Lipid nanoparticles
(LNPs) have emerged as potent carriers for
mRNA delivery, but several challenges remain before this approach
can offer broad clinical translation of mRNA therapeutics. To improve
their efficacy, a better understanding is required regarding how LNPs
are trapped and processed at the anionic endosomal membrane prior
to mRNA release. We used surface-sensitive fluorescence microscopy
with single LNP resolution to investigate the pH dependency of the
binding kinetics of ionizable lipid-containing LNPs to a supported
endosomal model membrane. A sharp increase of LNP binding was observed
when the pH was lowered from 6 to 5, accompanied by stepwise large-scale
LNP disintegration. For LNPs preincubated in serum, protein corona
formation shifted the onset of LNP binding and subsequent disintegration
to lower pH, an effect that was less pronounced for lipoprotein-depleted
serum. The LNP binding to the endosomal membrane mimic was observed
to eventually become severely limited by suppression of the driving
force for the formation of multivalent bonds during LNP attachment
or, more specifically, by charge neutralization of anionic lipids
in the model membrane due to their association with cationic lipids
from earlier attached LNPs upon their disintegration. Cell uptake
experiments demonstrated marginal differences in LNP uptake in untreated
and lipoprotein-depleted serum, whereas lipoprotein-depleted serum
increased mRNA-controlled protein (eGFP) production substantially.
This complies with model membrane data and suggests that protein corona
formation on the surface of the LNPs influences the nature of the
interaction between LNPs and endosomal membranes.
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