleaved caspase-3 were remarkably increased, and G2-M regulatory molecules, the phosphorylation of Cdc2 and Chk2, were significantly accentuated. Collectively, FPHPB has anti-tumor activities and may be a potential candidate for treating gastric cancers.

Molecular Cloning and Characterization of Small Viral Genome in Fission Yeast

Fission yeast is a single-cell eukaryote that has been used extensively as a model organism to study cell biology and virology of higher eukaryotes including plants and humans. In particular, it is a very well-tested model to study evolutionary highly conserved cellular activities such as cell proliferation, cell cycle regulation, and cell death. Some of the advantages of using fission yeast as a surrogate system: easy to carry out functional and genome-wide analysis of small viral genome, easy to maintain in the laboratory with a relatively short doubling time. It is genetically amendable and can be used to test the effect of gain-of-function or loss-of-function of a gene product. Here, we describe a streamlined and large-scale molecular cloning strategy for genome-wide characterization of small viruses in fission yeast.

Yeast for virus research

Budding yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are two popular model organisms for virus research. They are natural hosts for viruses as they carry their own indigenous viruses. Both yeasts have been used for studies of plant, animal and human viruses. Many positive sense (+) RNA viruses and some DNA viruses replicate with various levels in yeasts, thus allowing study of those viral activities during viral life cycle. Yeasts are single cell eukaryotic organisms. Hence, many of the fundamental cellular functions such as cell cycle regulation or programed cell death are highly conserved from yeasts to higher eukaryotes. Therefore, they are particularly suited to study the impact of those viral activities on related cellular activities during virus-host interactions. Yeasts present many unique advantages in virus research over high eukaryotes. Yeast cells are easy to maintain in the laboratory with relative short doubling time. They are non-biohazardous, genetically amendable with small genomes that permit genome-wide analysis of virologic and cellular functions. In this review, similarities and differences of these two yeasts are described. Studies of virologic activities such as viral translation, viral replication and genome-wide study of virus-cell interactions in yeasts are highlighted. Impacts of viral proteins on basic cellular functions such as cell cycle regulation and programed cell death are discussed. Potential applications of using yeasts as hosts to carry out functional analysis of small viral genome and to develop high throughput drug screening platform for the discovery of antiviral drugs are presented.

HIV-1 Protease in the Fission Yeast Schizosaccharomyces pombe

BACKGROUND

HIV-1 protease (PR) is an essential viral enzyme. Its primary function is to proteolyze the viral Gag-Pol polyprotein for production of viral enzymes and structural proteins and for maturation of infectious viral particles. Increasing evidence suggests that PR cleaves host cellular proteins. However, the nature of PR-host cellular protein interactions is elusive. This study aimed to develop a fission yeast (Schizosaccharomyces pombe) model system and to examine the possible interaction of HIV-1 PR with cellular proteins and its potential impact on cell proliferation and viability.

RESULTS

A fission yeast strain RE294 was created that carried a single integrated copy of the PR gene in its chromosome. The PR gene was expressed using an inducible nmt1 promoter so that PR-specific effects could be measured. HIV-1 PR from this system cleaved the same indigenous viral p6/MA protein substrate as it does in natural HIV-1 infections. HIV-1 PR expression in fission yeast cells prevented cell proliferation and induced cellular oxidative stress and changes in mitochondrial morphology that led to cell death. Both these PR activities can be prevented by a PR-specific enzymatic inhibitor, indinavir, suggesting that PR-mediated proteolytic activities and cytotoxic effects resulted from enzymatic activities of HIV-1 PR. Through genome-wide screening, a serine/threonine kinase, Hhp2, was identified that suppresses HIV-1 PR-induced protease cleavage and cell death in fission yeast and in mammalian cells, where it prevented PR-induced apoptosis and cleavage of caspase-3 and caspase-8.

CONCLUSIONS

This is the first report to show that HIV-1 protease is functional as an enzyme in fission yeast, and that it behaves in a similar manner as it does in HIV-1 infection. HIV-1 PR-induced cell death in fission yeast could potentially be used as an endpoint for mechanistic studies, and this system could be used for developing a high-throughput system for drug screenings.

Cytotoxic activity of the MK2 inhibitor CMPD1 in glioblastoma cells is independent of MK2

MAPK-activated protein kinase 2 (MK2) is a checkpoint kinase involved in the DNA damage response. MK2 inhibition enhances the efficacy of chemotherapeutic agents; however, whether MK2 inhibition alone, without concurrent chemotherapy, would attenuate survival of cancer cells has not been investigated. CMPD1 is a widely used non-ATP competitive inhibitor that prevents MK2 phosphorylation. We employed CMPD1 together with MK2 knock-down and ATP-competitive MK2 inhibitor III (MK2i) in a panel of glioblastoma cells to assess whether MK2 inhibition could induce cancer cell death. While CMPD1 was effective at selective killing of cancer cells, MK2i and MK2 knock-down had no effect on viability of glioblastoma cells. CMPD1 treatment induced a significant G2/M arrest but MK2i-treated cells were only minimally arrested at G1 phase. Intriguingly, at doses that were cytotoxic to glioblastoma cells, CMPD1 did not inhibit phosphorylation of MK2 and of its downstream substrate Hsp27. These results suggest that CMPD1 exhibits cytotoxic activity independently of MK2 inhibition. Indeed, we identified tubulin as a primary target of the CMPD1 cytotoxic activity. This study demonstrates how functional and mechanistic studies with appropriate selection of test compounds, combining genetic knock-down and pharmacological inhibition, coordinating timing and dose levels enabled us to uncover the primary target of an MK2 inhibitor commonly used in the research community. Tubulin is emerging as one of the most common non-kinase targets for kinase inhibitors and we propose that potential tubulin-targeting activity should be assessed in preclinical pharmacology studies of all novel kinase inhibitors.

Cell cycle checkpoints and pathogenesis of HIV-1 infection: a brief overview

To understand HIV pathogenesis or development is no simple undertaking and neither is the cell cycle which is highly complex that requires the coordination of multiple events and machinery. It is interesting that these two processes are interrelated, intersect and interact as HIV-1 infection results in cell cycle arrest at the G2 phase which is accompanied by massive CD4+ T cell death. For its own benefit, in an impressive manner and with the overabundance of tactics, HIV maneuvers DNA damage responses and cell cycle check points for viral replication at different stages from infection, to latency and to pathogenesis. Although the cell cycle is the most critical aspect involved in both viral and cellular replication, in this review, our main focus is on recent developments, including our own observations in the field of cell cycle proteins, checkpoints and strategies utilized by the viruses to manipulate these pathways to promote their own replication and survival. We will also discuss the emerging concept of targeting the replication initiation machinery for HIV therapy.

Cell cycle G2/M arrest through an S phase-dependent mechanism by HIV-1 viral protein R

Background

Cell cycle G2 arrest induced by HIV-1 Vpr is thought to benefit viral proliferation by providing an optimized cellular environment for viral replication and by skipping host immune responses. Even though Vpr-induced G2 arrest has been studied extensively, how Vpr triggers G2 arrest remains elusive.

Results

To examine this initiation event, we measured the Vpr effect over a single cell cycle. We found that even though Vpr stops the cell cycle at the G2/M phase, but the initiation event actually occurs in the S phase of the cell cycle. Specifically, Vpr triggers activation of Chk1 through Ser³⁴⁵ phosphorylation in an S phase-dependent manner. The S phase-dependent requirement of Chk1-Ser³⁴⁵ phosphorylation by Vpr was confirmed by siRNA gene silencing and site-directed mutagenesis. Moreover, downregulation of DNA replication licensing factors Cdt1 by siRNA significantly reduced Vpr-induced Chk1-Ser³⁴⁵ phosphorylation and G2 arrest. Even though hydroxyurea (HU) and ultraviolet light (UV) also induce Chk1-Ser³⁴⁵ phosphorylation in S phase under the same conditions, neither HU nor UV-treated cells were able to pass through S phase, whereas vpr-expressing cells completed S phase and stopped at the G2/M boundary. Furthermore, unlike HU/UV, Vpr promotes Chk1- and proteasome-mediated protein degradations of Cdc25B/C for G2 induction; in contrast, Vpr had little or no effect on Cdc25A protein degradation normally mediated by HU/UV.

Conclusions

These data suggest that Vpr induces cell cycle G2 arrest through a unique molecular mechanism that regulates host cell cycle regulation in an S-phase dependent fashion.

HIV-1 replication through hHR23A-mediated interaction of Vpr with 26S proteasome

HIV-1 Vpr is a virion-associated protein. Its activities link to viral pathogenesis and disease progression of HIV-infected patients. In vitro, Vpr moderately activates HIV-1 replication in proliferating T cells, but it is required for efficient viral infection and replication in vivo in non-dividing cells such as macrophages. How exactly Vpr contributes to viral replication remains elusive. We show here that Vpr stimulates HIV-1 replication at least in part through its interaction with hHR23A, a protein that binds to 19S subunit of the 26S proteasome and shuttles ubiquitinated proteins to the proteasome for degradation. The Vpr-proteasome interaction was initially discovered in fission yeast, where Vpr was shown to associate with Mts4 and Mts2, two 19S-associated proteins. The interaction of Vpr with the 19S subunit of the proteasome was further confirmed in mammalian cells where Vpr associates with the mammalian orthologues of fission yeast Mts4 and S5a. Consistently, depletion of hHR23A interrupts interaction of Vpr with proteasome in mammalian cells. Furthermore, Vpr promotes hHR23A-mediated protein-ubiquitination, and down-regulation of hHR23A using RNAi significantly reduced viral replication in non-proliferating MAGI-CCR5 cells and primary macrophages. These findings suggest that Vpr-proteasome interaction might counteract certain host restriction factor(s) to stimulate viral replication in non-dividing cells.

Human immunodeficiency virus type 1 Vpr: functions and molecular interactions

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) is an accessory protein that interacts with a number of cellular and viral proteins. The functions of many of these interactions in the pathogenesis of HIV-1 have been identified. Deletion of the vpr gene reduces the virulence of HIV-1 dramatically, indicating the importance of this protein for the virus. This review describes the current findings on several established functions of HIV-1 Vpr and some possible roles proposed for this protein. Because Vpr exploits cellular proteins and pathways to influence the biology of HIV-1, understanding the functions of Vpr usually involves the study of cellular pathways. Several functions of Vpr are attributed to the virion-incorporated protein, but some of them are attributed to the expression of Vpr in HIV-1-infected cells. The structure of Vpr may be key to understanding the variety of its interactions. Due to the critical role of Vpr in HIV-1 pathogenicity, study of the interactions between Vpr and cellular proteins may help us to understand the mechanism(s) of HIV-1 pathogenicity.

HIV-1 viral protein R (Vpr) and its interactions with host cell

Human immunodeficiency virus type 1 (HIV-1) is engaged in dynamic and antagonistic interactions with host cells. Once infected by HIV-1, host cells initiate various antiviral strategies, such as innate antiviral defense mechanisms, to counteract viral invasion. In contrast, the virus has different strategies to suppress these host responses to infection. The final balance between these interactions determines the outcome of the viral infection and disease progression. Recent findings suggest that HIV-1 viral protein R (Vpr) interacts with some of the host innate antiviral factors, such as heat shock proteins, and plays an active role as a viral pathogenic factor. Cellular heat stress response factors counteract Vpr activities and inhibit HIV replication. However, Vpr overcomes these heat-stress-like responses by preventing heat shock factor-1 (HSF-1)-mediated activation of heat shock proteins. In this review, we will focus on the virus-host interactions involving Vpr. In addition to heat stress response proteins, we will discuss interactions of Vpr with other proteins, such as EF2 and Skp1/GSK3, their involvements in cellular responses to Vpr, as well as strategies to develop novel antiviral therapies aimed at enhancing anti-Vpr responses of the host cell.

Kinases that control the cell cycle in response to DNA damage: Chk1, Chk2, and MK2

In response to DNA damage eukaryotic cells activate cell cycle checkpoints -- complex kinase signaling networks that prevent further progression through the cell cycle. Parallel to implementing a cell cycle arrest, checkpoint signaling also mediates the recruitment of DNA repair pathways. If the extent of damage exceeds repair capacity, additional signaling cascades are activated to ensure elimination of these damaged cells. The DNA damage response has traditionally been divided into two major kinase branches. The ATM/Chk2 module is activated after DNA double strand breaks and the ATR/Chk1 pathway responds primarily to DNA single strand breaks or bulky lesions. Both pathways converge on Cdc25, a positive regulator of cell cycle progression, which is inhibited by Chk1-mediated or Chk2-mediated phosphorylation. Recently a third effector kinase complex consisting of p38MAPK and MK2 has emerged. This pathway is activated downstream of ATM and ATR in response to DNA damage. MK2 has been shown to share substrate homology with both Chk1 and Chk2. Here we will discuss recent advances in our understanding of the eukaryotic DNA damage response with emphasis on the Chk1, Chk2, and the newly emerged effector kinases p38MAPK and MK2.