Interaction of c-Cbl with myosin IIA regulates Bleb associated macropinocytosis of Kaposi's sarcoma-associated herpesvirus

Gulping rather than sipping: macropinocytosis as a way of virus entry

Macropinocytosis has emerged as a major endocytic mechanism in the cell entry of animal viruses. The process differs fundamentally from other endocytic mechanisms involved in virus internalization. By activating growth factor receptors or other signaling molecules, plasma membrane-bound viruses trigger the activation of a signaling pathway. When amplified, this causes a transient, global change in cell behavior. The consequences of this change include the actin-dependent formation of membrane protrusions, the elevation of non-specific uptake of fluid, and the internalization of membrane together with surface-bound ligands and particles including viruses. Recent studies show that this strategy is used by a variety of enveloped and non-enveloped viruses.

African swine fever virus uses macropinocytosis to enter host cells

African swine fever (ASF) is caused by a large and highly pathogenic DNA virus, African swine fever virus (ASFV), which provokes severe economic losses and expansion threats. Presently, no specific protection or vaccine against ASF is available, despite the high hazard that the continued occurrence of the disease in sub-Saharan Africa, the recent outbreak in the Caucasus in 2007, and the potential dissemination to neighboring countries, represents. Although virus entry is a remarkable target for the development of protection tools, knowledge of the ASFV entry mechanism is still very limited. Whereas early studies have proposed that the virus enters cells through receptor-mediated endocytosis, the specific mechanism used by ASFV remains uncertain. Here we used the ASFV virulent isolate Ba71, adapted to grow in Vero cells (Ba71V), and the virulent strain E70 to demonstrate that entry and internalization of ASFV includes most of the features of macropinocytosis. By a combination of optical and electron microscopy, we show that the virus causes cytoplasm membrane perturbation, blebbing and ruffles. We have also found that internalization of the virions depends on actin reorganization, activity of Na⁺/H⁺ exchangers, and signaling events typical of the macropinocytic mechanism of endocytosis. The entry of virus into cells appears to directly stimulate dextran uptake, actin polarization and EGFR, PI3K-Akt, Pak1 and Rac1 activation. Inhibition of these key regulators of macropinocytosis, as well as treatment with the drug EIPA, results in a considerable decrease in ASFV entry and infection. In conclusion, this study identifies for the first time the whole pathway for ASFV entry, including the key cellular factors required for the uptake of the virus and the cell signaling involved.

ASFV is a highly pathogenic zoonotic virus, which can cause severe economic losses and bioterrorism threats. No vaccine against ASFV is available so far. A strong hazard of ASFV dissemination through EU countries from Caucasian areas has recently emerged, thus making urgent to acquire knowledge and tools for protection against this virus. Despite that, our understanding of how ASFV enters host cells is very limited. A thorough understanding of this process would enable to design targeted antiviral therapies and vaccine development. The present study clearly defines key steps of ASFV cellular uptake, as well as the host factors responsible for permitting virus entry into cells. Our results indicate that the primary mechanism of ASFV uptake is a macropinocytosis-like process, that involves cellular membrane perturbation, actin polarization, activity of Na⁺/H⁺ membrane channels, and signaling proceedings typical of the macropinocytic mechanism of endocytosis, such as Rac1-Pak1 pathways, PI3K and tyrosine-kinases activation. These findings help understanding how ASFV infects cells and suggest that disturbance of macropinocytosis may be useful in the impairment of infection and vaccine development.

The ubiquitin/proteasome system mediates entry and endosomal trafficking of Kaposi's sarcoma-associated herpesvirus in endothelial cells

Ubiquitination, a post-translational modification, mediates diverse cellular functions including endocytic transport of molecules. Kaposi's sarcoma-associated herpesvirus (KSHV), an enveloped herpesvirus, enters endothelial cells primarily through clathrin-mediated endocytosis. Whether ubiquitination and proteasome activity regulates KSHV entry and endocytosis remains unknown. We showed that inhibition of proteasome activity reduced KSHV entry into endothelial cells and intracellular trafficking to nuclei, thus preventing KSHV infection of the cells. Three-dimensional (3-D) analyses revealed accumulation of KSHV particles in a cytoplasmic compartment identified as EEA1+ endosomal vesicles upon proteasome inhibition. KSHV particles are colocalized with ubiquitin-binding proteins epsin and eps15. Furthermore, ubiquitination mediates internalization of both KSHV and one of its receptors integrin β1. KSHV particles are colocalized with activated forms of the E3 ligase c-Cbl. Knock-down of c-Cbl or inhibition of its phosphorylation reduced viral entry and intracellular trafficking, resulting in decreased KSHV infectivity. These results demonstrate that ubiquitination mediates internalization of both KSHV and one of its cognate receptors integrin β1, and identify c-Cbl as a potential E3 ligase that facilitates this process.

Ubiquitination, a post-translational modification, mediates important cellular functions including endocytic transport of molecules. Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus linked to the development of Kaposi's sarcoma, an endothelial malignancy commonly found in AIDS patients, and several other malignancies. KSHV enters endothelial cells primarily through clathrin-mediated endocytosis. In this study, we show that the proteasome activity is required for KSHV entry into endothelial cells and intracellular trafficking to nuclei. Inhibition of proteasome activity reduced KSHV infectivity and led to the accumulation of KSHV particles in EEA1+ early endosomal vesicles. Furthermore, we show that ubiquitination mediates the internalization of both KSHV and one of its receptors integrin β1. KSHV particles are colocalized with ubiquitin-binding proteins epsin and eps15, as well as activated forms of the E3 ligase c-Cbl. Knock-down of c-Cbl or inhibition of its phosphorylation blocked KSHV entry and trafficking, thus preventing KSHV infection of endothelial cells. Together, these results illustrate the essential role of ubiquitination during the internalization of KSHV and its cognate receptor integrin β1. The identification of an E3 ligase that mediates the ubiquitination of KSHV and its cognate receptor integrin β1 leading to viral entry provide a potential therapeutic target for this oncogenic virus.

Nonmuscle myosin-2: mix and match

Members of the nonmuscle myosin-2 (NM-2) family of actin-based molecular motors catalyze the conversion of chemical energy into directed movement and force thereby acting as central regulatory components of the eukaryotic cytoskeleton. By cyclically interacting with adenosine triphosphate and F-actin, NM-2 isoforms promote cytoskeletal force generation in established cellular processes like cell migration, shape changes, adhesion dynamics, endo- and exo-cytosis, and cytokinesis. Novel functions of the NM-2 family members in autophagy and viral infection are emerging, making NM-2 isoforms regulators of nearly all cellular processes that require the spatiotemporal organization of cytoskeletal scaffolding. Here, we assess current views about the role of NM-2 isoforms in these activities including the tight regulation of NM-2 assembly and activation through phosphorylation and how NM-2-mediated changes in cytoskeletal dynamics and mechanics affect cell physiological functions in health and disease.

Kaposi's sarcoma-associated herpesvirus interacts with EphrinA2 receptor to amplify signaling essential for productive infection

Kaposi's sarcoma-associated herpesvirus (KSHV), etiologically associated with Kaposi's sarcoma, uses integrins (α3β1, αVβ3, and αVβ5) and associated signaling to enter human dermal microvascular endothelial cells (HMVEC-d), an in vivo target of infection. KSHV infection activated c-Cbl, which induced the selective translocation of KSHV into lipid rafts (LRs) along with the α3β1, αVβ3, and xCT receptors, but not αVβ5. LR-translocated receptors were monoubiquitinated, leading to productive macropinocytic entry, whereas non-LR-associated αVβ5 was polyubiquitinated, leading to clathrin-mediated entry that was targeted to lysosomes. Because the molecule(s) that integrate signal pathways and productive KSHV macropinocytosis were unknown, we immunoprecipitated KSHV-infected LR fractions with anti-α3β1 antibodies and analyzed them by mass spectrometry. The tyrosine kinase EphrinA2 (EphA2), implicated in many cancers, was identified in this analysis. EphA2 was activated by KSHV. EphA2 was also associated with KSHV and integrins (α3β1 and αVβ3) in LRs early during infection. Preincubation of virus with soluble EphA2, knockdown of EphA2 by shRNAs, or pretreatment of cells with anti-EphA2 monoclonal antibodies or tyrosine kinase inhibitor dasatinib significantly reduced KSHV entry and gene expression. EphA2 associates with c-Cbl-myosin IIA and augmented KSHV-induced Src and PI3-K signals in LRs, leading to bleb formation and macropinocytosis of KSHV. EphA2 shRNA ablated macropinocytosis-associated signaling events, virus internalization, and productive nuclear trafficking of KSHV DNA. Taken together, these studies demonstrate that the EphA2 receptor acts as a master assembly regulator of KSHV-induced signal molecules and KSHV entry in endothelial cells and suggest that the EphA2 receptor is an attractive target for controlling KSHV infection.

Kaposi's Sarcoma Associated Herpesvirus Entry into Target Cells

Herpesvirus infection of target cells is a complex process involving multiple host cell surface molecules (receptors) and multiple viral envelope glycoproteins. Kaposi's sarcoma associated herpesvirus (KSHV or HHV-8) infects a variety of in vivo target cells such as endothelial cells, B cells, monocytes, epithelial cells, and keratinocytes. KSHV also infects a diversity of in vitro target cells and establishes in vitro latency in many of these cell types. KSHV interactions with the host cell surface molecules and its mode of entry in the various target cells are critical for the understanding of KSHV pathogenesis. KSHV is the first herpesvirus shown to interact with adherent target cell integrins and this interaction initiates the host cell pre-existing signal pathways that are utilized for successful infection. This chapter discusses the various aspects of the early stage of KSHV infection of target cells, receptors used and issues that need to be clarified, and future directions. The various signaling events triggered by KSHV infection and the potential role of signaling events in the different stages of infection are summarized providing the framework and starting point for further detailed studies essential to fully comprehend the pathogenesis of KSHV.

The cell biology of receptor-mediated virus entry

The cell imposes multiple barriers to virus entry. However, viruses exploit fundamental cellular processes to gain entry to cells and deliver their genetic cargo. Virus entry pathways are largely defined by the interactions between virus particles and their receptors at the cell surface. These interactions determine the mechanisms of virus attachment, uptake, intracellular trafficking, and, ultimately, penetration to the cytosol. Elucidating the complex interplay between viruses and their receptors is necessary for a full understanding of how these remarkable agents invade their cellular hosts.

c-Cbl-mediated selective virus-receptor translocations into lipid rafts regulate productive Kaposi's sarcoma-associated herpesvirus infection in endothelial cells

During target cell entry and infection, many enveloped and nonenveloped viruses utilize cell surface receptors that translocate into lipid rafts (LRs). However, the mechanism behind this translocation is not known. Kaposi's sarcoma-associated herpesvirus (KSHV) interacts with the human microvascular dermal endothelial (HMVEC-d) cell surface heparan sulfate (HS), integrins α3β1, αVβ3, and αVβ5, and the amino acid transporter x-CT protein and enters via c-Cbl-bleb-mediated macropinocytosis (Veettil et al., J. Virol. 82:12126-12144, 2008; Veettil et al., PLoS Pathog. 6:e1001238, 2010). Here we have demonstrated that very early during infection (1 min postinfection), c-Cbl induced the selective translocation of KSHV into the LR along with the α3β1, αVβ3, and x-CT receptors but not αVβ5. Activated c-Cbl localized with LRs at the junctional base of macropinocytic blebs. LR-translocated α3β1 and αVβ3 were monoubiquitinated, leading to productive macropinocytic entry, whereas non-LR-associated αVβ5 was polyubiquitinated, leading to clathrin entry that was targeted to lysosomes. c-Cbl knockdown blocked the macropinocytosis and receptor translocation and diverted KSHV to a clathrin-lysosomal noninfectious pathway. Similar results were also seen by LR disruption with MβCD. These studies provide the first evidence that c-Cbl regulates selective KSHV-α3β1, -αVβ3, and -x-CT receptor translocations into the LRs and differential ubiquitination of receptors which are critical determinants of the macropinocytic entry route and productive infection of KSHV. Our studies suggest that interventions targeting c-Cbl and LRs are potential avenues to block KSHV infection of endothelial cells.

Actin' up: herpesvirus interactions with Rho GTPase signaling

Herpesviruses constitute a very large and diverse family of DNA viruses, which can generally be subdivided in alpha-, beta- and gammaherpesvirus subfamilies. Increasing evidence indicates that many herpesviruses interact with cytoskeleton-regulating Rho GTPase signaling pathways during different phases of their replication cycle. Because of the large differences between herpesvirus subfamilies, the molecular mechanisms and specific consequences of individual herpesvirus interactions with Rho GTPase signaling may differ. However, some evolutionary distinct but similar general effects on Rho GTPase signaling and the cytoskeleton have also been reported. Examples of these include Rho GTPase-mediated nuclear translocation of virus during entry in a host cell and Rho GTPase-mediated viral cell-to-cell spread during later stages of infection. The current review gives an overview of both general and individual interactions of herpesviruses with Rho GTPase signaling.