Coronavirus Nucleocapsid Protein Suppresses Innate Immune Response Through Disruption Of Nucleocytoplasmic Trafficking

COVID-19 News: Coronaviruses, notorious for their ability to evade host immune defenses, have emerged as a significant global health threat. The hallmark of coronavirus infection lies in its adept manipulation of the nucleocytoplasmic trafficking system, a crucial component for the expression of antiviral genes. In this COVID-19 News report, we delve into a new study by researchers from the Chinese Academy of Agricultural Sciences-China and Yangzhou University-China that explored the intricate details of how the infectious bronchitis virus (IBV), a representative coronavirus, strategically disrupts the nuclear translocation of key transcription factors, hindering the expression of vital antiviral genes. The findings shed light on a novel and highly effective mechanism conserved across pan-coronaviruses, involving the interaction between the coronavirus nucleocapsid protein (N) and the scaffold protein RACK1, leading to the recruitment of activated protein kinase C alpha (p-PKCα) and subsequent suppression of the innate immune response.


Coronavirus Nucleocapsid Protein Suppresses Innate Immune Response Through Disruption Of Nucleocytoplasmic Trafficking
IBV N protein alters the morphology of nuclear envelope and the ring signal of importin β1, and reduces the cytoplasmic signal of importin α1. Vero cells were transfected with either the vector PXJ40 or a plasmid encoding IBV N protein. At
24 h post-transfection, cells were harvested and subjected to immunofluorescence analysis.
 
Nucleocytoplasmic Trafficking: A Fundamental Cellular Process
The nucleocytoplasmic trafficking system, orchestrated by the nuclear pore complex (NPC), plays a pivotal role in various cellular functions, including gene transcription, RNA export, protein translation, and antiviral innate immunity. Comprising the nuclear envelope, NPC, and nuclear transport receptors, this intricate system ensures the regulated movement of molecules between the nucleus and cytoplasm. The NPC, a complex structure consisting of nucleoporins (Nups), acts as the exclusive gateway for molecules entering and leaving the nucleus. The dynamics of NPC are finely regulated by cell cycle-dependent phosphorylation events, with hyperphosphorylation of key Nups leading to NPC disintegration.
 
Key Signaling Transducers and Antiviral Gene Expression
The NPC governs the nuclear translocation of critical signaling transcription factors, such as IFN regulatory factor 3 (IRF3), signal transducer and activator of transcription 1/2/9 (STAT1/STAT2/IRF9), and nuclear factor-kappa B (NF-κB), crucial for the activation of antiviral genes. The type I interferon (IFN) signaling pathway, a central component of the antiviral innate immune response, relies on the nuclear translocation of these transcription factors. In response to infection signals, the phosphorylation and nuclear translocation of p38 mitogen-activated prote in kinase (MAPK) contribute to inflammation by regulating stress-related transcription processes. The NPC also serves as the gate for exporting mRNA from the nucleus into the cytoplasm, influencing the expression of antiviral genes.
 
Coronaviruses and Innate Immune Suppression
Coronaviruses, with their positive-sense, single-stranded RNA genomes, employ diverse strategies to suppress the host innate immune response. Our study focuses on IBV, a coronavirus causing infectious bronchitis in chickens, and reveals a shared mechanism across pan-coronaviruses to disrupt the host nucleocytoplasmic trafficking system, thereby compromising the expression of antiviral genes. Despite extensive vaccination efforts, controlling diseases caused by coronaviruses remains challenging due to the continuous emergence of new serotypes and variants.
 
The Role of IBV Nucleocapsid Protein in Immune Suppression
The study identifies the IBV nucleocapsid protein (N) as a key player in disrupting nucleocytoplasmic trafficking and suppressing the innate immune response. During the infection process, the IBV N protein induces cytoplasmic dispersion of key components of the NPC, particularly FG-Nups, leading to compromised nuclear translocation of transcription factors and subsequent dampening of antiviral gene expression. Interactome analysis reveals that the IBV N protein interacts with the scaffold protein RACK1, facilitating the recruitment of activated p-PKCα to RACK1 and relocating the complex to the cytoplasm. Importantly, this mechanism is conserved across pan-coronaviruses, indicating its evolutionary significance.
 
The Interplay Between IBV N Protein, RACK1, and p-PKCα
The study findings elucidate the critical role of the N-RACK1-p-PKCα complex in inhibiting nucleocytoplasmic trafficking. The interaction between the IBV N protein and RACK1 enhances the binding of p-PKCα to RACK1, leading to the phosphorylation and cytoplasmic dispersion of NUP62, a key component of the NPC. The disruption of nuclear trafficking inhibits the nuclear entry of transcription factors critical for antiviral gene expression. Furthermore, the study highlights the conserved nature of this mechanism across pan-coronaviruses, underscoring its effectiveness in countering the host immune response.
 
Implications for Antiviral Drug Development
Understanding the intricacies of how coronaviruses manipulate the host immune response provides valuable insights for the development of antiviral therapeutics. Targeting the N-RACK1-p-PKCα complex emerges as a promising strategy to impede viral replication and bolster the host innate immune defense. The conserved nature of this mechanism across pan-coronaviruses suggests its potential as a broad-spectrum antiviral target.
 
Conclusion
In conclusion, the study unveils a novel and highly effective mechanism employed by coronaviruses, with a focus on the infectious bronchitis virus, to suppress the host innate immune response. The interaction between the N protein, RACK1, and p-PKCα disrupts nucleocytoplasmic trafficking, inhibiting the nuclear translocation of key transcription factors and compromising the expression of antiviral genes. This conserved mechanism across pan-coronaviruses highlights its evolutionary significance and provides a foundation for the development of antiviral therapeutics targeting the N-RACK1-p-PKCα complex. As the world continues to face challenges posed by coronaviruses, unraveling the intricacies of their immune evasion strategies becomes imperative for devising effective countermeasures.
 
The study findings were published on a preprint server and is currently being peer reviewed.
https://www.biorxiv.org/content/10.1101/2024.03.06.583677v1
 
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