Transporter phenotyping

Yankaskas, J. R., and R. C. Boucher. 1990. Transformation of airway epithelial cells with persistence of cystic fibrosis or normal ion transport phenotypes. Methods Enzymol 192 565-71. 

Crucial data regarding the physiological relevance of MDR transporters came from knockout mice studies. Surprisingly, loss of these genes does not result in an obvious phenotype MDR knockout mice are viable and... 

Even though dynein, kinesin, and myosin serve similar ATPase-dependent chemomechanical functions and have structural similarities, they do not appear to be related to each other in molecular terms. Their similarity lies in the overall shape of the molecule, which is composed of a pair of globular heads that bind microtubules and a fan-shaped tail piece (not present in myosin) that is suspected to carry the attachment site for membranous vesicles and other cytoplasmic components transported by MT. The cytoplasmic and axonemal dyneins are similar in structure (Hirokawa et al., 1989 Holzbaur and Vallee, 1994). Current studies on mutant phenotypes are likely to lead to a better understanding of the cellular roles of molecular motor proteins and their mechanisms of action (Endow and Titus, 1992). 

Cells Binding, adsorption, partitioning Physical dimensions Metabolism Monolayer integrity Membrane domain characteristics (polarity) surface area transporters, receptors lipid composition charge Cell phenotype and culture conditions... 

Acetylcholine synthesis and neurotransmission requires normal functioning of two active transport mechanisms. Choline acetyltransferase (ChAT) is the enzyme responsible for ACh synthesis from the precursor molecules acetyl coenzyme A and choline. ChAT is the neurochemical phenotype used to define cholinergic neurons although ChAT is present in cell bodies, it is concentrated in cholinergic terminals. The ability of ChAT to produce ACh is critically dependent on an adequate level of choline. Cholinergic neurons possess a high-affinity choline uptake mechanism referred to as the choline transporter (ChT in Fig. 5.1). The choline transporter can be blocked by the molecule hemicholinium-3. Blockade of the choline transporter by hemicholinium-3 decreases ACh release,... 

The Fur protein regulates iron uptake systems in many Gram-negative bacteria. The striking phenotype of the first fur mutants isolated was the overexpression of the outer membrane receptors for siderophore iron transport. In addition, excretion of siderophores under iron-rich growth conditions was observed in these mutants, indicating that the biosynthesis of siderophores is also regulated by Fur. 

Another human colonic cancer cell line is T84 this develops monolayers of high TER ( 1000 Q cm2) when grown on permeable supports, but cells are not well differentiated and have been described as resembling a colonic crypt cell phenotype. Hence, these cells have been used mainly in studies of epithelial ion transport and are generally not considered to be adequate for drug transport studies, particularly with respect to carrier-mediated processes [10, 79, 82-84]. 

Wang, Y., et al. Functional analysis of mutations in the OCTN2 transporter causing primary carnitine deficiency lack of genotype-phenotype correlation. Hum. Mutat. 2000, 16, 401-407. 

As SNP discovery technologies become mainstream research tools, the importance of genotype-phenotype relationships will continue to be a focus in pharma-cogenomic research. Not only will characterization of drug transporter polymorphisms enhance our insight of the molecular mechanisms involved in transporter function, it is likely that such findings will become important components of individualized drug therapy in the future. 

Fetsch P, Abati A, Ross DD et al. The multidrug-resistant phenotype associated with overexpression of the new ABC half-transporter, MXR (ABCG2). J Cell Sci 2000 113(Pt 11) 2011-2021. 

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