Reactivation and Lytic Replication of Kaposi's Sarcoma-Associated Herpesvirus: An Update

Abstract

The life cycle of Kaposi's sarcoma-associated herpesvirus (KSHV) consists of two phases, latent and lytic. The virus establishes latency as a strategy for avoiding host immune surveillance and fusing symbiotically with the host for lifetime persistent infection. However, latency can be disrupted and KSHV is reactivated for entry into the lytic replication. Viral lytic replication is crucial for efficient dissemination from its long-term reservoir to the sites of disease and for the spread of the virus to new hosts. The balance of these two phases in the KSHV life cycle is important for both the virus and the host and control of the switch between these two phases is extremely complex. Various environmental factors such as oxidative stress, hypoxia, and certain chemicals have been shown to switch KSHV from latency to lytic reactivation. Immunosuppression, unbalanced inflammatory cytokines, and other viral co-infections also lead to the reactivation of KSHV. This review article summarizes the current understanding of the initiation and regulation of KSHV reactivation and the mechanisms underlying the process of viral lytic replication. In particular, the central role of an immediate-early gene product RTA in KSHV reactivation has been extensively investigated. These studies revealed multiple layers of regulation in activation of RTA as well as the multifunctional roles of RTA in the lytic replication cascade. Epigenetic regulation is known as a critical layer of control for the switch of KSHV between latency and lytic replication. The viral non-coding RNA, PAN, was demonstrated to play a central role in the epigenetic regulation by serving as a guide RNA that brought chromatin remodeling enzymes to the promoters of RTA and other lytic genes. In addition, a novel dimension of regulation by microPeptides emerged and has been shown to regulate RTA expression at the protein level. Overall, extensive investigation of KSHV reactivation and lytic replication has revealed a sophisticated regulation network that controls the important events in KSHV life cycle.

Keywords: Kaposi’s sarcoma-associated herpesvirus (KSHV); Rta; human herpesvirus 8 (HHV-8); lytic replication; viral reactivation.

FIGURE 1

The central role of RTA in reactivation of Kaposi’s sarcoma-associated herpesvirus (KSHV) from latency leading to lytic cycle. Different types of signaling pathways and several physiological factors including hypoxia, oxidative stress, and reactive oxygen species (ROS) can disrupt KSHV latency and reactivate the virus by activating the RTA promoter. RTA activation sets up the cascade of gene expression leading to viral lytic replication and virion assembly. RTA auto-regulates its expression at the transcriptional level by using cellular RBP-Jκ notch signaling pathway repressor and by Oct-1, as well as at post-translational level by self-ubiquitylation. The microPeptide vSP-1 blocks the self-ubiquitylation of RTA and promotes viral lytic replication. Green arrows represent activation and black arrows show inhibition.

FIGURE 2

Schematic representation of RTA and functional domains. The RTA nuclear localization signals (NLS), RING finger-like domain, protein abundant regulatory signal (PARS), and transactivation domain are shown with color codes as indicated. RTA undergoes auto-ubiquitylation by the E3 ligase activity of the ring finger-like domain for a self-control of its abundance. A microPeptide (vSP1) interacts with PARS II domain and lifts the abundance restriction by blocking RTA self-ubiquitylation.

FIGURE 3

Chromatin remodeling of KSHV RTA promoter region during latency and lytic reactivation. During latency, the chromatin of the immediate-early (IE) promoter region contains bivalent histone marks including both activating acH3, H3K4me3 and repressive H3K27me3. Polycomb repressive complex 2 (PRC2) colocalizes with H3K27me3 on RTA and PRC1 complex generates H2AK119Ub2 to repress RTA transcription. When the virus enters lytic life cycle, PRC2 dissociates from the genomic regions of IE and delayed-early (DE) genes. PAN RNA recruits histone demethylases UTX and JMJD3 and histone-lysine N-methtltransferase 2D (KMT2D or MLL2) to the chromatin. As a result, the decrease in H3K27me3 and increase in H3K4me3 and acH3 results in activation of the IE and DE promoters and lytic gene expression.

FIGURE 4

Structure of the KSHV origin of DNA replication (ori-Lyt) and formation of viral replication initiation complex formation. (A) Ori-Lyt-R and ori-Lyt-L are superimposed to show their commonalities. The positions of various characteristic motifs (TATA boxes, C/EBP binding motifs, AT palindrome, RRE and GC tandem repeats) are as indicated. The homologies of subregions between two ori-Lyts are compared and shown on the bottom. (B) Model for formation of pre-replication complex and replication initiation complex on KSHV ori-Lyt. Six core replication proteins form pre-replication complex. The pre-replication complex is then loaded at a KSHV ori-Lyt by a two-point-contact through RTA and K8, each of which interacts with their binding motifs in the ori-Lyt. The interaction may lead to looping and distortion of the ori-Lyt DNA. Furthermore, some cellular proteins are also recruited to the complexes. RecQL is likely to be a component of pre-replication complex and recruited to ori-Lyt together with viral core replication proteins in the complex through K8 and RTA. The loading of the pre-replication complex on ori-Lyt may cause structural changes of ori-Lyt DNA that facilitates the recruiting of more cellular proteins, including MSH2/6 and DNA-PK/Ku86/70, to the ori-Lyt. Scaffold attachment factor A (SAF-A) binds directly to ori-Lyt DNA and may tether the ori-Lyt DNA to the nuclear scaffold or matrix for efficient DNA replication (Modified from Wang Y. et al., 2004).

FIGURE 5

Model for role of ORF45 in lipid raft (LR)-localization and KSHV final envelopment. After budding into the cytoplasm, nucleocapsids gain tegument in the cytoplasm. Tegument protein ORF45 directs tegumented capsid targeting LR for viral assembly in Golgi complex, and budding through Golgi-derived vesicles. LRs serve as a platform for KSHV assembly. Mutation in ORF45 (K297R) results in immature virion particles that fail to target LR, but are degraded in lysosomes (Adapted from Wang et al., 2015).