Apical cytoplasm

About: Apical cytoplasm is a research topic. Over the lifetime, 1080 publications have been published within this topic receiving 36131 citations.

Ezrin, a membrane-organizing protein, as a polarization marker of the retinal pigment epithelium in vertebrates.

Abstract: Immunoreactivity for ezrin, a membrane-organizing phosphoprotein that tethers actin microfilaments to cell membrane proteins, was evaluated as a polarization marker in the intraocular neuroepithelial cells of vertebrates, especially in the retinal pigment epithelium (RPE). Six fetal human eyes representing the 14th–28th gestational weeks, 9 normal adult eyes, 12 eyes with intraocular tumors, and 26 eyes from 15 other vertebrate species were analyzed by immunohistochemistry using the avidin-biotinylated peroxidase complex (ABC) method and monoclonal antibody (mAb) 3C12 to ezrin. The apical cytoplasm and microvilli of the human RPE always reacted with mAb 3C12, but the basal cytoplasm was labeled in reactive RPE only. In autopsy eyes and if fixation was delayed, ezrin immunoreactivity in RPE was more diffuse. Developing RPE became gradually immunoreactive from the 14th week of gestation onward. The microvilli of the baboon, pig, raccoon dog, cow, and rat RPE cells were likewise labeled, and their basal cytoplasm was variably immunoreactive as well, but the microvilli of the avian RPE did not react with the antibody used. In all six mammals mentioned, both layers of the ciliary epithelium and the anterior iris epithelium reacted for ezrin, and the posterior epithelium was weakly labeled in pig, cow, and rat eyes. Normal peripheral and reactive human retina, and normal baboon, pig, raccoon dog, cow, rat, black grouse, and jay eyes, showed immunoreaction for ezrin in Muller cells, usually in their microvilli. Ezrin is widely found in RPE and anterior segment neuroepithelia of the mammalian eye, in which it may segregate membrane proteins to specific membrane surfaces, especially to the apical microvilli of the RPE, which intimately interact with outer segments of photoreceptor cells. The ezrin gene on human chromosome 6q25–26 is consequently a candidate gene for causing retinal degenerations.

The fine structure of the aglomerular nephron of the toadfish, Opsanus tau.

Abstract: Kidney cell fine structure in the aglomerular fish, Opsanus tau, varies profoundly with different fixation procedures. An extensive system of cisternae, tubules and irregular-shaped elements of smooth-surfaced membranes are seen in the basal cytoplasm after fixation with 2% OsO4 buffered with s-collidine. Permanganate fixation demonstrates these membranes as extensions of the basal plasmalemma. Mitochondria and homogeneous bodies surrounded by a single dense membrane lie in close association with the basal membranes. The apical cytoplasm contains an abundance of smooth-surfaced elements whose morphology varies with the fixation procedure used. The fine structure of these cells is discussed with respect to that of other ion transporting tissues and with respect to the concept that basal infoldings are related to “water reabsorption”.

Synthesis of secretory and plasma membrane glycoproteins by striated duct cells of rat salivary glands as visualized by radioautography after 3H-fucose injection.

Abstract: The ability of the striated ducts of rat salivary glands to incorporate 3H-fucose into glycoprotein was studied by light and electron microscope radioautography. At 3.5 to 20 minutes after intravenous injection, the majority of the radioautographic grains in the ducts of the parotid gland were localized to the Golgi apparatus. By 40 minutes, the percentage of grains over the Golgi apparatus had decreased; a corresponding increase in grains occurred over small (0.1-0.4 micrometer) apical granules and the highly infolded basal and lateral plasma membranes. By two hours, less than 10% of the label was associated with the Golgi apparatus, while 26% and 28% were attributed to the apical granules and plasma membrane, respectively. By 8 to 12 hours after injection, the number of grains over the apical cytoplasm had decreased, suggesint luminal discharge of the apical granules. In contrast, the basal and lateral plasma membranes remained labeled up to 30 hours after injection as judged by the distribution of grains in light microscope radioautographs. Mitochondria appeared capable of independent incorporation of fucose, accounting for about 20% of the grains from ten minutes to two hours after injection. Comparable results were obtained in the striated ducts of the submandibular and sublingual glands. These results indicate that the striated duct cells readily incorporate 3H-fucose into newly-synthesized glycoproteins. A portion of these are secretory glycoproteins which are packaged and stored in the apical granules, and a portion are membrane glycoproteins which are incorporated into the extensive plasma membrane of these cells.

Naked2 Acts as a Cargo Recognition and Targeting Protein to Ensure Proper Delivery and Fusion of TGF-α–containing Exocytic Vesicles at the Lower Lateral Membrane of Polarized MDCK Cells

Abstract: Transforming growth factor- (TGF-) is the major autocrine EGF receptor ligand in vivo. In polarized epithelial cells, proTGF- is synthesized and then delivered to the basolateral cell surface. We previously reported that Naked2 interacts with basolateral sorting determinants in the cytoplasmic tail of a Golgi-processed form of TGF- and that TGF- is not detected at the basolateral surface of Madin-Darby canine kidney (MDCK) cells expressing myristoylation-deficient (G2A) Naked2. By high-resolution microscopy, we now show that wild-type, but not G2A, Naked2-associated vesicles fuse at the plasma membrane. We further demonstrate that Naked2-associated vesicles are delivered to the lower lateral membrane of polarized MDCK cells independent of 1B adaptin. We identify a basolateral targeting segment within Naked2; residues 1-173 redirect NHERF-1 from the apical cytoplasm to the basolateral membrane, and internal deletion of residues 37-104 results in apical mislocalization of Naked2 and TGF-. Short hairpin RNA knockdown of Naked2 leads to a dramatic reduction in the 16-kDa cell surface isoform of TGF- and increased cytosolic TGF- immunoreactivity. We propose that Naked2 acts as a cargo recognition and targeting (CaRT) protein to ensure proper delivery, tethering, and fusion of TGF-– containing vesicles to a distinct region at the basolateral surface of polarized epithelial cells.