A MCF7 cell (breast cancer cell) incubated with fluorescently labelled rodshaped nanoparticles (green signal) that has its nucleus stained with Hoechst 33342 (blue signal). We tracked whether this nanoparticle morphology could traverse the nuclear pore complex (inset): the gatekeeper to the cell nucleus (target of DNA intercalating chemotherapeutics such as doxorubicin).The size, surface charge, and composition of a nanoparticle determine how easily it can slip inside a cell. Now Australian researchers have demonstrated that the same holds true of nanoparticle shape [Hinde et al., Nature Nanotechnology (2016), doi: 10.1038/nnano.2016.160].
Nanoparticles made from biocompatible polymers could make effective delivery systems taking anti-cancer drugs, nucleic acids used in gene therapy, and imaging agents right into the heart of cells − the nucleus. But particles carrying active agents must be able to gain access to cells quickly and easily.
To find out whether the shape of a nanoparticle could make a difference, the researchers from the University of New South Wales created spherical, rod, vesicle, and S-shaped worm-like particles from the same polymeric material with identical surface chemistry. The particles were loaded with the same anticancer agent, doxorubicin, and tracked using microscopy.
“We used a fluorescence microscopy method called pair correlation microscopy to show that polymeric nanoparticles with different shapes, but identical surface chemistries, move across the various cellular barriers at different rates,” explains first author of the study, Elizabeth Hinde. “Using this technology, we were able to pinpoint where drugs were being released and how they spread throughout the cell.”
The results show marked differences between the various nanoparticles, according to J. Justin Gooding and Katharina Gaus, who led the study. The teams’ observations indicate that all the nanoparticles find the outer plasma membrane of the cell an equal barrier, regardless of shape. Once inside the cell, however, spherical nanoparticles appear more mobile – showing shorter transit times – than higher-aspect-ratio rod- and worm-shaped particles. All the nanoparticles show a similar ability to escape from the endosomal system – which can be limit the efficiency of drug delivery.
But the crucial difference, found by the researchers, is that rod and worm-shaped particles enter the cell nucleus via passive diffusion more readily than spherical particles and vesicles. As the nuclear membrane presents more of a barrier to the particles than the plasma membrane, particles that enter the nucleus tend to accumulate there.
The routes taken by different nanoparticles inside the cell indicate where drug release ultimately takes place. So while all nanoparticles can deliver their cargoes to the cellular cytoplasm, significantly higher amounts of doxorubicin were deposited in the nucleus by higher aspect ratio rod- and worm-shaped particles.
“We found that the cancer drug was most effectively delivered when the nanoparticle carrier could breach the envelope protecting the nucleus – the cell’s control center – and release the drug,” explains Hinde.
The researchers also managed to improve on the inherent ability of rod- and worm-shaped particles to enter the cell nucleus and deliver their cargo by adding a nuclear localization signal (NLS) – an amino acid ‘tag’ that facilitates passage through nuclear membrane.
“Our results demonstrate that drug delivery across the major cellular barrier, the nuclear envelope, is important for doxorubicin efficiency and can be achieved with appropriately shaped nanoparticles,” says Hinde.
Ultimately, if the dimensions of rod-shaped nanoparticles could be fine-tuned to facilitate preferential entry to cancer cells rather than healthy ones, drugs could be delivered more efficiently with reduced side effects. The researchers’ analysis of the role of nanoparticle shape in determining their ability to traverse intracellular transport barriers is an interesting one, believes Samir Mitragotri, director of the Center for Bioengineering at the University of California, Santa Barbara.
“This analysis provides novel insights into and guidelines for the design of nanoparticles for drug delivery,” he says.
This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.10.012