What property of nanoparticles determines the triggering of clathrin-mediated endocytosis: size or mass?

Clathrin-mediated endocytosis (CME) is one of the most important pathways for transporting nanoparticles (NPs) into cells. It has been demonstrated that a single large NP can effectively trigger CME when it exceeds a certain threshold, thereby carrying nearby small NPs into cells and significantly enhancing their cellular uptake. However, it is unclear what property of the NPs, mass or size, contributes more to the CME triggering. Herein, we demonstrate that size is more important than mass in CME triggering by using a special trio of accurately synthesized silica NPs. One of the trio (Ag@SNP80) with a silver core has the same size as the 80 nm silica NPs (SNP80) and the same mass as the 100 nm silica NPs (SNP100). SNP80 is smaller than the CME triggering threshold, whereas SNP100 exceeds it. By comparing the different enhancing effects of this particular trio on the cellular uptake of 40 nm silica NPs (FSNP40), we are able to compare their different capabilities in triggering CME. Results from both HeLa and A549 cells showed the same order of the enhancing effect, with SNP100 > Ag@SNP80 ≈ SNP80, clearly demonstrating that size is more important than mass in triggering CME. This conclusion is further confirmed by the enhanced cellular uptake of 70 nm polystyrene NPs (FPS70) by the trio of SNPs in the same order. This finding improves our understanding of how NPs and other materials are internalized by cells and sheds light on the fundamental mechanisms of CME.