Insights into cell entry and intracellular trafficking of peptide and protein drugs provided by electron microscopy

Abstract

For widening the arsenal of protein and peptide therapeutics that act within cells, their cell-entry mechanisms, intracellular trafficking and distribution need to be characterized in detail. Immunofluorescence microscopy has been a prevalent tool for these studies. However, due to the limited resolution, it is often complemented with other methods. This article focuses on the perspectives of electron microscopy in tracking the intracellular delivery and trafficking of proteins, peptides and their carriers. This review introduces the electron microscopy techniques and labeling methods currently used for studying the cellular whereabouts of peptides and proteins with a focus on their intracellular trafficking. Since cell-penetrating peptides have widely been harnessed as carriers for proteins and peptides, and their usage is rapidly expanding, a particular emphasis has been placed on their applications and cell-entry mechanisms.

Keywords: CPP; Cell-penetrating peptides; EM; ET; Electron microscopy; HPF; IFM; Immunolabeling; Intracellular trafficking; Peptide and protein therapeutics; TEM; TP; Tat-SPIONs; cell-penetrating peptide; cryo-EM; cryo-electron microscopy; electron microcopy; electron tomography; high pressure freezing; immuno-EM; immuno-electron microscopy; immunofluorescence microscopy; mAb; monoclonal antibody; superparamagnetic iron oxide nanoparticles modified with Tat peptide; transmission electron microscopy; transportan.