Cell-Penetrating Peptides Selectively Cross the Blood-Brain Barrier In Vivo

Cell-penetrating peptides (CPPs) are a group of peptides, which have the ability to cross cell membrane bilayers. CPPs themselves can exert biological activity and can be formed endogenously. Fragmentary studies demonstrate their ability to enhance transport of different cargoes across the blood-brain barrier (BBB). However, comparative, quantitative data on the BBB permeability of different CPPs are currently lacking. Therefore, the in vivo BBB transport characteristics of five chemically diverse CPPs, i.e. pVEC, SynB3, Tat 47–57, transportan 10 (TP10) and TP10-2, were determined. The results of the multiple time regression (MTR) analysis revealed that CPPs show divergent BBB influx properties: Tat 47–57, SynB3, and especially pVEC showed very high unidirectional influx rates of 4.73 μl/(g × min), 5.63 μl/(g × min) and 6.02 μl/(g × min), respectively, while the transportan analogs showed a negligible to low brain influx. Using capillary depletion, it was found that 80% of the influxed peptides effectively reached the brain parenchyma. Except for pVEC, all peptides showed a significant efflux out of the brain. Co-injection of pVEC with radioiodinated bovine serum albumin (BSA) did not enhance the brain influx of radiodionated BSA, indicating that pVEC does not itself significantly alter the BBB properties. A saturable mechanism could not be demonstrated by co-injecting an excess dose of non-radiolabeled CPP. No significant regional differences in brain influx were observed, with the exception for pVEC, for which the regional variations were only marginal. The observed BBB influx transport properties cannot be correlated with their cell-penetrating ability, and therefore, good CPP properties do not imply efficient brain influx.细胞穿透肽（cell-penetrating peptides, CPPs）是一类能够跨细胞膜双层结构的肽类物质，自身可发挥生物学活性且可内源性生成。现有零散研究证实，这类肽能够增强不同负载物跨血脑屏障（blood-brain barrier, BBB）的转运能力，但目前仍缺乏不同CPPs血脑屏障通透性的对比性定量数据。为此，本研究针对五种化学结构各异的CPPs——即pVEC、SynB3、Tat 47–57、转运肽10（transportan 10, TP10）以及TP10-2——的体内血脑屏障转运特性展开测定。多次时间回归（multiple time regression, MTR）分析结果显示，不同CPPs的血脑屏障内流特性存在显著差异：Tat 47–57、SynB3，尤其是pVEC，展现出极高的单向内流速率，分别为4.73 μl/(g × min)、5.63 μl/(g × min)及6.02 μl/(g × min)；而转运肽类似物的脑内流速率则处于可忽略至较低水平。采用毛细管清除法（capillary depletion）检测发现，内流进入脑内的肽类中有80%可有效抵达脑实质。除pVEC外，其余所有肽类均表现出显著的脑内外流现象。将pVEC与放射性碘标记牛血清白蛋白（radioiodinated bovine serum albumin, BSA）共注射后，并未提升放射性标记BSA的脑内流效率，这表明pVEC自身并不会显著改变血脑屏障的特性。通过共注射过量非放射性标记CPPs的实验，未证实存在可饱和的转运机制。除pVEC仅存在微小的区域差异外，未观察到脑内流存在显著的区域差异。本研究观测到的血脑屏障内流转运特性与其细胞穿透能力并无关联，因此，优异的CPP性能并不意味着其具备高效的脑内转运能力。