Intracellular delivery of nanomaterials: How to catch endosomal escape in the act

Abstract

Successful cytosolic delivery of nanomaterials is becoming more and more important, given the increase in intracellular applications of quantum dots, gold nanoparticles, liposomal drug formulations and polymeric gene delivery vectors. Most nanomaterials are taken up by the cell via endocytosis, yet endosomal escape has long been recognized as a major bottleneck in cytosolic delivery. Although it is essential to detect and reliably quantify endosomal escape, no consensus has been reached so far on the methods to do so. This review will summarize and discuss for the first time the different assays used to investigate this elusive step to date.

Cytosolic Delivery of Nanolabels Prevents Their Asymmetric Inheritance and Enables Extended Quantitative in Vivo Cell Imaging

“…1a. Cells are first incubated with plasmonic NPs, such as gold NPs (AuNPs), for [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] min. During that time the AuNP can adsorb to the cells and are internalized just below the plasma membrane by endocytic uptake (see TEM images in Figure S1).…”

“…Instead, cells labelled by endocytosis exhibited a much more rapid decrease of the cell signal (Fig. 2a, b), due to other processes than cell division alone, most importantly label degradation (CdSe QD) or fluorescence quenching (FD) in the acidic endolysosomes 26,28 .…”

“…A second problem is that entrapment of contrast agents in the acidic endolysosomes may lead to increased cytotoxicity, for instance by creating reactive groups on the NP surface or by leaching of toxic metal ions, such as Cd 2+ in case of QDs [25][26][27] . A third disadvantage of endocytic cell labelling is that the vast majority of the contrast agents are trafficked to the endolysosomes where they are exposed to an acidic and overall degrading environment 26,28 . This can lead to degradation of the contrast agent, lowering the signal intensity, as reported for fluorescent labels [28][29][30] as well as MRI contrast agents [31][32][33] .…”

“…A third disadvantage of endocytic cell labelling is that the vast majority of the contrast agents are trafficked to the endolysosomes where they are exposed to an acidic and overall degrading environment 26,28 . This can lead to degradation of the contrast agent, lowering the signal intensity, as reported for fluorescent labels [28][29][30] as well as MRI contrast agents [31][32][33] . In addition, it has been reported that nanomaterials trapped in endolysosomal vesicles are not distributed equally over daughter cells upon cell division 30,34,35 .…”

Therapeutic and diagnostic applications of extracellular vesicles

“…Also for synthetic nanomedicines the endolysosomal entrapment is one of the major hurdles for efficient cellular delivery of membrane impermeable drugs. The delivery efficiency of nanomedicines hinges on strategies to cross the endosomal barrier, such as the so-called proton sponge effect and/or lipid bilayer fusion [141]. As many of the effects mediated by EVs have been attributed to the functional delivery of miRNA and mRNAs, [87] this implies that (subtypes of) EVs might contain built-in mechanisms to stimulate endosomal escape.…”