Imaging plasma membranes without cellular internalization: multisite membrane anchoring reagents based on glycol chitosan derivatives

Plasma membrane imaging has received substantial attention due to its capability for dynamically tracing significant biological processes including cell trafficking, vesicle transportation, apoptosis, etc. However, cellular internalization of staining molecules poses challenges to the development of fluorescent dyes to specifically label plasma membranes rather than intracellular organelles. In this work, glycol chitosan, a multifunctional biomaterial derived from natural polymers, was used for the first time to image the plasma membranes based on a strategy of multisite membrane anchoring. A glycol chitosan derivative, glycol chitosan-cholesterol-FITC (Chito-Chol-FITC), was synthesized by using glycol chitosan as the backbone, and PEG-cholesterols and FITC molecules as side chains. The cholesterol groups and FITC molecules serve as hydrophobic anchoring units and fluorescent units, respectively. Benefitting from the strategy, this molecular probe could rapidly stain the cell membrane within 5 min as well as effectively restrain the cellular uptake process-it could tolerate an incubation time of 6 h without substantial cellular internalization. Its imaging performance far exceeds that of the current commercial plasma membrane imaging reagents based on small molecules (such as DiD and FM families), which will be easily internalized by the cells within 10-15 min. The present work shows the biomacromolecular assembly of the glycol chitosan derivative on the cell surface, which may shed new light on the interactions of biomaterials with biological systems. Besides, the multisite membrane anchoring strategy developed herein also provides a novel platform for future cell surface engineering studies.