Endocytic processes

A process in which a substance gains entry into a cell. Endocytic mechanisms are crucial for a variety of cellular functions such as the uptake of nutrients, regulation of cell surface expression of receptors, maintenance of cell polarity, and more. Receptor-mediated endocytosis via clathrin-coated pits is the most studied endocytic process, which is important for regulation of the time and magnitude of signals generated by a variety of cell-surface receptors. 

Most pathways in the endocytic system are shared with cells in general, but a special case exists in synaptic vesicle cycling, which is unique to neurons and a keystone in neuronal function. Three categories will be considered here endocytic processes important for degradation of macromolecules and uptake of nutrients constitutive and regulated neuroendocrine secretion and receptor-mediated endocytosis. Synaptic vesicle cycling will then be considered separately and in greater detail. 

This refers to the transport across the epithelial cells, which can occur by passive diffusion, carrier-mediated transport, and/or endocytic processes (e.g., transcytosis). Traditionally, the transcellular route of nasal mucosa has been simply viewed as primarily crossing the lipoidal barrier, in which the absorption of a drug is determined by the magnitude of its partition coefficient and molecular size. However, several investigators have reported the lack of linear correlation between penetrant lipophilicity and permeability [9], which implies that cell membranes of nasal epithelium cannot be regarded as a simple lipoidal barrier. Recently, compounds whose transport could not be fully explained by passive simple diffusion have been investigated to test if they could be utilized as specific substrates for various transporters which have been identified in the... 

Since the uptake of particles in nasal epithelial tissue is known to be mostly mediated by M cells, nasal administration has been investigated as a noninva-sive delivery of vaccines [37], However, since the uptake of naked DNA by endocytocis is limited, use of either nanoparticles as mucosal delivery systems [37] or hypotonic shock [38] is reported for the efficient transfection of gene and vaccine into the nasal epithelium. It was also reported that polypeptides and polypeptide-coated nanospheres (diameter about 500 nm) are transported through endocytic process in rat M cells [39],... 

Due to the close structural and functional relationships between yeast and mammalian endocytic processes, it is worthwhile to review the action of some of the major molecular players in endocytosis of yeast membrane proteins. These are the ubiquitin ligase Rsp5p (Nedd4 in mammals), Panlp, the putative ortholog of Eps-15 of mammals, and Vps23p, whose ortholog is Tsg-101. 

Then, by comparing the normalized EPIi and TIRFi curves in the presence of Baf A1, we calculated the kinetic of the movement of SLMVs out of EW field that we can consider as the first step of the endocytic process. The first derivative of the endocytosis curve shows the evolution of the speed of endocytic events during and after the stimulus they start at 600 ms during the stimulus, reach two maxima at 1.2 and 2.8 s and then end after 6 s. The bimodal distribution indicates the existence of two distinct phases of endocytosis and most probably, of two distinct modes of secretion. 

Active transport mechanisms exist in the gastrointestinal tract and other epithelial sites, for the absorption of di- and tri-peptides. As described above, a greater understanding of the molecular specificity of this carrier could provide important leads for the delivery of peptides. Proteins and large peptides may be transported across cells via endocytic processes. Transcytosis is achieved if the endocytic vesicles can reach the basal membrane without fusion with lysosomes. However, various studies have shown that in the majority of cases the internalized protein is degraded, indicating that the transcytotic pathway is a minor one and most of the endocytosed protein is subject to lysosomal degradation. 

Other transcellular mechanisms of absorption include carrier-mediated transport and endocytic processes. Although it is well known that carrier-mediated transport systems exist for di- and tripeptides in the intestine, there is still no evidence for carrier-mediated transport of peptides across the vaginal mucosa, although prostaglandins have been demonstrated to utilize such a mechanism. Although there must be some type of endocytic transport of endogenous peptides into the epithelial cells in order to regulate proliferation, no receptor-mediated or bulk-fluid mechanisms have been reported. 

Hydrophilic compounds may be absorbed via the paracellular route, moving between the epithelial cells via passive diffusion whereas lipid soluble drags are usually absorbed transcellularly, at rates which correlate with their lipid/water diffusion coefficients. Macromolecules may be absorbed via endocytic processes. 

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