Epithelial cell apical membrane

Gonzalez RF, Dobbs LG (1998) Purification and analysis of RTI40, a type I alveolar epithelial cell apical membrane protein. Biochim Biophys Acta 1429(1) 208-216... 

Alveolar epithelial cell apical membrane protein (type I) Cardiac membrane proteolipids... 

Airway cross-sections have the nominal anatomy shown in Fig. 5.16. Airway surface liquid (AST), primarily composed of mucus gel and water, surrounds the airway lumen with a thickness thought to vary from 5 to 10 mm. AST lies on the apical surface of airway epithelial cells (mostly columnar ciliated epithelium). This layer of cells, roughly two to three cells thick in proximal airways and eventually thinning to a single cell thickness in distal airways, rests along a basement membrane on its basal surface. Connective tissue (collagen fibers, basement membranes, elastin, and water) lies between the basement membrane and airway smooth muscle. Edema occurs when the volume of water within the connective tissue increases considerably. Interspersed within the smooth muscle are respiratory supply vessels (capillaries, arteriovenous anastomoses), nerves, and lymphatic vessels. 

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... 

In the tongue, ENaC is expressed in taste bud epithelial cells. The expression of a, (3 and y subunits at the apical membrane of taste buds is observed under low salt diet, known to greatly increase plasma aldosterone. This observation suggests that ENaC could play a significant role in the transduction of salt sensation. 

The cytoskeleton is involved in the maintenance of cell shape and cytoplasmic processes (e.g., microvilli). In polarized epithelial cells, distinct cyto-cortical cytoskeletal complexes are associated with the apical and basal-lateral domains of the plasma membrane (Rodriguez-Boulan and Nelson, 1989 Mays et al., 1994). 

Although a uniform nomenclature for Na /H exchanger isoforms has not yet been adopted, we will refer to the amiloride-sensitive type of Na" /H exchanger that is present in the basolateral membrane of epithelia (apical membrane of placental syncytiotrophoblast) and also widely distributed in non-epithelial cells as the sensitive-type. The relatively amiloride-resistant isoform present in apical mem-... 

Biochemical studies of plasma membrane Na /H exchangers have been directed at two major goals (1) identification of amino acids that are involved in the transport mechanism and (2) identification and characterization of the transport pro-tein(s). To date, most studies have been performed on the amiloride-resistant form of Na /H" exchanger that is present in apical or brush border membrane vesicles from mammalian kidney, probably because of the relative abundance of transport activity in this starting material. However, some studies have also been performed on the amiloride-sensitive isoform present in non-epithelial cells. 

Next, Reilly et al. [65] localized the Na /H exchanger gene product in renal epithelial cells where the distributions of the kinetic isoforms was well-established. The strategy was based on the observation that the resistant- and sensitive-types are restricted to the apical and basolateral membranes, respectively, in confluent LLC-PK]/Clone 4 cells [8]. Thus, if proteins encoded by the cloned cDNAs localized to the apical membrane this would indicate that they represent the resistant-type. Localization to the basolateral membrane would prove they were the sensitive-type and presence on both membranes would suggest that the two functional isoforms had identical primary structures. Na exchanger proteins were localized by... 

The third mucosal layer is that lining the entire length of the small intestine and which represents a continuous sheet of epithelial cells. These epithelial cells (or enterocytes) are columnar in shape, and the luminal cell membrane, upon which the microvilli reside, is called the apical cell membrane. Opposite this membrane is the basal (or basolateral) plasma membrane, which is separated from the lamina propria by a basement membrane. A sketch of this cell is shown in Fig. 5. The primary function of the villi is absorption. 

Fig. 9 Schematic representation depicting the movement of molecules from the absorbing (mucosal or apical) surface of the GIT to the basolateral membrane and from there to blood. (A) transcellular movement through the epithelial cell. (B) Paracellular transport via movement between epithelial cells. (Q Specialized carrier-mediated transport into the epithelial cell. (D) Carrier-mediated efflux transport of drug out of the epithelial cell. (Copyright 2000 Saguaro Technical Press, Inc., used with permission.)...

However, in interphase delaminating neuroblasts, which are known to have completed S-phase and are at the G2 stage of the cell cycle, this codependence of Baz/Insc/Pins seen in mitotic neuroblasts does not apply. Delaminating neuroblasts possess an apical membrane stalk which retains contact with the epithelial surface and this is where apical cortical localization of Insc is initially seen (see Fig. 2). This initial localization of Insc to the apical stalk occurs... 

Figure 2 Comparison of intestinal epithelial cells in culture and in situ. (A) Human colon Caco-2 cells grown in culture for 16 days on a semiporous filter. (B) Epithelial layer of rat jejunum. AP, apical or luminal membrane B, basal or abluminal membrane BM, basement membrane G, goblet cell LS, lateral space mv, microvilli Nu, nucleus TJ, tight junction. Bars equal 10 pm. 

The transfer step involves the passage of iron that has been taken up at the apical, brush border membrane across the mucosal epithelial cell to the basolateral membrane, where it is transferred to the circulation. However, not all the iron taken up from the lumen into the cell is transferred. As a function of the body s... 

Proulx [30] summarized the published lipid compositions of BBM isolated from epithelial cells from pig, rabbit, mouse and rat small intestines. Table 3.1 shows the lipid make-up for the rat, averaged from five reported studies [30], On a molar basis, cholesterol accounts for about 50% of the total lipid content (37% on a weight basis). Thus, the cholesterol content in BBM is higher than that found in kidney epithelial (MDCK) and brain endothelial cells (Table 3.1). Slightly different BBM lipid distribution was reported by Alcorn et al. [31] here, the outer (luminal) leaflet of the BBM was seen to be rich in sphingomyelin content, while the inner leaflet (cytosol) was rich in PE and PC. Apical (brush border) and basolateral lipids are different in epithelia. The basolateral membrane content (not reported by... 

T., Simmons, N. L., Hirst, B. H., Functional expression of P-glycoprotein in apical membranes of human intestinal epithelial Caco-2 cells kinetics of vinblastine secretion and interaction with modulators, J. Biol. Chem. 1993, 268, 14991-14997. 

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