A novel mechanism of natural immune regulation of gamma herpesvirus replication

Gamma herpes viruses, including KSHV and EBV, are clinically associated with the development of various malignancies such as Kaposi's sarcoma, primary exudative lymphoma, and nasopharyngeal carcinoma. Gamma herpes viruses have a biphasic life cycle, namely the latent infection phase and the lytic replication phase. During the latent infection period, the viral genome exists in the form of an episome, expressing only a small amount of viral genes to escape from the body's immune surveillance. When the body is under pathophysiological conditions such as hypoxia, microbial co-infection, and immune deficiency, the virus can be reactivated to enter the lytic replication phase, expressing a large number of viral genes and assembling and releasing infectious offspring viruses.

The primary reason why the virus cannot be removed by the organism is that the life cycle of gamma herpes viruses consists of two distinct phases. Finding effective solutions to prevent and control viral infections requires a thorough understanding of the regulatory mechanism of the gamma herpesvirus life cycle transition, but this mechanism is extremely complex, and many questions remain unanswered. The KSHV life cycle is regulated by viral proteins such as RTA and LANA and host epigenetic modifiers, although the involvement of the host's innate immunity in this process is poorly understood.

On March 15, 2023, Hongyu Deng's team at the Institute of Biophysics, Chinese Academy of Sciences published online in PNAS a paper entitled "RNF213 modulates γ-herpesvirus infection and reactivation via targeting the viral Replication and Transcription Activator. Transcription Activator". The study reveals for the first time the molecular mechanism by which the interferon-inducible gene RNF213 acts as an E3 ubiquitin ligase to promote the ubiquitination modification and degradation of the KSHV "molecular switch" protein RTA, thereby inhibiting viral infection and lysis reactivation.

To identify the effectors of interferon regulation of KSHV cleavage replication and latent infection, researchers first constructed an expression library of interferon-stimulated genes (ISGs), and screened the model virus MHV-68, which is used to study KSHV and EBV, and identified multiple MHV-68 replication-inhibiting. Among them, RNF213 had a significant inhibitory function. The detection of key steps of MHV-68 replication revealed that RNF213 significantly inhibited early gene transcription and genome replication, and that the viral protein RTA was the "molecular switch" that initiated these downstream steps.

Further experiments confirmed that RNF213 inhibits transcriptional activation of early genes by RTA and replication of the viral genome through downregulating the expression level of RTA protein. More importantly, it was found that RNF213 could similarly downregulate the expression and inhibit the transcriptional activation function of the RTA protein encoded by KSHV, suggesting that RNF213 has the potential function of regulating latent infection and cleavage replication of KSHV. Subsequent functional experiments also confirmed that RNF213 significantly inhibited KSHV de novo infection and lytic reactivation processes.

So, how does RNF213 downregulate RTA protein expression levels? The researchers first clarified that RNF213 degrades RTA protein via the proteasome pathway rather than the lysosomal pathway and that there is a direct interaction between RNF213 and RTA. Subsequent molecular biochemical experiments demonstrated that RNF213 acts as an E3 ubiquitin ligase in a ubiquitin K48-linked manner to promote polyubiquitination modification of RTA, which in turn degrades RTA protein via the proteasome.

In summary, this work is the first to identify the function of the interferon-inducible gene RNF213 in inhibiting γ herpesvirus replication, and to reveal the molecular mechanism by which RNF213 interacts with the viral "molecular switch" protein RTA and mediates the ubiquitination modification and degradation of RTA. This work will help with the understanding of how natural immunity, especially interferon, regulates the latent infection and lytic replication of gamma herpesvirus, and provide new ideas for effective prevention and control of viral infections.

Reference: https://viroantibody.creative-biolabs.com/viral-infection-process.htm