細胞穿膜肽-說明
      穿透細胞膜進入細胞內(nèi)是許多作用靶點在細胞內(nèi)的生物大分子發(fā)揮作用的先決條件，然而生物膜的生物屏障作用阻止了許多高分子物質(zhì)進入細胞內(nèi)，從而很大程度地限制了這些物質(zhì)在治療領(lǐng)域的應(yīng)用。因此，如何引導(dǎo)這些物質(zhì)穿透細胞膜是一個迫切需要解決的問題，目前介導(dǎo)生物大分子穿透細胞膜的方法主要包括細胞穿透肽（cell penetrating peptides，CPPs）、脂質(zhì)體、腺病毒、納米顆粒、影細胞等，而CPPs是一類以非受體依賴方式，非經(jīng)典內(nèi)吞方式直接穿過細胞膜進入細胞的多肽，它們的長度一般不超過30個氨基酸且富含堿性氨基酸，氨基酸序列通常帶正電荷。
      1型人免疫缺陷病毒轉(zhuǎn)錄激活因子TAT（human immunodeficiency virus-1 transcription activator， HIV-1 TAT）是第一個被發(fā)現(xiàn)的細胞穿透肽，它憑借一種無毒的、高效的方式進入細胞。
      細胞穿透肽（cell penetrating peptides，CPPs）的一個重要特點是可以攜帶多種不同大小和性質(zhì)的生物活性物質(zhì)進入細胞，包括小分子化合物、染料、多肽、多肽核酸（peptide nucleo acid, PNA）、蛋白質(zhì)、質(zhì)粒DNA、siRNA、200nm的脂質(zhì)體、噬菌體顆粒和超順磁性粒子等，這一性質(zhì)為其成為靶向藥物的良好載體提供了可能。
      CPPs作為載體的優(yōu)勢在于低毒性和無細胞類型的限制，盡管CPPs可輸送不同類型的物質(zhì)進入細胞，但其實際應(yīng)用多集中于寡肽、蛋白質(zhì)、寡聚核苷（oligonucleotides，ONs)或類似物的細胞轉(zhuǎn)運。

跨膜機理
不同的細胞穿透肽（CPP）跨膜機制不同，一個細胞穿透肽（CPP）的具體機制有賴于幾個參數(shù)，如分子大?。〝y帶物質(zhì)）、溫度、細胞類型和細胞內(nèi)外的穩(wěn)定性等。細胞穿透肽（CPP）進入細胞的具體機制目前還不清楚，比較流行的推測包括以下三種：
A: 倒置膠粒模型（inverted micelle model），CPPs通過細胞膜上磷脂分子的移動形成倒置膠粒結(jié)構(gòu)，而進入胞漿。
B: 直接穿透，即孔隙結(jié)構(gòu)模型 (pore formation model)，CPPs在細胞膜上組成跨膜的孔隙結(jié)構(gòu)而進入胞漿 。
C: 內(nèi)吞方式進行細胞攝取。
來源: Cell-penetrating peptides and their therapeutic applications, Victoria Sebbage, BioscienceHorizons, Volume 2, Number 1, March 2009.

細胞穿透肽 HIV TAT
      細胞穿透肽（如HIV TAT）可以以直接穿透和內(nèi)吞兩種方式進入細胞。HIV TAT或者簡單的多聚精氨酸可被設(shè)計作為有效的藥物載體，但CPP（如HIV TAT）是如何實現(xiàn)胞膜轉(zhuǎn)運，目前仍不清楚。
簡單的HIV TAT是如何促進象直接穿透和內(nèi)吞作用的入胞機制的呢？來自Gerard Wong實驗室的研究人員研究了在不同的條件下，HIV TAT是如何與細胞質(zhì)膜、細胞骨架、特異的胞膜受體相互作用，從而誘導(dǎo)了多重轉(zhuǎn)運途徑。

      有趣的是，TAT在不同條件下可與同一序列發(fā)生多種不同的反應(yīng)，因而與胞膜、細胞骨架、特異受體相互作用可產(chǎn)生多種轉(zhuǎn)運途徑。
      CPP的跨膜機制與多肽序列存在很敏感的關(guān)系，如果在一個純親水性的CPP中增加一個疏水殘基，就能徹底地改變其轉(zhuǎn)運機制，例如，最簡單的CPP原型-多聚精氨基（polyR），可以誘導(dǎo)細胞膜上形成跨膜的孔隙結(jié)構(gòu)。疏水氨基酸通過插入胞膜來形成正曲率，精氨酸可同時形成正曲率和負曲率，賴氨酸只能沿一個方向形成負曲率，這就意味著在精氨酸與賴氨酸/疏水物之間存在補償關(guān)系。
      如果疏水性有助于形成負高斯曲率（Gaussian curvature），那為什么TAT肽中的疏水含量相對較低呢？其原因是CPPs都是利用盡可能少的疏水基去形成saddle-splay curvature。序列上的差異很可能只會在膜上誘導(dǎo)短暫的類似孔隙的跨膜結(jié)構(gòu)，從而形成對CPP來說更短的孔隙壽命。由于CPP的氨基酸組成不同，TAT肽在有或無受體情況下都可以介導(dǎo)細胞內(nèi)吞作用。

     專肽生物提供各類細胞穿膜肽序列，部分由現(xiàn)貨，例如TAT，R8，R4等，具體可咨詢銷售人員。

Definition

Cell permeable peptides (CPPs) are carriers with small peptide domains that can freely cross cell membranes.  They are mainly used as carriers of proteins and nucleic acids into the cell1.

Discovery

The first CPP was discovered independently by two laboratories in 1988 when it was found that the trans-activating transcriptional activator (Tat) from Human Immunodeficiency Virus 1 (HIV-1) could be efficiently taken up from the surrounding media by numerous cell types in culture2. 

Structural Characteristics

CPPs typically have an amino acid composition containing either a high relative abundance of positively charged, cationic amino acids such as lysine or arginine, or have sequences that contain an alternating pattern of polar/charged amino acids and non-polar, hydrophobic amino acids3.  Some examples include: TAT peptide-YGRKKRRQRRR, lipid membrane translocating peptide-KKAAAVLLPVLLAAP and Antennapedia leader peptide-KKWKMRRNQFWVKVQRG.

Classification

Numerous CPPs have been identified to date and they belong to a wide variety of protein families. For example, some CPPs are amphipathic protein family members3.

Mode of action

CPPs enter the cell with their carrier by either of three mechanisms:  Direct delivery that involves energy independent entry of the CPPs in to the cell4, endocytosis where the cells take up the CPPs by imbibing them with their cell membranes5 and translocation through the formation of transient structures which is yet to be understood6. 

Functions

CPPs have found numerous applications in medicine as drug delivery agents in the treatment of different diseases including cancer, virus inhibitors, contrast agents for cell labeling a classical example is Green Fluorescent protein GFP, as MRI contrast agents, quantum dots7.  TAT is very effective in delivering drugs in vitro and in vivo and so far a peptide that matches its efficiency has not been found7.

References

1.     Wagstaff KM and David JA (2006). Protein Transduction: Cell Penetrating Peptides and Their Therapeutic Applications, Current Medicinal Chemistry, 13 (12), 1371-1387.

2.     Feng S and Holland EC (1988). HIV-1 Tat trans-activation requires the loop sequence within Tar. Nature 334, 165–167.

3.     Stewart KM, Horton KL, Kelley SO (2008). Cell-penetrating peptides as delivery vehicles for biology and medicine, Org Biomol Chem., 6(13), 2242-55.

4.     Luo D, Saltzman WM (2000). Synthetic DNA delivery systems. Nat. Biotechnol, 18, 33-37.

5.     Lundberg M., Wikstrom S and Johansson M (2003). Cell surface adherence and endocytosis of protein transduction domains, Mol. Ther., 8, 143–150.

6.     Deshayes S, Gerbal-Chaloin S, Morris MC, Aldrian-Herrada G, Charnet P, Divita G (2004). On the mechanism of non-endosomial peptide-mediated cellular delivery of nucleic acids, Biochim. Biophys. Acta, 1667, 141–147.

7.     Temsamani J and Vida P (2004). The use of cell-penetrating peptides for drug delivery, Drug Discovery Today, 9 (23), 1012-1019.