Schulz, F. H. | Search bioRxiv

bioRxiv2026

Tuning siRNA packing order in lipid nanoparticles modulates oligonucleotide functional delivery

Efficient siRNA delivery by lipid nanoparticles (LNPs) is widely attributed to carrier composition, yet how intraparticle packing governs function remains unclear. Here, we developed a single-particle fluorescence microscopy assay that simultaneously quantifies size and siRNA loading of individual, chromophore-labeled LNPs. Imaging ~0.5M particles per condition per hour uncovered two major packing modes: a high and a low order corroborated by cryo-EM. Quantitative live cell imaging combined with systematic variation of LNPs lipid composition and N/P ratio allowed deconvolution of siRNA packing, internalization, and silencing and its dependance on lipid composition and electrostatics. Surprisingly, low-order particles while encapsulating modest RNA mediate more efficient knockdown of a fluorescent reporter than their high-order counterparts. Guided by these findings we predicted and experimentally validated that tuning composition and N/P ratio to favor less compact siRNA packing enhances silencing potency. This framework offers actionable guidance for the rational optimization of LNP formulations for RNA therapeutics.

biophysics↗

bioRxiv2025

Single-Particle Tracking and Positional Phenotyping Reveals Variant-Specific Early Checkpoints in SARS-CoV-2 Cell Entry

SARS-CoV-2 entry is governed by Spike (S) protein-mediated engagement of ACE2 and subsequent activation of either plasma membrane fusion mediated by TMPRSS2 or endocytic uptake. Currently, most insights into these pathways come from bulk assays that obscure the fate of individual virions, thereby concealing intricate mechanistic details that can inform on therapeutic intervention strategies. Here, we applied single-particle fluorescence imaging to directly observe the early checkpoints of SARS-CoV-2 cell entry pathways and separate binding from internalization. Fluorescent virus-like particles (VLPs) pseudotyped with either G614 or Omicron BA.5 S protein variants were imaged on HEK293T-ACE2 (TMPRSS2-negative) and classified at the single-particle level as surface-interacting, crossing, or internal. At baseline, G614 VLPs show higher binding and a larger internalized share than BA.5 VLPs, revealing general divergence in early entry behavior between variants. A trivalent anti-S receptor-binding domain aptamer reduces G614 binding and lowers its internalization. Conversely, the aptamer does not block BA.5 VLP cell binding but increases its internalization efficiency. Pitstop 2, an inhibitor of clathrin-mediated endocytosis, causes no significant change in this observation window, consistent with early clathrin-sensitive events having already progressed. Quantification of trajectories reveals variant-specific mobility: BA.5 displays higher step length than G614, consistent with greater lateral scanning and surface retention. Together, these compact single-particle readouts expose variant-resolved early checkpoints in entry and provide a simple platform to test how ligands and pathway probes shift binding and internalization.

biophysics↗