Caveolin mediated endocytosis

It has also been proposed that CPP are taken up by clathrin-independent mechanisms such as caveolin-mediated endocytosis (100). TAT-containing liposomes were colocalized with caveolin 1, a marker for caveolar endocytosis, but not with markers for clathrin (101). An inhibitor of caveolin and nystatin reduced TAT peptide reporter in HepG2 and CHO cells by 50% (80). However, it was pointed out by Brooks et al. that HepG2 cell lines do not contain caveolin 1 (102,103). The caveolin pathways relevance to CPP uptake may depend on the cell type because nystatin inhibits... 

It has been reported that NP internalised via clathrin-dependent endocytosis were generally entrapped within the endosomes and then fused with lysosomes, resulting in lysosomal degradation of the load [74]. On the other hand, NP internalised by caveolin-mediated endocytosis could normally bypass lysosomal degradation [75]. It was also shown that polymeric micelles incorporating nile red (M-NR) were capable of colocalisation with tubulin, an important protein that shapes the microtubule network of cells this colocalisation might be advantageous for M-NR uptake via dynamin and caveolin-mediated endocytosis [76, 77]. 

Apart from clathrin- and caveolin-mediated endocytosis, there also exist multiple other pathways independent of clatherin and caveoli-1 that also play an important role in nanoparticle endocytosis. Exploring the clathrin- and caveolin-independent uptake mechanisms has only recently gained attention in endocytosis research, and these less-understood uptake mechanisms might have implications for dmg delivery research. 

Figure 3.8 (a) Clathrin-mediated endocytosis in which typical vesicles are approximately 200 nm diameter. Reproduced with permission from Ref [32] (b) Caveolin-mediated endocytosis in which typical vesicles are approximately 80nm diameter. Both mechanisms have potential for the uptake of nanostructures. Reproduced with permission from Ref [33],... 

Depending on the mechanism, endocytosis can also be classified into clathrin-mediated endocytosis [34], caveolin-mediated endocytosis [35], and clathrin- and caveolin-independent endocytosis [36]. Clathrin-mediated endocytosis is a major entry mechanism, and also the most intensely investigated mechanism among the various forms of endocytosis. Although clathrin molecules are involved in most receptor-mediated endocytosis and nonspecific endocytosis, the detailed endocytic mechanism of nonviral carriers has not been well understood yet and has been shown to vary with the size, surface charges, and other characteristics of the carriers. Many carriers use several mechanisms simultaneously, probably due to the intrinsic heterogeneity of the supramolecular structure [37]. 

LDH-FITC is well overlapped with red fluorescent clathrin-TR, but not with caveolin-1-TR (Figure 13.10). This is dear evidence that clathrin-mediated endocytosis is the prindpal mechanism for the cellular internalization of LDH particles. Caveolae-mediated endocytosis, if any, seems not to be responsible for LDH uptake. 

Receptor-Mediated Endocytosis. Mammalian cells have developed an assortment of mechanisms to facilitate the internalization of specific substrates and target these to defined locations inside the cytoplasm. Collectively, these processes of membrane deformations are termed "endocytosis," consisting of phagocytosis, pinocytosis, receptor-mediated endocytosis (clathrin-mediated), and potocytosis [nonclathrin (caveolin)-mediated RME]. The emphasis of this section is receptor-mediated endocytosis in the intestinal tract, but the interested student may consult alternative reviews covering the complete spectrum of endo-cytotic processes in other cell types (17,18). 

Caveolin Family of proteins mediating caveolin-dependent endocytosis. 

Vesicular transport occurs when a membrane completely surrounds a compound, particle, or cell and encloses it into a vesicle. When the vesicle fuses with another membrane system, the entrapped compounds are released. Endocytosis refers to vesicular transport into the cell, and exocytosis to transport out of the cell. Endocytosis is further classified as phagocytosis if the vesicle forms around particulate matter (such as whole bacterial cells or metals and dyes from a tattoo), and pinocy-tosis if the vesicle forms around fluid containing dispersed molecules. Receptor-mediated endocytosis is the name given to the formation of clathrin-coated vesicles that mediate the internalization of membrane-bound receptors in vesicles coated on the intracellular side with subunits of the protein clathrin (Eig. 10.14). Potocytosis is the name given to endocytosis that occurs via caveolae (small invaginations or caves ), which are regions of the cell membrane with a unique lipid and protein composition (including the protein caveolin-1). 

A typical feature of caveola-mediated endocytosis is the formation of non-coated invaginations composed of detergent-resistant membrane components rich in cholesterol and sphingolipids, known as lipid rafts [56]. The importance of the caveola-mediated mechanism in the PTD-mediated internalization was confirmed in an experiment where the cellular uptake of Tat peptide was affected by drugs that either disrupt lipid rafts or alter caveolar trafficking [57]. Moreover, Tat-PTD-fused protein showed colocalization with a marker of caveolar uptake, caveolin, further strengthening the importance of the mechanism in the PTD-mediated internalization [57]. 

Cellular internalization mechanism can be grouped as follows (i) phagocytosis, that is, uptake of large particles (in the order of a few micrometers), which is restricted to specialized cells (e.g., macrophages) and (ii) pinocytosis, that is, molecules are taken up by cells by means of fluid-phase endocytosis, clathrin-assisted and receptor-mediated endocytosis (-120 nm), caveolin-assisted and receptor-mediated endocytosis (-60 nm), and clathrin and caveoUn-independent endocytosis (-90 nm) (Alberola and Radler 2009, Bareford and Swaan 2007, Conner and Schmid 2003). 

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