Cell-surface expression

Recent studies indicate that - like many other receptors - G-protein-coupled receptors may form dimers, either homodimers or dimers with another type of receptor. The role of dimer formation in the cell surface expression of receptors and in their signalling and the resultant pharmacology are currently under intensive investigation [1]. 

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. 

PAMPs and various tissue factors can prime DCs to produce T-cell-polarizing factors [21], IL-12 is a pro-inflammatory cytokine that induces IFN-y and promotes the development of Thl-cell differentiation [31], Other Thl-polarizing factors are IFN-a and IFN-(3 [32] and cell-surface expressed intracellular adhesion molecule (ICAM)-l [33]. On the other hand, it has been shown that NF-kB inducing kinase (NIK), which is known to regulate B-cell maturation and lymphoid organogenesis, is important for the induction of Thl7 cells [34],... 

HFE has been shown to be located in cells in the crypts of the small intestine, the site of iron absorption. There is evidence that it associates with P2 niicroglobu-lin, an association that may be necessary for its stability, intracellular processing, and cell surface expression. The complex interacts with the transferrin receptor (TfR) how this leads to excessive storage of iron when HFE is altered by mutation is under close smdy. The mouse homolog of HFE has been knocked out, resulting in a potentially useful animal model of hemochromatosis. 

N-Glycosylation secures folding and cell surface expression. 

Daws L., Callaghan P., Morom J. et al. Cocaine increases dopamine uptake and cell surface expression of dopamine transporters. Biochem. Biophys. Res. Commun. 290 1545, 2002. 

Park, M. and Tenner, A.J. (2003) Cell surface expression of ClqRP/CD93 is stabilized by O-glycosylation. Journal of Cellular Physiology, 196 (3), 512—522. 

C-termini and a large glycosylated extracellular loop between transmembrane domains 3 and 4. The proteins show the most homology in their transmembrane spanning domains, particularly domains 1, 2, and 4-8, which may be involved in moving the transmitter across the membrane. The transporters are substrates for PKC-dependent phosphorylation, which reduces their activity. The dopamine transporter is phosphorylated on the extreme end of the N-terminal tail, and consensus phosphorylation sites for various other kinases are present in the intracellular loops and domains [20-22] (Fig. 12-4). The dopamine and norepinephrine transporters form functional homo-oligomers, although it is not known if this is required for transport activity, and the transporters also interact with many other membrane proteins that may control their cell-surface expression or other properties. 

Ramprasad, M.R, et al., Cell surface expression of mouse macrosialin and humand CD68 and their role as macrophage receptors for oxidized low density lipoprotein, Proc. Natl. Acad. Sci. USA, 93, 14833, 1996. 

NE and EPI stimulate a- and (TAR on the cell surface of target tissues. P2-AR are expressed on almost all types of immune cells, with the notable exception of T-helper (Th)2 clones [3], P-AR on immunocytes are coupled with Gs proteins and adenylate cyclase, with subsequent activation increasing intracellular adenosine 3 , 5 -cyclic monophosphate (cAMP) and protein kinase A (PKA). Under normal conditions, P-AR cell surface expression up- and down-regulates in response to reduced and increased catecholamine... 

Fan, G.-F., Ray, K., Zhao, X., Goldsmith, R K and Spiegel, A. M. (1998) Mutational analysis of the cysteines in the extracellular domian of the human Ca2+ receptor effects on cell surface expression, dimerization and signal transduction. FEBSLett. 436, 353-356. 

Connolly, C. N., Krishek, B. J., McDonald, B. J., Smart, T. G., and Moss, S. J. (1996) Assembly and cell surface expression of heteromeric and homomeric y-aminobutyric acid type A receptors. J. Biol. Chem. 271, 89-96. 

Davis, K. E., Straff, D. J., Weinstein, E. A., Bannerman, P. G., Correale, D. M., Rothstein, J. D and Robinson, M. B. (1998) Multiple signaling pathways regulate cell surface expression, and activity of the excitatory amino acid carrier 1 subtype of Glu transporter in C6 glioma. J. Neurosci. 18,2475-2485. 

Difficulties in detecting nucleolin on the cell surface could be explained by its very low concentration in this compartment (Hovanessian et al, 2000). Moreover, this cell surface expressed nucleolin protein has a different isoelectric point and it is recognized by only one monoclonal antibody (mAb D3) in its native conformation (Hovanessian et al, 2000). This probably reflects specific post-translational modifications undergone by nucleolin on the cell surface. Consistent with this hypothesis, extracellular nucleolin is a substrate of ecto-protein kinases including casein kinase II (Dumler et al, 1999 Jordan et al., 1994). Interestingly, indirect evidence suggests... 

Hovanessian AG, Puvion-Dutilleul F, Nisole S, Svab J, Perret E, Deng JS, Krust B (2000) The cell-surface-expressed nucleolin is associated with the actin cytoskeleton. Exp Cell Res 261 312—328 Iftode C, Daniely Y, Borowiec JA (1999) Replication protein A (RPA) the eukaryotic SSB. Crit Rev Biochem Mol Biol 34 141-180... 

The above discussion provides only a brief overview of how fluorescence techniques can be used to study the interactions of ligands with their receptors. We have focused on the quantitation of the binding parameters and compared the data with that which may be obtained with those from radiolabelled ligand binding studies. The number of applications of fluorescence in the study of neurochemistry and molecular biology is ever increasing. Outside the scope of this review is, for example, the use of fluorescence microscopy to monitor cell surface expression and targeting of receptors or the use of fluorescence probes to monitor ion transport into and out of cells. 

Kumar, A., Dhawan, S., Hardegen, N.J., and Aggarwal, B.B., Curcumin (diferuloylmethane) inhibition of tumor necrosis factor (TNF)-mediated adhesion of monocytes to endothelial cells by suppression of cell surface expression of adhesion molecules and of nuclear factor-KB activation, Biochem. Pharmacol, 55, 775, 1998. 

It is also important to note that the coagulation mechanism in vivo does not occur in solution, but is localized to activated cell surfaces expressing anionic phospholipids such as phosphatidylserine, and is mediated by Ca2+ bridging between the anionic phospholipids and 7-carboxyglutamic acid residues of the clotting factors. This is the basis for using calcium chelators such as ethylenediamine tetraacetic acid (EDTA) or citrate to prevent blood from clotting in a test tube. 

From electrophysiological studies with in vitro expressed NMDA subunits in cellular systems, it has been concluded that a conventional NMDA receptor must consist of a mixed combination of NR1 splice variants and NR2A-d subunits in order to have full physiological activity (Monyer et al., 1992). This NR1/NR2 expression pattern has also been reported to be a prerequisite for adequate cell surface expression of NMDA receptors (Mcllhinney et al., 1996). Given the tetrameric stoichiometry, any conventional NMDA receptor might consist of two NR1 and two NR2 subunits. 

Mcllhinney, R. A. J., Molnar, E., Atack, J. R., Whiting, P. J. Cell surface expression of the human N-methyl-D-aspartate receptor subunit 1a requires the co-expression of the NR2A subunit in transfected cells, Neuroscience 1996, 70, 989-997. 

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