Rab9 depletion enhances human adenovirus type 26 transduction efficiency through increased internalization and reduced late endosomal/lysosomal retention

1. Problem Statement

Human Adenovirus type 26 (HAdV-D26) is a critical platform for vaccines (e.g., Ebola, COVID-19) and gene therapy due to its low seroprevalence in the human population compared to the traditional vector, HAdV-C5. However, HAdV-D26 exhibits significantly lower transduction efficiency in epithelial cells. While the receptor usage (αvβ3 integrin) and entry mechanisms (caveolin/clathrin-dependent) are partially understood, the intracellular trafficking pathways and the specific endosomal compartments involved in HAdV-D26 post-entry transport remain poorly characterized. Understanding these mechanisms is essential for optimizing vector design and improving therapeutic efficacy.

2. Methodology

The study employed a multi-faceted approach using A549 epithelial cells and U2OS cells (stably expressing mCherry-tubulin) to compare HAdV-D26 with the well-characterized HAdV-C5.

• Imaging Techniques:
  • Confocal Microscopy: Used to track fluorescently labeled viruses and assess co-localization with endosomal markers (EEA1 for early endosomes, LAMP1 for late endosomes/lysosomes, LysoTracker for live lysosomes).
  • Transmission Electron Microscopy (TEM): Visualized the ultrastructural localization of viral particles within cellular compartments (plasma membrane, endosomes, cytosol, degradative compartments).
  • Live-Cell Microscopy: Tracked viral particle velocity and directionality along microtubules.
  • Streptolysin O (SLO) Assay: Distinguished between cytosolic and endosomal viral particles by allowing antibody access to the cytosol of permeabilized cells.
• Molecular & Biochemical Assays:
  • qPCR: Quantified viral genome delivery to the nucleus by measuring viral DNA ratios in nuclear fractions versus total cell lysates over time.
  • Transduction Efficiency Assays: Measured eGFP expression (via flow cytometry or spectrophotometry) as a proxy for successful infection.
  • Pharmacological Inhibition: Used agents to modulate endosomal function:
    • Bafilomycin A1, Chloroquine, NH4Cl: To inhibit lysosomal acidification.
    • EGA: To block transport from early to late endosomes.
  • Genetic Manipulation:
    • Dominant-Negative (DN) Mutants: Transfection of DN forms of Rab5, Rab7, Rab9, and Rab11 to inhibit specific GTPase functions.
    • siRNA Knockdown: Depletion of Rab5, Rab7, and Rab9 to assess the impact of reduced protein levels on trafficking and transduction.

3. Key Contributions

• Characterization of HAdV-D26 Trafficking Kinetics: The study provides the first detailed temporal map of HAdV-D26 intracellular trafficking, revealing a prolonged residence in endosomal compartments compared to HAdV-C5.
• Identification of Rab9 as a Critical Regulator: The paper identifies Rab9 as a previously unrecognized host factor that negatively regulates HAdV-D26 transduction by promoting its retention in late endosomes/lysosomes.
• Mechanistic Insight into Serotype Differences: It demonstrates that HAdV-D26 and HAdV-C5 utilize distinct endosomal pathways, challenging the assumption that all adenoviruses follow a uniform entry/escape route.
• Therapeutic Strategy Proposal: The finding that Rab9 depletion enhances transduction offers a novel strategy for improving HAdV-D26 vector performance, particularly in oncolytic virotherapy or gene delivery contexts.

4. Key Results

A. Prolonged Intracellular Trafficking and Reduced Nuclear Entry

• Delayed Nuclear Arrival: Unlike HAdV-C5, which reaches the nucleus within ~60 minutes, HAdV-D26 remains dispersed in the cytoplasm for extended periods. Even at 240 minutes post-infection (p.i.), significant nuclear accumulation is not observed.
• Genome Delivery Defect: qPCR analysis showed that while HAdV-D26 DNA reaches the nucleus within the first hour, the amount subsequently decreases, suggesting degradation or failure of nuclear import, whereas HAdV-C5 DNA levels in the nucleus increase over time.
• Microtubule Transport: HAdV-D26 moves along microtubules but at a slower average velocity (12 µm/min) compared to HAdV-C5, and only ~34% of rapidly moving particles reach the nuclear surface within one hour.

B. Endosomal Retention and Lysosomal Accumulation

• Early vs. Late Endosomes: HAdV-D26 transiently visits early endosomes (EEA1+) but rapidly shifts to late endosomes/lysosomes (LAMP1+).
• Lysosomal Trapping: Approximately 40% of viral particles are found in degradative compartments within 60 minutes. By 240 minutes, over 50% of the virus remains co-localized with LAMP1 and LysoTracker, indicating prolonged retention in lysosomes.
• SLO Assay: Only ~44% of HAdV-D26 particles are cytosolic at 45 minutes p.i., compared to higher escape rates for HAdV-C5.

C. Impact of Endosomal Modulation on Transduction

• Acidification Inhibition: Treating cells with lysosomotropic agents (Bafilomycin A1, Chloroquine, NH4Cl) modestly increased transduction efficiency, suggesting that lysosomal degradation contributes to the loss of infectivity.
• Transport Blockade: Inhibiting the transition from early to late endosomes (using EGA) did not significantly improve transduction, implying that the virus does not need to reach the lysosome to escape, but rather that retention in the lysosome is detrimental.
• Co-infection: Co-infection with HAdV-C5 did not rescue HAdV-D26 transduction, confirming they do not share the same endosomal escape machinery.

D. The Role of Rab GTPases

• Dominant-Negative Effects: Expression of DN-Rab5, DN-Rab7, and DN-Rab9 reduced transduction efficiency, with DN-Rab9 showing the most pronounced effect (~40% reduction).
• Knockdown Effects (The Critical Finding):
  • Rab5/Rab7 KD: Increased retention of HAdV-D26 in early endosomes but had minimal impact on overall transduction efficiency.
  • Rab9 KD: Dramatically increased HAdV-D26 transduction efficiency by 62%.
    • Mechanism: Rab9 knockdown reduced HAdV-D26 co-localization with LAMP1 (lysosomes) from ~60% to ~24% at 120 minutes p.i.
    • Outcome: Rab9 depletion led to a >30% increase in viral internalization and a significant boost in nuclear genome delivery.
• Divergence of Mechanisms: The authors note that while DN-Rab9 (which sequesters GEFs) inhibits infection, Rab9 knockdown (which reduces protein levels) enhances it. This suggests Rab9 normally facilitates the sorting of HAdV-D26 into the degradative lysosomal pathway; removing Rab9 diverts the virus to an alternative, productive escape route.

5. Significance

• Vector Optimization: The study reveals that Rab9 acts as a "brake" on HAdV-D26 infection. Targeting Rab9 (e.g., via siRNA or small molecules) could be a viable strategy to boost the efficacy of HAdV-D26-based vaccines and gene therapies.
• Mechanistic Understanding: It clarifies that HAdV-D26 has a unique, slower trafficking profile characterized by lysosomal trapping, distinguishing it from species C adenoviruses (like HAdV-C5).
• Immunological Implications: The prolonged residence in lysosomes may actually benefit HAdV-D26 as a vaccine vector by facilitating antigen processing for MHC class II presentation and engaging innate immune sensors, acting as a self-adjuvant.
• Future Directions: The findings open new avenues for investigating the specific receptors or sorting mechanisms that direct HAdV-D26 to Rab9-dependent pathways, potentially leading to engineered capsids with improved nuclear delivery kinetics.