表面电荷对toxicity和uptake的影响及原理

Technorati Tags: drug delivery,cytotoxicity,cell uptake,surface charge

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把一个这么简单的事情描述的这么复杂，看的头都晕了。不就是正电荷增加了uptake同时增加了toxicity吗？而且没有给出最佳的正电荷比例。

亮点是第二张图，我觉得这么个办法调整表面电荷挺聪明的。

Nanoparticle penetration into cell membranes is an interesting phenomenon that may have crucial implications on the nanoparticles’ biomedical applications. In this paper, a coarse-grained model for gold nanoparticles (AuNPs) is developed (verified against experimental data available) to simulate their interactions with model lipid membranes. Simulations reveal that AuNPs with different signs and densities of surface charges spontaneously adhere to the bilayer surface or penetrate into the bilayer interior. The potential of mean force calculations show that the energy gains upon adhesion or penetration is significant. In the case of penetration, it is found that defective areas are induced across the entire surface of the upper leaflet of the bilayer and a hydrophilic pore that transports water molecules was formed with its surrounding lipids highly disordered. Penetration and its concomitant membrane disruptions can be a possible mechanism of the two observed phenomena in experiments:

1. AuNPs bypass endocytosis during their internalization into cells and cytotoxicity of AuNPs.
2. It is also found that both the level of penetration and membrane disruption increase as the charge density of the AuNP increases, but in different manners. The findings suggest a way of controlling the AuNP−cell interactions by manipulating surface charge densities of AuNPs to achieve designated goals in their biomedical applications, such as striking a balance between their cellular uptake and cytotoxicity in order to achieve optimal delivery efficiency as delivery agents.

It is clear that the severe disruption on the bilayer is caused by the strong attractions between the terminals of AuNPs’ cationic ligands (ammonium) and the phosphate groups of DPPC and DPPG. In a recent experiment, it is found that highly charged cationic AuNPs are able to generate holes on supported lipid bilayers.These bilayers are supported on a mica surface, which carries negative net charges that provide the bilayer with a similar electric feature to that of a PC/PG bilayer. Although the size of holes observed in the experiment are larger than that the simulation, they both indicate a highly disruptive nature of cationic AuNPs shown to negative bilayers, which shows qualitative agreement to each other.

Both the level of penetration and disruption goes higher as the AuNPs’ surface charge increases but in different manners. At the cationic coverage below 50%, the increase of the penetration is prominent. . The particle is already “inside” the bilayer when the coverage has reached 50%. However, further increase of coverage promotes penetration to a much lesser degree. Even at 100% cationic coverage, the particle still resides in the bilayer interior and does not move downward to breakout the lower leaflet of the bilayer.

By contrast, the disruption on the membrane is not significant until the coverage reaches around 60%. Further increase of coverage results in severe membrane disruption which is clearly visible.

the influence of surface charge density of a AuNP on membrane can be divided into two stages.

1. In the first stage where the AuNPs have lower charge densities, the effect of surface charge mainly contributes to penetration.
2. In the second stage where AuNPs have higher charge densities, the effect of surface charge mainly contributes to membrane disruption since further penetration is not possible. The finding may provide us a clue on how to avoid high toxic effect of AuNPs while achieve certain goal in their biomedical applications.

A recent experimental study showed that the membrane affinity constant of cationic AuNPs is three times greater than that of anionic AuNPs in human cancer cell lines.(Cho, E. C.; Xie, J.; Wurm, P. A.; Xia, Y. Understanding the Role of Surface Charges in Cellular Adsorption versus Internalization by Selectively Removing Gold Nanoparticles on the Cell Surface with a I2/KI Etchant Nano Lett. 2009, 9, 1080– 1084)

cationic AuNPs are, on average, 27 times more toxic than their anionic counterparts in three different cell lines.(Hauck, T. S.; Ghazani, A. A.; Chan, W. C. Assessing the Effect of Surface Chemistry on Gold Nanorod Uptake, Toxicity, and Gene Expression in Mammalian Cells Small 2008, 4, 153– 159)

TOXICITY vs surface charge

1. First, cationic AuNPs have higher adhesion to cell membranes than anionic AuNPs do, which is also a reason for their high uptake;
2. second, their membrane disruption ability is far more significant than that of their anionic counterparts. AuNPs with high cationic surface coating can disrupt a bilayer membrane to a great extent, which subsequently compromises the membrane integrity and thus breach the hydrophobic barrier. The hydrated channel will lead to the exchange of medium between extracellular fluid and cytosol, which may cause acute cytotoxicity.(21)
3. Although not enough to compromise bilayer integrity, anionic AuNPs are capable of altering cell functions by inducing changes on membrane protein properties as well as bilayer properties, which can in turn affect the functioning of membrane proteins (see ref 37 for an in-depth review). Changes on the functioning of membrane proteins are able to alter cell functions substantially, which may be one of the reasons for the observed minor cytotoxicity of anionic AuNPs, since the AuNPs are still able to induce negative effects once inside the cells.(9)