Nonviral Gene Delivery Vectors Use Syndecan-Dependent Transport Mechanisms in Filopodia To Reach the Cell Surface

Lipoplexes and polyplexes, that is, assemblies of cationic lipids and polymers with nucleic acids, respectively, are popular nanocarriers for delivery of genes or siRNA into cells for therapeutic or cell biological purposes. Although endocytosis represents a major mechanism for their cellular entry, very little is known about parameters that govern early events in the initial interaction of such delivery devices with the cell surface. Here, we demonstrate that prior to entry, poly- and lipoplexes are captured by thin, actin-rich filopodial extensions, protruding from the cell surface. Subsequent additional recruitment and local clustering of filopodia-localized syndecans, presumably driven by multivalent interactions with the polycationic nanocarriers, appear instrumental in their processing to the cell body. Detailed microscopic analyses reveal that the latter relies on either directional surfing along or retraction of the filopodia. By interfering with actin polymerization or inhibiting the motor protein myosin II, localized at the base of filopodia, our data reveal that the binding of the nanocarriers to and subsequent clustering of syndecans initiates actin retrograde flow, which moves the syndecan-bound nanocarriers to the cell body. At the present experimental conditions, inhibition of this process inhibits nanocarrier-mediated transfection by 50–90%. The present findings add novel insight to our understanding of the mechanism of nanocarrier-cell surface interaction, which may be instrumental in further improving delivery efficiency. In addition, the current experimental approach may also be of relevance to improving our understanding of cellular infection by viruses and pathogenic bacteria, given a striking parallel in filopodia-mediated processing of these infectious particles and nanocarriers.