Kaposi's Sarcoma Associated Herpesvirus Entry into Target Cells

Abstract

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.

Keywords: KSHV; endocytosis; entry; integrins; receptors; signaling; tropism.

Figure 1

Schematic model depicting the different overlapping phases of KSHV entry and infection in endothelial (HMVEC-d) target cell. KSHV infection is initiated by binding to the cell surface proteoglycan heparan sulfate [HS; Stage 1: binding; inset electron microscopic, (EM) picture], followed by subsequent temporal associations with integrins (α3β1, αVβ3, αVβ5) and xCT molecules in the non-lipid raft (NLR) parts of the membranes. KSHV’s interactions with integrins activate FAK at tyrosine 397, which creates a binding site for the SH2 domain containing Src family kinases, subsequently leading to the activation of PI3-K and Rho-GTPases (Stage 2: signal induction). These rapid overlapping host cell signal induction play roles in actin modulation, formation of endocytic vesicles, and virus entry and trafficking through the cytosol leading into a productive infection (Blue arrows). KSHV infection induces the phosphorylation of c-Cbl and the phosphorylated c-Cbl forms a complex with p85-PI3-K, leading into the interaction of c-Cbl with downstream molecules. c-Cbl mediates a rapid selective translocations of KSHV into the lipid rafts (LRs) along with the α3β1, αVβ3, and xCT receptors. KSHV-associated αVβ5 remains in the NLR parts of the membranes. Activated c-Cbl localizes with LRs, associates with myosin IIA and actin, and is rapidly recruited to membrane blebs. This also leads into c-Cbl mediated ubiquitination of actin and myosin. c-Cbl mediated monoubiquitination of translocated receptors is followed by productive macropinocytic entry. Myosin IIA interactions with actin may be providing the ATP-dependent force to generate actomyosin contraction, bleb retraction to form macropinosomes along with KSHV (Stage 3: internalization; inset EM picture). NLR associated KSHV bound αVβ5 and other receptors are polyubiquitinated and directed to a clathrin-dependant lysosomal non-productive pathway. RhoA activates Dia-2 aiding in formation and movement of endosomes through the cytoplasm. KSHV capsid is released from the endocytic vesicles by fusion of viral envelope with endosomal vesicles (Stage 3: internalization; inset EM picture). Released capsids are transported toward the nucleus utilizing a Rho-GTPase dependant pathway that involves acetylation of microtubules (MT; Stage 4: movement in cytoplasm). KSHV capsid disassembly at or near the nuclear pore results in the delivery of KSHV DNA into the infected cell nucleus (Stage 5: nuclear delivery) followed by viral and host gene expression (Stage 6) initiated by KSHV binding and entry induced ERK and NF-κB pathways. All these events demonstrate that KSHV has evolved to utilize its interactions with cellular receptors to manipulate host cell signaling and to induce an environment that is conducive for a productive infection.