CHIKV nsP3 Hijacks the Host Hsp70-TOM20 Axis to Trigger Mitophagy and Degrade MAVS for Immune Evasion

Technical Summary: CHIKV nsP3 Hijacks the Host Hsp70-TOM20 Axis to Trigger Mitophagy and Degrade MAVS for Immune Evasion

Problem Statement
Chikungunya virus (CHIKV) is a re-emerging mosquito-borne alphavirus causing acute febrile illness and chronic, debilitating arthritis. While viral persistence in joint tissues is implicated in long-term pathology, the molecular mechanisms by which CHIKV establishes persistent infection and evades the host innate immune system remain incompletely understood. Specifically, the strategies CHIKV employs to disrupt the mitochondrial antiviral signaling (MAVS) hub—a critical scaffold for type I interferon (IFN) production—have not been fully elucidated. Although CHIKV is known to suppress the IFN response, the direct link between viral proteins, mitochondrial quality control mechanisms (mitophagy), and the degradation of MAVS requires further definition.

Methodology
The study employed a multi-faceted approach using HEK293T and Vero-E6 cell models to dissect the molecular interactions between CHIKV and host factors.

• Viral Models: Cells were infected with CHIKV (strain BN51) or transfected with plasmids expressing individual non-structural proteins (nsP1–4) to isolate specific viral functions.
• Omics Profiling: Transcriptomic (RNA-seq) and proteomic (TMT-based mass spectrometry) analyses were conducted on nsP3-overexpressing cells to identify enriched pathways and interacting partners.
• Interaction Mapping: Co-immunoprecipitation (Co-IP), GST pull-down assays, and domain deletion mutagenesis were used to map physical interactions between viral nsP3 and host proteins (Hsp70, TOM20, SMURF1).
• Functional Assays: Transmission electron microscopy (TEM) and immunofluorescence (IF) were utilized to visualize mitochondrial ultrastructure and autophagic flux. Western blotting and qPCR assessed markers of mitophagy (LC3B, p62, PINK1), MAVS levels, and IFN-stimulated gene (ISG) expression.
• Pharmacological Intervention: Specific inhibitors targeting Hsp70 (VER-155008), the ubiquitin-activating enzyme E1 (TAK-243), and the E3 ligase SMURF1 (SMURF1-IN-A01) were used to validate the dependency of the viral mechanism on these host factors.
• Reporter Assays: Dual-luciferase assays measured IFN-β promoter activity to quantify immune suppression.

Key Contributions and Results

1. nsP3 is the Primary Driver of CHIKV-Induced Mitophagy:
   The study identified the viral non-structural protein 3 (nsP3) as the key factor triggering mitophagy. Overexpression of nsP3 alone recapitulated the effects of CHIKV infection, including the accumulation of full-length PINK1, conversion of LC3B-I to LC3B-II, degradation of p62 and TIM23, and the formation of autophagosomes containing damaged mitochondria. TEM confirmed mitochondrial swelling, cristae loss, and engulfment by autophagosomes in nsP3-expressing cells.

2. Mechanism of Host Hijacking: The nsP3-Hsp70-TOM20 Axis:

   • Interaction: nsP3 directly binds to the host chaperone Hsp70 via its Alphavirus Unique Domain (AUD, aa 166–325). The interaction is mediated by the substrate-binding domain (SBD) of Hsp70.
   • Trafficking: Hsp70 acts as a chaperone to transport the nsP3 complex to the mitochondrial outer membrane (OMM). Inhibition of Hsp70 ATPase activity prevented this mitochondrial localization.
   • Docking: At the OMM, the nsP3-Hsp70 complex engages the translocase subunit TOM20.
   • Ubiquitination: This complex recruits the E3 ubiquitin ligase SMURF1, which catalyzes the polyubiquitination of TOM20. This process initiates a PINK1/Parkin-independent mitophagic cascade.

3. Immune Evasion via MAVS Degradation:
   The induced mitophagy leads to the selective clearance of mitochondria, resulting in the concomitant degradation of MAVS, which resides on the OMM. Consequently, the RIG-I/MDA5-MAVS signaling axis is disrupted, leading to:

   • Reduced phosphorylation of IRF3.
   • Suppression of IFN-β promoter activity.
   • Downregulation of key ISGs (e.g., IFNβ1, ISG54, MX1).
   • Enhanced viral replication.

4. Therapeutic Potential:
   Pharmacological inhibition of the nsP3-Hsp70 axis demonstrated therapeutic potential. Treatment with Hsp70 inhibitors (VER-155008) or ubiquitination inhibitors (TAK-243) restored MAVS levels, reactivated the IFN response, and significantly reduced CHIKV genomic RNA and infectious viral titers.

Significance and Claims
The paper claims to unveil a novel "proviral strategy" wherein CHIKV actively manipulates the host chaperone system and mitochondrial import machinery to induce mitophagy. Unlike other viruses that cleave MAVS or block its aggregation, CHIKV employs a unique, multi-step scaffolding mechanism (nsP3-Hsp70-TOM20-SMURF1) to physically remove the MAVS signaling platform via lysosomal degradation.

The authors position the nsP3-Hsp70-TOM20 axis as a "druggable host dependency." By targeting host factors (Hsp70 or the ubiquitination cascade) rather than rapidly mutating viral proteins, this approach offers a potential strategy for broad-spectrum antiviral therapy against CHIKV and related alphaviruses. The study establishes a direct mechanistic link from a specific viral-host protein interaction to organelle-targeted immune subversion, providing a molecular foundation for future host-directed antiviral interventions.

The authors note limitations, acknowledging that the findings are primarily derived from cell culture models (HEK293T) and that the contribution of this axis to viral persistence and chronic arthritis in vivo, particularly in physiologically relevant cell types like macrophages and fibroblasts, requires further investigation in animal models.