Protein targeting receptor-mediated endocytosis

Figure 41 -15. Two types of endocytosis. An endocytotic vesicle (V) forms as a result of invagination of a portion of the plasma membrane. Fluid-phase endocytosis (A) is random and nondirected. Receptor-mediated endocytosis (B) is selective and occurs in coated pits (CP) lined with the protein clathrin (the fuzzy material). Targeting is provided by receptors (black symbols) specific for a variety of molecules. This results in the formation of a coated vesicle (CV).

Targeting to SECs should be directed at specific receptors present on this ceU type. A wide range of proteins and other molecules can be taken up by SECs through receptor-mediated endocytosis. For example, SECs play an important role in the uptake of degradation prodncts of the extracellular matrix. For this purpose they have hyaluronan [6], (pro)coUagen, and fi-bronectin receptors [7]. The first two receptors are nniqnely located on SECs. Elevated levels of serum hyaluronan and fibronectin, that are often fonnd in Uver disease [8], are nsnally the result of dysfunction of the clearance capacity of SECs combined with an increased production by HSCs [9]. 

Y. Kato and Y. Sugiyama, Targeted delivery of peptides, proteins, and genes by receptor-mediated endocytosis, Crit. Rev. Ther. Drug Carrier Syst. 74 287-331 (1997). 

Many putative receptors for anti-dsDNA on the membrane of various cell types have been noted. Some workers found a 30-kDa protein involved in the binding and internalization of [3H]DNA via receptor-mediated endocytosis (B12, B13). Other possible receptors include nucleosomes on human leukocytes (R8), Fc receptors on human T cells (A6), DNA on mouse and human mononuclear cells (06), a 94-kDa protein on several cells lines (J2), DNase-resistant target on human fibroblasts and PK 15 cells (K9), membrane determinant precisely resembling DNA in murine renal tubular cells (Zl), Hp8 on human and murine tubular cells (Z2), ribosomal P protein on rat and human glomerular mesangial cells (S30, S31), brush border myosin 110 kDa on rat hepatoma cells, and a diverse set of membrane proteins on a series of human tumor cell lines (R3). 

Not all lysosomal proteins take the normal route of protein targeting some end up being exported by the cell and must be retrieved. This scavenger pathway works as follows. The lysosomal glycoprotein binds to mannose 6-phosphate receptors in the plasma membrane and is internalized again by endocytosis (Fig. 5). This process, called receptor-mediated endocytosis, creates an endocytic vesicle (or endosome) that then delivers the lysosomal protein to the lysosome by fusion (see Topic E4). 

VLDLs are synthesized in the liver and transport triacylglycerols, cholesterol and phospholipids to other tissues, where lipoprotein lipase hydrolyzes the triacylglycerols and releases the fatty acids for uptake. The VLDL remnants are transformed first to IDLs and then to LDLs as all of their apoproteins other than apoB-100 are removed and their cholesterol esterified. The LDLs bind to the LDL receptor protein on the surface of target cells and are internalized by receptor-mediated endocytosis. The cholesterol, which is released from the lipoproteins by the action of lysosomal lipases, is either incorporated into the cell membrane or re-esterified for storage. High levels of intracellular cholesterol decrease the synthesis of the LDL receptor, reducing the rate of uptake of cholesterol, and inhibit HMG CoA reductase, preventing the cellular synthesis of cholesterol. 

The third subcategory of endocytosis is referred to as receptor-mediated. Receptors belong to a special class of cell surface proteins that utilize the endocytosis machinery to carry exogenous ligands into cells. When a specific cell surface receptor is overexpressed on a pathological cell, receptor-mediated endocytosis can be exploited for targeted drug delivery. 

Folate as a Targeting Device for Proteins Utilizing Folate Receptor-Mediated Endocytosis... 

Cholesterol, which is largely insoluble in aqueous m a, travels through the blood circulation in the form of Upoprotein complexes. The plasma lipoproteins are a family of globular particles that share common structural features. A core of hydrophobic lipid, principally triacylglycerols (triglycerides) and cholesterol esters, is surrounded by a hydrophilic monolayer of phospholipid and protein (the apolipoproteins) [1-3]. Lipid-apolipoprotein interactions, facihtated byi amphi-pathic protein helices that segregate polar from nonpolar surfaces [2,3], provide the mechanism by which cholesterol can circulate in a soluble form. In addition, the apolipoproteins modulate the activities of certain enzymes involved in Upoprotein metabolism and interact with specific cell surface receptors which take up Upopro-teins by receptor-mediated endocytosis. Differences in the Upid and apoUpoprotein compositions of plasma Upoproteins determine their target sites and classification based on buoyant density. 

In conformity with the sequential processing of bacterial protein toxins such as diphtheria or cholera toxin, the action of BoNT involves multiple discrete steps binding to surface receptors, internalization via receptor-mediated endocytosis, transport from endosome to cytosol, and cleavage of target proteins in the cytosol. " Binding and internalization are mediated by the C- and N-terminal domains of the BoNT H-chain, respectively. The L-chains have zinc metalloprotease activity, targeted selectively to one of three proteins that are required for the docking and fusion of synaptic vesicles with active zones at the cytoplasmic surface of the nerve terminal. 

The LDL lipoprotein complexes bind to receptor cells at specialized sites called LDL receptors. The receptors are clustered in structures called coated pits, which are abundant in the protein called clathrin. After binding to the LDL receptor, the LDL is internalized into the target cell by a process called receptor mediated endocytosis (Figure 18.10). 

---