Cell membrane coated biomimetic nanoparticles (NPs) have been widely studied in nanomedicine because of their unique properties such long blood circulation, specific molecular recognition, and efficient cancer targeting, indicating a great potential in targeted cancer therapy. However, the integrity of the cell membrane coating on NPs, a key metric related to the quality of these biomimetic-systems and to the resulting biomedical function, has remained largely unexplored.

Now researchers at the University of Eastern Finland report the development of a fluorescence quenching assay to probe the integrity of the cell membrane coating. Their study (“Cell membrane coating integrity affects the internalization mechanism of biomimetic nanoparticles”), published in Nature Communications, shows that the great majority of the cell membrane coated NPs were only partially coated when traditional coating techniques were applied.

The scientists, who carried out their work in the department of applied physics under the direction of Vesa-Pekka Lehto, PhD, says that this information is essential as the coating degree impacts the biological fate of NPs.

“Here, we report a fluorescence quenching assay to probe the integrity of cell membrane coating. In contradiction to the common assumption of perfect coating, we uncover that up to 90% of the biomimetic NPs are only partially coated. Using in vitro homologous targeting studies, we demonstrate that partially coated NPs could still be internalized by the target cells,” write the investigators.

“By combining molecular simulations with experimental analysis, we further identify an endocytic entry mechanism for these NPs. We unravel that NPs with a high coating degree (≥50%) enter the cells individually, whereas the NPs with a low coating degree (<50%) need to aggregate together before internalization.

“This quantitative method and the fundamental understanding of how cell membrane coated NPs enter the cells will enhance the rational designing of biomimetic nanosystems and pave the way for more effective cancer nanomedicine.”

“The present methods for characterizing the cell membrane coating are only qualitative and fail to statistically evaluate the degree and variability of the coating,” notes Lizhi Liu, a researcher and first author of the publication. “When we applied the developed quantification method to evaluate the success of the commonly used protocols to produce fully coated NPs, we found that the fraction never exceeded 20%.”

“Our discovery is a big surprise to whole scientific community in nanomedicine because it has been generally accepted that the cell membrane coating is perfect. Despite of the partial coating, biomimetic NPs could still be internalized by the target cells via different pathways,” adds Wujun Xu, PhD, senior research and team leader and one of the corresponding authors of the paper.

To explain this, the authors proposed a new endocytic entry mechanism for these partially coated NPs by computational simulations. Specifically, the NPs with a high coating degree (≥ 50%) entered the cells individually, whereas the NPs with a low coating degree (< 50%) needed to aggregate together before internalization.

“The present study highlights some of the limitations of the current cell membrane coating protocols and motivates the efforts to improve the protocols. The developed quantification method is a practical tool to assess the success of these efforts and establish a standard for comparing the different coating designs,” says Vesa-Pekka Lehto.