On The Retrograde Transport of RNA-Loaded Lipid Nanoparticles Designed for Brain Delivery

Lipid nanoparticles (LNP) have been extensively studied for their ability to encapsulate and protect RNA molecules from degradation. More recently, a few studies have begun to explore their applications as carriers for brain drug delivery via various administration routes. Nose-to-brain delivery represents a promising alternative to both invasive local injections and systemic administration, offering the possibility to bypass the blood-brain barrier and directly access the brain, achieve rapid absorption, reduce systemic exposure, and allow for ease of administration. In order to evaluate the viability of this alternative route, it is essential to acquire a better understanding of the intraneuronal mass transport of LNP, particularly in terms of how effectively and efficiently they deliver their payloads from the periphery to neuronal cell bodies. However, most previous studies have focused primarily on the delivery vector itself rather than on the fate of the transported cargo. In this study, we investigate the retrograde trafficking of nucleic acid-loaded LNP in primary cortical neurons, focusing on the transport of both the particle and the payload. Three distinct LNP were formulated to characterize different aspects of their interaction with the cells, with the major LNP player of this study containing a red-fluorescent Rhodamine B-tagged lipid and a green fluorescently FAM-tagged RNA. Flow cytometry was used to document LNP uptake by primary cortical neurons over time. Additionally, confocal microscopy was then used to investigate the colocalization of LNP and RNA after a conventional 2D culture treatment. As a final step, a compartmentalized chip that separates the somal and the axonal regions of cortical neurons was used to study the intraneuronal dynamics of LNP and their cargo. In this second setup, LNP were selectively administered at the axonal compartment, and the fluorescent signals from the vector (red) and the payload (green) were imaged through time-lapse microscopy. The progressive accumulation of RNA found at cellular bodies also in the absence of the red signal suggested an efficient retrograde transport of the LNP payload toward the soma. Comprehensively, this work demonstrates that primary cortical neurons are capable of efficiently uptaking LNP and of intracellularly transporting both LNP and their RNA cargo. Interestingly, a different colocalization trend (LNP-RNA) emerged depending on the followed setup. Localized axonal transfection appeared to favor dissociation of RNA from the LNP and subsequent accumulation at the soma. Overall, our work provides a fundamental in vitro proof of concept of the RNA delivery to the cellular bodies of primary cortical neurons via the retrograde transport of LNP vectors administered at the axonal termini. This finding, together with the image-analysis-based quantification of the RNA accumulation described in our work, paves the way for future studies aimed at designing lipid-based nanoparticles for RNA therapeutic delivery to the brain via peripheral administration.