Invagination processes

The concept of receptor-mediated endocytosis was new, and pointed the way ahead for cell biology of the 20th century. This general mechanism enables cells to incorporate large molecules, bound to specific cell-surface receptors, via a membrane invagination process. LDL receptors are found on almost all cells of the body. Those in the fiver are however crucial for homoeostasis, since this is where excess of cholesterol is metabolised to bile adds and excreted. 

On cooling the hollow spheres gelatinize and the spontaneous degeneration now no longer follows. Invagination processes now occur in these gelatinized objects under all sorts of conditions (Fig. 32). Among these phenomena are some, the cause of which one is inclined to seek in an osmotic abstraction of water from the vacuole. The inv ination is then very deep (for example Fig. 32d). This occurs, for example... 

This process occurs in the mitochondria, organelles present in the cells of all multicellular organisms (see Figs 6.8 and 6.26). Mitochondria have two membranes. The invaginations of the internal membrane into the inner space of the organelle (matrix space) are termed crests (from the Latin, cristae). 

In the classic model of synaptic vesicle recycling in nerve terminals, synaptic vesicles fuse completely with the plasma membrane and the integrated vesicle proteins move away from the active zone to adjacent membrane regions (Fig. 9-9A). In these regions, clathrin-mediated synaptic vesicle endocytosis takes place rapidly after neurotransmitter release (within seconds) [64]. The process starts with the formation of a clathrin-coated pit that invaginates toward the interior of the cell and pinches off to form a clathrin-coated vesicle [83]. Coated vesicles are transient organelles that rapidly shed their coats in an ATP/chaperone dependent process. Once uncoated, the recycled vesicle fuses with a local EE for reconstitution as a synaptic vesicle. Subsequently, the recycled synaptic vesicle is filled with neurotransmitter and it returns to the release site ready for use. This may be the normal pathway when neurotransmitter release rates are modest. Clathrin/ EE-based pathways become essential when synaptic proteins have been incorporated into the presynaptic plasma membrane. 

Most amoeboid protozoa assimilate particles by invagination of the plasma membrane in an actin-based phagocytotic process and digest them in food... 

The process of transcytosis is illustrated in Figure 2.3 for the transferrin receptor (TfR) [37]. The receptor is heavily expressed at the BBB compared to other vascular beds [38]. Transferrin or a monoclonal antibody to the extracellular domain of the receptor protein will bind from the luminal side of the BBB. This triggers cellular uptake by the mechanism of receptor-mediated endocytosis, i.e. the invagination and budding off of parts of the cell membrane as a result of the formation of small vesicles (endosomes). The transceUular passage of ligand (transcytosis) is completed by exocytosis at the abluminal membrane, and the whole process is completed within minutes in vivo. 

VLDLs, IDLs, and LDLs are closely related to one another. VLDLs formed in the liver (see p. 312) transport triacylglycerols, cholesterol, and phospholipids to other tissues. Like chylomicrons, they are gradually converted into IDL and LDL under the influence of lipoprotein lipase [1]. This process is also stimulated by HDL. Cells that have a demand for cholesterol bind LDL through an interaction between their LDL receptor and ApoB-100, and then take up the complete particle through receptor-mediated endocytosis. This type of transport is mediated by depressions in the membrane ( coated pits"), the interior of which is lined with the protein clathrin. After LDL binding, clathrin promotes invagination of the pits and pinching off of vesicles ( coated vesicles"). The clathrin then dissociates off and is reused. After fusion of the vesicle with ly-sosomes, the LDL particles are broken down (see p. 234), and cholesterol and other lipids are used by the cells. 

Uptake of low-density lipoprotein has been studied479 as a prototype of a receptor-mediated pathway for internalization of external macromolecules. It is a coupled process by which selected, extracellular proteins or peptides are first bound to specific, cell-surface receptors, and then rapidly internalized by the cell. Internalization follows clustering of receptors in specialized regions of the cell surface, called coated pits, that invaginate, to form coated vesicles. 

A primary function of the lysosome is to digest protein-containing particles derived from the extracellular space. One mechanism of delivery is the process of endocy-tosis. Endocytosis is the invagination of a group of occupied... 

Growth Pattern Bacteria reproduce predominantly by a process known as binary fission as illustrated in Figure 5.3. This process involves several steps cell elongation, invagination of the cell wall, distribution of nuclear material, formation of the transverse cell wall, distribution of cellular material into two cells, and separation into two new cells. This is an asexual reproductive process. 

Certain cells have the ability to transport substances across their membranes through processes such as endocytosis. Here the drug is engulfed by the cell via an invagination of the cell membrane. Although limited in scope, this method does allow certain large, nonlipid-soluble drugs to enter the cell. 

Once a drug is conjugated to a macromolecule, it cannot enter the cell by simple diffusion but rather requires endocytosis, which could be enhanced in cancerous cells by overexpression of receptors. Therefore, to achieve cellular targeting, macromolecules require access to the interior of a cell through endocytosis, a process in which a portion of the plasma membrane invaginates to create a new intracellular vesicle (Fig. 12.6). Internalization via endocytosis can occur in one of two ways ... 

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