The size-dependent penetration of silica nanoparticles through the blood-brain barrier
The blood-brain barrier (BBB) is essential for maintaining the homeostasis of the central nervous system (CNS). However, the intact structure of the BBB limits the positive diffusion of molecules, including small molecules (MW < 500 Da) and macromolecules. With the rapid development of bio-nanotechnology, the application of nanomaterials has opened up an expansive and new research field in brain-targeted nano-delivery therapy. Many investigations have suggested that the complex cerebrovascular system displays high selectivity for size-dependent physicochemical properties ranging from nanometer to micrometer.
Our investigation was conducted to identify the optimal size of a nano-delivery system, and to explore the size-dependent BBB permeability efficiency of silica nanoparticles (SiNPs). Owing to the excellent biocompatibility and size controllability of SiNPs, they were prepared in different sizes (35 nm, 58 nm, 68 nm, 122 nm, 142 nm, and 164 nm) and were modified with PEG to improve the half a life and blood circulation.
Firstly, the physicochemical properties of SiNPs with different sizes were characterized using dynamic light scattering (DLS) and transmission electron microscopy (TEM).
Then, SiNPs were injected at the dosage of 20 mg/kg by tail intravenous administration for 4 h blood circulation in BALB/c mice. The biodistributions of silica element in different organs, i.e., heart, liver, spleen, lung, kidney, and brain, were examined using ICP-OES (inductively coupled plasma - optical emission spectrometer). The liver and lung easily trapped SiNP_58 nm, whereas SiNP_68 nm was mostly captured by the kidney. SiNP_122 nm had the highest concentration in the heart and spleen. SiNP_142 nm and SiNP_164 nm showed they either the second lowest or the lowest accumulation in the surveyed organs. ICP-OES results from the brain tissues indicated that the brain mainly captured SiNP_58 nm, and the accumulation of silica decreased as particle size increased from SiNP_68 nm to SiNP_164 nm. Moreover, TEM results from brain sections indicated that the number of vesicles and the size of vesicles were increased in the endothelial cells, and that the transcytosis was related to mitochondria.
Next, to examine size-dependent endothelium permeability, the crossing efficiency of the SiNPs was examined in vitro with an hCMEC/D3 BBB transwell model. ICP-OES measurements of the amount of silica transported through the BBB transwell model indicated that SiNP_58 nm had the highest efficiency in penetrating the BBB. As the size of silica nanoparticles increase from 68 nm to 164 nm, the transport efficiency of the silica nanoparticles decreases. TEM observations of hCMEC/D3 cells indicated that they trapped more SiNP_58 nm than SiNP_35 nm or SiNP_68 nm, and a larger percentage of the SiNP_58 nm particles were found in vesicles. Therefore, from the cellular point of view, among the six sizes of SiNPs, 58 nm SiNPs crossed the BBB with the highest efficiency.
Finally, the cellular fates of SiNP_58 nm and the biological mechanisms by which it penetrates the BBB were investigated by conducting RNA-Sequencing, western blot (WB), and real-time PCR (RT-PCR) and compared with results from SiNP_35 nm. The results confirmed that both sizes of SiNPs penetrated the BBB via caveolin-mediated transcytosis. However, SiNP_58 nm induced greater enhancement of transcytosis process. On the other hand, SiNP_35 nm activated the up-regulation of some genes that suppressed transcytosis, such as MFSD2A. The results of RNA-Seq suggested that particles of both sizes induced inflammatory responses, but no obvious cell death or individual abnormalities were observed.
Overall, this study systematically characterizes size-dependent transcytosis across the BBB using different sizes of SiNPs. The results provide guidance for the design of nanoparticles for targeted drug delivery to the brain.