Apical membrane polarity

ENaC is located in the apical membrane of polarized epithelial cells where it mediates Na+ transport across tight epithelia [3], The most important tight epithelia expressing ENaC include the distal nephron of the kidney, the respiratory epithelium, and the distal colon. The basic function of ENaC in polarized epithelial cells is to allow vectorial transcellular transport of Na+ ions. This transepithelial Na+ transport through a cell involves... 

The taste bud is a polarized structure with a narrow apical opening, termed the taste pore, and basolateral synapses with afferent nerve fibers. Solutes in the oral cavity make contact with the apical membranes of the TRCs via the taste pore. There is a significant amount of lateral connectedness between taste cells within a bud both electrical synapses between TRCs and chemical synapses between TRCs and Merkel-like basal cells have been demonstrated to occur [39]. Furthermore, there are symmetrical synapses between TRCs and Merkel-like basal cells [39]. In addition, these basal cells synapse with the afferent nerve fiber, suggesting that they may function in effect as interneurons [39]. The extensive lateral interconnections... 

In the transport assays, the permeability of a compound in both absorption and secretion directions is measured using polarized epithelial cells that constitutively express high levels of P-gp (e.g. Caco-2) or have been transfected with the gene for a specific P-gp (e.g. MDR1-transfected MDCK or LLC-PK1 cells). Since P-gp is expressed on the apical membrane, ratios ofbasolateral-to-apical (B —> A) permeability versus apical-to-basolateral (A —> B) permeability greater than 1 may indicate an active efflux transport process. Bidirectional permeability measurements can also be performed in the presence of a specific P-gp inhibitor. Thus, apical-to-basolateral permeability increases and basolateral-to-apical permeability decreases such that... 

Olsen JC, Johnson LG, Stutts MJ, Sarkadi B, Yankaskas JR, Swanstrom R, Boucher RC (1992) Correction of the apical membrane chloride permeability defect in polarized cystic fibrosis airway epithelia following retroviral-mediated gene transfer. Hum Gene Ther 3(3) 253-266. 

So far GPI biosynthesis raises many questions as its correct regulation is crucial for viability in yeast and for embryonic development in mammals. Many steps remain unclear, also due to the fact that most of the proteins involved still have to be characterized. The same is true for the biological function of the GPI anchor apart from membrane insertion. Several suggestions have been made according to which GPI anchors are targeting lipid rafts, specific intracellular compartments, or the apical membrane of polarized epithelial cells. 

Burton PS, Conradi RA, Hilgers AR, et al. Evidence for a polarized efflux system for peptides in the apical membrane of Caco-2 cells. Biochem Biophys Res Com-mun 1993 190(3) 760-766. 

Iron transport across the intestinal cell occurs at both the apical and basolateral interfaces. Figure 31-1 highlights the importance of this polarity in both iron transport into the cell and the sensing of iron stores in both the villus and crypt enterocytic apical and basolateral interfaces. The apical membrane is specialized to transport heme and ferrous iron into the cell through three major pathways. The first is via DMT-l, which transports ferrous iron and divalent metal ions into the enterocyte. Iron can also be absorbed as the intact heme moiety,... 

Meder D, Moreno MJ, Verkade P, Vaz WLC, Simons K. Phase coexistence and connectivity in the apical membrane of polarized epithelial cells. PNAS 2006 103 329-334. 

Schuck S, Simons K. Polarized sorting in epithelial cells raft clustering and the biogenesis of the apical membrane. J. Cell Sci. 2004 117 5955-5964. 

As a class of tissue, epithelia demarcate body entry points, predisposing a general barrier function with respect to solute entry and translocation. The intestine is lined with enterocytes, which are polarized cells with their apical membrane facing the intestinal lumen that is separated by tight junctions from the basolateral membrane that faces the subepithelial tissues. In addition to their barrier function, the epithelia that line the GI tract serve specialized functions that promote efficient nutrient digestion and absorption and support other organs of the body in water, electrolyte, and bile salt homeostasis. The homeostatic demand on GI tissue that results from this dual function may pose special transport consideration compared with solute translocation across biologically inert barriers. 

Cell death due to apoptosis or necrosis is not the only form of tubular injury in AKI. There is also sub-lethal injury causing cell dysfunction. For example, alterations in proximal tubular cell polarity occur during renal ischemia. Tubule polarity is essential for its primary function of selective reabsorption of ions from the tubular fluid. Sodium-potassium-ATPase (NaK-ATPase), the enzyme, normally localized to the basolateral membrane, maintains tubular polarity by regulation of cellular transport sodium and potassium in proximal tubules. NaK-ATPase is hnked to the cytoskeleton/ membrane complex by a variety of proteins including spectrin. It has been demonstrated that in early reperfusion period spectrin dissociates from the cytosleleton and NaK-ATPase moves from the basolateral membrane into the cytoplasm and apical membrane [54-58]. 

Achler C,Filmer D, Merte C, Drenckhahn D. 1989. Role of microtubules in polarized delivery of apical membrane proteins to the brush border of the intestinal epithelium. J. Cell Biol. 109 179-89... 

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