Bovine viral diarrhea virus (BVDV) relies on cellular lipid droplet biogenesis and lipolysis to provide energy for viral replication

Technical Summary: Bovine Viral Diarrhea Virus (BVDV) Hijacks Lipid Droplet Biogenesis and Lipolysis via Core Protein to Fuel Replication

Problem Statement

Bovine viral diarrhea virus (BVDV) is a significant pathogen causing immunosuppression, reproductive disorders, and persistent infections in cattle, leading to severe economic losses. While previous research has established a link between viral infections and lipid metabolism, the specific pathogenic mechanisms by which BVDV reprograms host lipid metabolism remain unclear. Specifically, the role of lipid droplets (LDs) in the BVDV life cycle, the molecular mechanisms governing LD biogenesis and degradation during infection, and the interaction between LDs and mitochondria for energy production in the context of BVDV infection are undefined.

Methodology

The study employed a combination of virological, biochemical, and cell biological approaches using Madin-Darby Bovine Kidney (MDBK) cells and bovine endometrial epithelial cells (BEECs).

• Viral Infection Models: Cells were infected with BVDV strain AV67 at various multiplicities of infection (MOI) and time points (up to 24 hours post-infection).
• Lipid Visualization and Quantification: LDs were visualized using BODIPY 493/503, Nile Red, and fluorescent fatty acid analogues (BODIPY 500/510 for uptake; BODIPY 558/568 C12 for transfer). Confocal microscopy was used to assess LD number, area, and colocalization with organelles (mitochondria, lysosomes) and proteins.
• Pharmacological Inhibition: Specific inhibitors were used to dissect metabolic pathways:
  • De novo lipogenesis (DNL): Orlistat (FASN inhibitor), Torin 1 (mTORC1 inhibitor).
  • Esterification: DGAT1 and DGAT2 inhibitors (PF-04620110, PF-006424439).
  • Lipolysis: Atglistatin (ATGL inhibitor), CAY10499 (HSL inhibitor).
  • Lipophagy: Bafilomycin A1 (BafA, lysosomal acidification inhibitor).
  • Fatty Acid Oxidation (FAO): Etomoxir (CPT1A inhibitor).
  • Palmitoylation: 2-Bromopalmitate (2-BP).
  • FAO Activation: AICAR.
• Rescue Experiments: Exogenous fatty acids (Palmitic acid-BSA) were added to test if they could reverse the effects of metabolic inhibitors.
• Molecular Biology: qRT-PCR and Western blotting were used to quantify mRNA and protein levels of lipogenic, lipolytic, and mitochondrial genes. Immunoprecipitation (IP) and co-localization studies were performed to identify protein-protein interactions, specifically focusing on the BVDV Core protein.
• Protein Engineering: EGFP-tagged BVDV Core protein (N-terminal and C-terminal fusions) and amphipathic helix analysis were used to determine LD targeting mechanisms.

Key Results

1. BVDV Induces LD Accumulation via Multiple Metabolic Pathways
BVDV infection significantly increased the number and area of LDs in bovine cells. This accumulation was driven by:

• Enhanced De Novo Lipogenesis (DNL): Upregulation of lipogenic genes (Acly, Acc1, Fasn, Scd1) and increased mRNA levels of DGAT1 and DGAT2. Inhibition of FASN (Orlistat) or mTORC1 (Torin 1) blocked LD formation and viral replication, which was rescued by exogenous palmitate.
• Increased Exogenous Fatty Acid Uptake: BVDV infection increased the uptake of exogenous fatty acids, accompanied by upregulation of FATP and FABP4 mRNA.
• Esterification: DGAT1 and DGAT2 were upregulated, and inhibition of these enzymes (particularly DGAT1) reduced LD formation and viral replication.

2. BVDV Mobilizes LDs via Lipolysis, Not Lipophagy
Following accumulation, BVDV promoted the mobilization of stored lipids.

• Lipolysis Dominance: Inhibition of lipolysis enzymes ATGL (Atglistatin) and HSL (CAY10499) reversed the reduction in LD numbers caused by infection and significantly inhibited viral replication. Conversely, inhibition of lipophagy (BafA) did not rescue LD levels or viral replication.
• Mechanism: BVDV infection increased the protein and mRNA levels of ATGL and HSL and promoted their recruitment to the LD surface, while lysosomal acid lipase (LAL) levels remained unchanged.

3. LD-Mitochondria Interaction and Fatty Acid Oxidation (FAO)
The study demonstrated that BVDV facilitates the transfer of free fatty acids (FFAs) from LDs to mitochondria for oxidation.

• Enhanced Contact: BVDV infection increased the physical proximity and colocalization of LDs and mitochondria.
• FA Transfer: Pulse-chase assays with Red C12 showed that BVDV accelerated the transfer of fatty acids from LDs to mitochondria. This transfer was dependent on lipolysis (blocked by Atglistatin) and correlated with increased ATP levels.
• FAO Dependency: Inhibition of mitochondrial FAO (Etomoxir) significantly reduced viral replication and could not be rescued by exogenous palmitate, indicating that the oxidation of fatty acids, not just their presence, is critical. BVDV also upregulated genes related to mitochondrial biogenesis (PPAR-A, PGC1-A, TFAM) and FAO (CPT1, CPT2, Acyl-CoA dehydrogenases), compensating for infection-induced mitochondrial fragmentation and depolarization.

4. The BVDV Core Protein as a Central Regulator
The BVDV Core protein was identified as the primary viral factor reprogramming lipid metabolism.

• LD Targeting: The Core protein localizes to the surface of LDs, a process mediated by two amphipathic helices (AH1 and AH2), with AH2 being crucial for LD association.
• Enzyme Recruitment: The Core protein interacts directly with FASN and ATGL, recruiting them to the LD surface to promote both lipid synthesis and lipolysis.
• Mitochondrial Link: The Core protein also interacts with the mitochondrial outer membrane protein TOMM20. This interaction physically links mitochondria to LDs, facilitating the efficient transfer of FFAs for oxidation.
• Functional Impact: Overexpression of the Core protein alone was sufficient to upregulate lipogenic and lipolytic genes, recruit ATGL to LDs, and accelerate FA transfer to mitochondria, mimicking the effects of full viral infection.

Significance and Claims

The authors claim that this study elucidates a novel pathogenic mechanism where BVDV reprograms host cellular lipid metabolism to support its replication. Specifically:

• BVDV creates a "lipid cycle" where it first drives the accumulation of LDs through enhanced synthesis and uptake, and then mobilizes them via lipolysis.
• The mobilized fatty acids are not merely stored but are rapidly transferred to mitochondria for beta-oxidation to generate ATP, which fuels viral replication.
• The BVDV Core protein acts as a molecular bridge, targeting LDs to recruit metabolic enzymes (FASN, ATGL) and linking LDs to mitochondria (via TOMM20) to ensure efficient energy production.
• These findings contribute to understanding the pathogenesis of BVDV and identify the Core protein and the LD-mitochondria axis as potential targets for developing antiviral strategies to control BVDV transmission.

The paper maintains a modest tone, noting that while the Core protein's role is clear, further structural studies (e.g., point mutations in amphipathic helices) and direct interaction assays (e.g., GST pull-down) are required to fully validate the mechanistic details. The study does not propose immediate clinical applications but frames these findings as foundational for future therapeutic development.