Differential Localization and Dynamics of Class I Myosins in the Enterocyte Microvillus

Epithelial cells lining the intestinal tract build an apical array of microvilli known as the brush border. Each microvillus is a cylindrical membrane protrusion that is linked to a supporting actin bundle by myosin-1a (Myo1a). Mice lacking Myo1a demonstrate no overt physiological symptoms, suggesting that other myosins may compensate for the loss of Myo1a in these animals. To investigate changes in the microvillar myosin population that may limit the Myo1a KO phenotype, we performed proteomic analysis on WT and Myo1a KO brush borders. These studies revealed that WT brush borders also contain the short-tailed class I myosin, myosin-1d (Myo1d). Myo1d localizes to the terminal web and striking puncta at the tips of microvilli. In the absence of Myo1a, Myo1d peptide counts increase twofold; this motor also redistributes along the length of microvilli, into compartments normally occupied by Myo1a. FRAP studies demonstrate that Myo1a is less dynamic than Myo1d, providing a mechanistic explanation for the observed differential localization. These data suggest that Myo1d may be the primary compensating class I myosin in the Myo1a KO model; they also suggest that dynamics govern the localization and function of different yet closely related myosins that target common actin structures.     

A two-photon FRAP analysis of the cytoskeleton dynamics in the microvilli of intestinal cells

The molecular structure of the brush-border of enterocytes has been investigated since the 1980s, but the dynamics of this highly specialized subcellular domain have been difficult to study due to its small size. To perform a detailed analysis of the dynamics of cytoskeleton proteins in this domain, we developed two-photon fluorescence recovery after photobleaching and a theoretical framework for data analysis. With this method, fast dynamics of proteins in the microvilli of the brush border of epithelial intestinal cells can be measured on the millisecond timescale in volumes smaller than 1 microm3. Two major proteins of the cytoskeleton of the microvilli, actin and myosin 1a (Myo1a; formerly named brush border myosin I), are mobile in the brush-border of Caco-2 cells, an enterocyte-like cellular model. However, the mobility of actin is very different from that of Myo1a and they appear to be unrelated (diffusion coefficient of 15 microm2 s(-1) with a mobile fraction of 60% for actin, and 4 microm2 s(-1) with a mobile fraction of 90% for Myo1a). Furthermore, we show for the first time, in vivo, that the dynamics of Myo1a in microvilli reflect its motor activity.     

Myosin Ia is required for CFTR brush border membrane trafficking and ion transport in the mouse small intestine

In enterocytes of the small intestine, endocytic trafficking of CFTR channels from the brush border membrane (BBM) to the subapical endosomes requires the minus-end motor, myosin VI (Myo6). The subapical localization of Myo6 is dependent on myosin Ia (Myo1a) the major plus-end motor associated with the BBM, suggestive of functional synergy between these two motors. In villus enterocytes of the Myo1a KO mouse small intestine, CFTR accumulated in syntaxin-3 positive subapical endosomes, redistributed to the basolateral domain and was absent from the BBM. In colon, where villi are absent and Myo1a expression is low, CFTR exhibited normal localization to the BBM in the Myo1a KO similar to WT. cAMP-stimulated CFTR anion transport in the small intestine was reduced by 58% in the KO, while anion transport in the colon was comparable to WT. Co-immunoprecipitation confirmed the association of CFTR with Myo1a. These data indicate that Myo1a is an important regulator of CFTR traffic and anion transport in the BBM of villus enterocytes and suggest that Myo1a may power apical CFTR movement into the BBM from subapical endosomes. Alternatively, it may anchor CFTR channels in the BBM of villus enterocytes as was proposed for Myo1a's role in BBM localization of sucrase-isomaltase.     

Myosin-1A targets to microvilli using multiple membrane binding motifs in the tail homology 1 (TH1) domain

One of the most abundant components of the enterocyte brush border is the actin-based monomeric motor, myosin-1a (Myo1a). Within brush border microvilli, Myo1a carries out a number of critical functions at the interface between membrane and actin cytoskeleton. Proper physiological function of Myo1a depends on its ability to bind to microvillar membrane, an interaction mediated by a C-terminal tail homology 1 (TH1) domain. However, little is known about the mechanistic details of the Myo1a-TH1/membrane interaction. Structure-function analysis of Myo1a-TH1 targeting in epithelial cells revealed that an N-terminal motif conserved among class I myosins and a C-terminal motif unique to Myo1a-TH1 are both required for steady state microvillar enrichment. Purified Myo1a bound to liposomes composed of phosphatidylserine and phosphoinositol 4,5-bisphosphate, with moderate affinity in a charge-dependent manner. Additionally, peptides of the N- and C-terminal regions required for targeting were able to compete with Myo1a for binding to highly charged liposomes in vitro. Single molecule total internal reflection fluorescence microscopy showed that these motifs are also necessary for slowing the membrane detachment rate in cells. Finally, Myo1a-TH1 co-localized with both lactadherin-C2 (a phosphatidylserine-binding protein) and PLCδ1-PH (a phosphoinositol 4,5-bisphosphate-binding protein) in microvilli, but only lactaderin-C2 expression reduced brush border targeting of Myo1a-TH1. Together, our results suggest that Myo1a targeting to microvilli is driven by membrane binding potential that is distributed throughout TH1 rather than localized to a single motif. These data highlight the diversity of mechanisms that enable different class I myosins to target membranes in distinct biological contexts.     

Molecular Model of the Microvillar Cytoskeleton and Organization of the Brush Border

Background

Brush border microvilli are ∼1-µm long finger-like projections emanating from the apical surfaces of certain, specialized absorptive epithelial cells. A highly symmetric hexagonal array of thousands of these uniformly sized structures form the brush border, which in addition to aiding in nutrient absorption also defends the large surface area against pathogens. Here, we present a molecular model of the protein cytoskeleton responsible for this dramatic cellular morphology.

Methodology/Principal Findings

The model is constructed from published crystallographic and microscopic structures reported by several groups over the last 30+ years. Our efforts resulted in a single, unique, self-consistent arrangement of actin, fimbrin, villin, brush border myosin (Myo1A), calmodulin, and brush border spectrin. The central actin core bundle that supports the microvillus is nearly saturated with fimbrin and villin cross-linkers and has a density similar to that found in protein crystals. The proposed model accounts for all major proteinaceous components, reproduces the experimentally determined stoichiometry, and is consistent with the size and morphology of the biological brush border membrane.

Conclusions/Significance

The model presented here will serve as a structural framework to explain many of the dynamic cellular processes occurring over several time scales, such as protein diffusion, association, and turnover, lipid raft sorting, membrane deformation, cytoskeletal-membrane interactions, and even effacement of the brush border by invading pathogens. In addition, this model provides a structural basis for evaluating the equilibrium processes that result in the uniform size and structure of the highly dynamic microvilli.     

Myosin-1a powers the sliding of apical membrane along microvillar actin bundles

Microvilli are actin-rich membrane protrusions common to a variety of epithelial cell types. Within microvilli of the enterocyte brush border (BB), myosin-1a (Myo1a) forms an ordered ensemble of bridges that link the plasma membrane to the underlying polarized actin bundle. Despite decades of investigation, the function of this unique actomyosin array has remained unclear. Here, we show that addition of ATP to isolated BBs induces a plus end-directed translation of apical membrane along microvillar actin bundles. Upon reaching microvillar tips, membrane is "shed" into solution in the form of small vesicles. Because this movement demonstrates the polarity, velocity, and nucleotide dependence expected for a Myo1a-driven process, and BBs lacking Myo1a fail to undergo membrane translation, we conclude that Myo1a powers this novel form of motility. Thus, in addition to providing a means for amplifying apical surface area, we propose that microvilli function as actomyosin contractile arrays that power the release of BB membrane vesicles into the intestinal lumen.     

Cytoskeletal protein and mRNA accumulation during brush border formation in adult chicken enterocytes

We have explored the development of the brush border in adult chicken enterocytes by analyzing the cytoskeletal protein and mRNA levels as enterocytes arise from crypt stem cells and differentiate as they move toward the villus. At the base of the crypt, a small population of cells contain a rudimentary terminal web and a few short microvilli with long rootlets. These microvilli appear to arise from bundles of actin filaments which nucleate on the plasma membrane. The microvilli apparently elongate via the addition of membrane supplied by vesicles that fuse with the microvillus and extend the membrane around the actin core. Actin, villin, myosin, tropomyosin and spectrin, but not myosin I (previously called 110 kD; see Mooseker and Coleman, J. Cell Biol. 108, 2395-2400, 1989) are already concentrated in the luminal cytoplasm of crypt cells, as seen by immunofluorescence. Using quantitative densitometry of cDNA-hybridized RNA blots from cells isolated from crypts, villus middle (mid), or villus tip (tip), we found a 2- to 3-fold increase in villin, calmodulin and tropomyosin steady-state mRNA levels; an increase parallel to morphological brush border development. Actin, spectrin and myosin mRNA levels did not change significantly. ELISA of total crypt, mid and tip cell lysates show that there are no significant changes in actin, myosin, spectrin, tropomyosin, myosin I, villin or alpha-actinin protein levels as the brush border develops. The G-/F-actin ratio also did not change with brush border assembly. We conclude that, although the brush border is not fully assembled in immature enterocytes, the major cytoskeletal proteins are present in their full concentration and already localized within the apical cytoplasm. Therefore brush border formation may involve reorganization of a pool of existing cytoskeletal proteins mediated by the expression or regulation of an unidentified key protein(s).     

The organization of actin filaments in human placental villi

The surface of the syncytial trophoblast of the human placenta is covered by a microvillous (brush) border that is in direct contact with maternal blood. Because of this location, it is the site of a variety of transport, enzymatic and receptor activities vital to many placental functions. The organization of the brush border as well as other features of placental villus organization may well be influenced by the distribution of cytoplasmic actin filaments. In order to determine the distribution of actin filaments in human placenta, small pieces of villi were briefly fixed in glutaraldehyde, permeabilized with saponin, and incubated in solutions containing subfragment 1 of myosin (S1). After S1 decoration of actin filaments, tissue was fixed in glutaraldehyde containing tannic acid in order to better visualize the polarity of the filaments, and prepared for electron microscopic examination. The microvilli each contained a core of actin filaments running from the tip of the microvillus to the apical cytoplasm. Most of the actin filaments displayed a distinct polarity, with the S1 arrowheads pointing away from the microvillar tips. These filaments extended only a short distance into the apical cytoplasm. There appeared to be another group of actin filaments in a matlike arrangement in the apical cytoplasm. Coated pits and vesicles were often observed between the microvilli. There appeared to be no clear association between the coated pits and decorated actin filaments, but this was difficult to establish with certainty because of the close proximity of the microvilli. Bundles of actin filaments were sometimes observed near the basal cell surface of the syncytial trophoblast, and in pericytes and capillary endothelial cells in the cores of the villi.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of N-linked glycosylation affects organic cation transport across the brush border membrane of opossum kidney (OK) cells.

Many organic cations are transported across the apical membrane of the proximal tubule by specific saturable mechanisms. The goal of this study was to determine if the transporter for tetraethylammonium (TEA) in the brush border membrane of an established opossum kidney (OK) cell line is glycosylated and to elucidate the function of this glycosylation. The uptake of TEA was determined in OK cell monolayers treated with tunicamycin (TM), a compound that prevents synthesis of the core oligosaccharide precursor molecules. TM exposure significantly decreased the incorporation of [3H]mannose in OK cell proteins and significantly reduced TEA uptake in a time and a concentration dependent manner. No effect of TM exposure on cellular protein synthesis, DNA content, cell viability, or on [3H]proline uptake was observed. The transport of TEA in control cells was characterized by a Km of 26.9 +/- 16.4 microM and a Vmax of 378 +/- 39 pmol/mg of protein/min. TM treatment (1 microgram/ml for 21 h) significantly increased the Km by over 4-fold to 111.5 +/- 18.4 microM while not affecting the Vmax. The apparent KI values of other organic cations known to interact with this transport system were also significantly increased by TM exposure. Estimated KI values of N1-methylnicotinamide, cimetidine, and mepiperphenidol increased by 6-fold, 4-fold, and 2-fold, respectively, after exposure of OK cells to TM. An increased KI for protons was also observed. Additional inhibitors of the N-linked glycosylation pathway, castanospermine, deoxynojirimycin, and deoxymannojirimycin significantly decreased TEA transport, whereas swainsonine had no effect. Our results suggest that the organic cation transporter is glycosylated. The N-linked oligosaccharide side chain appears to be of the hybrid type, and it either directly or indirectly affects the binding site of the transporter for both organic cations and protons. This is the first report describing the importance of glycosylation in the function of the organic cation transporter in the apical membrane of OK cells.     

Organization of an actin filament-membrane complex. Filament polarity and membrane attachment in the microvilli of intestinal epithelial cells

The association of actin filaments with membranes is now recognized as an important parameter in the motility of nonmuscle cells. We have investigated the organization of one of the most extensive and highly ordered actin filament-membrane complexes in nature, the brush border of intestinal epithelial cells. Through the analysis of isolated, demembranated brush borders decorated with the myosin subfragment, S1, we have determined that all the microvillar actin filaments have the same polarity. The S1 arrowhead complexes point away from the site of attachment of actin filaments at the apical tip of the microvillar membrane. In addition to the end-on attachment of actin filaments at the tip of the microvillus, these filaments are also connected to the plasma membrane all along their lengths by periodic (33 nm) cross bridges. These bridges were best observed in isolated brush borders incubated in high concentrations of Mg++. Their visibility is attributed to the induction of actin paracrystals in the filament bundles of the microvilli. Finally, we present evidence for the presence of myosinlike filaments in the terminal web region of the brush border. A model for the functional organization of actin and myosin in the brush border is presented.     

Roles for Drosophila melanogaster myosin IB in maintenance of enterocyte brush-border structure and resistance to the bacterial pathogen Pseudomonas entomophila

Drosophila myosin IB (Myo1B) is one of two class I myosins in the Drosophila genome. In the larval and adult midgut enterocyte, Myo1B is present within the microvillus (MV) of the apical brush border (BB) where it forms lateral tethers between the MV membrane and underlying actin filament core. Expression of green fluorescent protein-Myo1B tail domain in the larval gut showed that the tail domain is sufficient for localization of Myo1B to the BB. A Myo1B deletion mutation exhibited normal larval gut physiology with respect to food uptake, clearance, and pH regulation. However, there is a threefold increase in terminal deoxynucleotidyl transferase dUTP nick-end labeling-positive enterocyte nuclei in the Myo1B mutant. Ultrastructural analysis of mutant midgut revealed many perturbations in the BB, including membrane tethering defects, MV vesiculation, and membrane shedding. The apical localization of both singed (fascin) and Dmoesin is impaired. BBs isolated from mutant and control midgut revealed that the loss of Myo1B causes the BB membrane and underlying cytoskeleton to become destabilized. Myo1B mutant larvae also exhibit enhanced sensitivity to oral infection by the bacterial pathogen Pseudomonas entomophila, and severe cytoskeletal defects are observed in the BB of proximal midgut epithelial cells soon after infection. Resistance to P. entomophila infection is restored in Myo1B mutant larvae expressing a Myo1B transgene. These results indicate that Myo1B may play a role in the local midgut response pathway of the Imd innate immune response to Gram-negative bacterial infection.     

Characterization and localization of myosin in the brush border of intestinal epithelial cells

The brush border of intestinal epithelial cells consists of a tightly packed array of microvilli, each of which contains a core of actin filaments. It has been postulated that microvillar movements are mediated by myosin interactions in the terminal web with the basal ends of these actin cores (Mooseker, M.S. 1976. J. Cell. Biol. 71:417-433). We report here that two predictions of this model are correct: (a) The brush border contains myosin, and (b) myosin is located in the terminal web. Myosin is isolated in 70 percent purity by solubilization of Triton-treated brush borders in 0.6 M KI, and separation of the components by gel filtration. Most of the remaining contaminants can be removed by precipitation of the myosin at low ionic strength. This yield is approximately 1 mg of myosin/30 mg of solubilized brush border protein. The molecule consists of three subunits with molecular weights of 200,000, 19,000, and 17,000 daltons in a 1:1:1 M ratio. At low ionic strength, the myosin forms small, bipolar filaments with dimensions of 300 X 11nm, that are similar to filaments seen previously in the terminal web of isolated brush borders. Like that of other vertebrate, nonmuscle myosins, the ATPase activity of isolated brush border myosin in 0.6 M KCI is highest with EDTA (1 μmol P(i)/mg-min; 37 degrees C), intermediate with Ca++ (0.4 μmol P(i)/mg-min), and low with Mg++ (0.01 μmol P(i)/mg-min). Actin does not stimulate the Mg-ATPase activity of the isolated enzyme. Antibodies against the rod fragment of human platelet myosin cross-react by immunodiffusion with brush border myosin. Staining of isolated mouse or chicken brush borders with rhodamine-antimyosin demonstrates that myosin is localized exclusively in the terminal web.     

The dual role of annexin II in targeting of brush border proteins and in intestinal cell polarity

Functional intestinal epithelium relies on complete polarization of enterocytes marked by the formation of microvilli and the accurate trafficking of glycoproteins to relevant membrane domains. Numerous transport pathways warrant the unique structural identity and protein/lipid composition of the brush border membrane. Annexin II (Ca(2+)-dependent lipid-binding protein) is an important component of one of the apical protein transport machineries, which involves detergent-resistant membranes and the actin cytoskeleton. Here, we investigate in intestinal Caco-2 cells the contribution of annexin II to the sorting and transport of brush border hydrolases and role in intestinal cell polarity. Downregulation of annexin II in Caco-2-A4 cell line results in a severe reduction of the levels of the brush border membrane resident enzyme sucrase isomaltase (SI) as well as structural components such as ezrin. This reduction is accompanied by a redistribution of these proteins to intracellular compartments and a striking morphological transition of Caco-2 cells to rudimentary epithelial cells that are characterized by an almost flat apical membrane with sparse and short microvilli. Concomitant with this alteration is the redistribution of the intermediate filament protein keratin 19 to the intracellular membranes in Caco-2-A4 cells. Interestingly, keratin 19 interacts with annexin II in wild type Caco-2 cells and this interaction occurs exclusively in lipid rafts. Our findings suggest a role for annexin II and K19 in differentiation and polarization of intestinal cells.     

Single amino acid residue influences the distribution pattern of an inwardly rectifying potassium channel in polarized cells

We used immunohistochemistry to identify the localization of the inwardly rectifying potassium channels K(ir) 2.1 and its mutant K(ir) 2.1 M84K, and P2X(2) receptors heterologously expressed in Xenopus oocytes, opossum kidney (OK) cells and NG108-15 cells. K(ir) 2.1 wild-type channels were unevenly distributed over the surface of the oocytes and the density was higher at the vegetal pole than at the animal pole. In OK cells the protein was detected at the basolateral membrane, and in NG108-15 cells the protein was found in the soma of the cells and in long thick outgrowing neurites. In contrast, mutant K(ir) 2.1 M84K channels were evenly distributed over the membrane of the oocytes, whereas in OK cells the protein was only detected at the tips of the brush border. In NG108-15 cells the protein was found in the soma of the cells and in all growing neurites. The density of P2X(2) was higher at the animal pole of the oocytes and was restricted to the tips of the brush border in OK cells. In NG108-15 cells the protein was restricted to thinner outgrowing structures and the soma of the cells. We conclude that the exchange of a single amino acid residue in the N-terminus of K(ir) 2.1 changes the distribution pattern in all of the cell types studied. Furthermore, we were able to show that another ion channel sharing the same topology with inwardly rectifying potassium channels showed a different distribution pattern in these cell types.     

Na(+)/H(+) exchanger 3 is in large complexes in the center of the apical surface of proximal tubule-derived OK cells

Cell biological approaches were usedto examine the location and function of the brush border (BB)Na⁺/H⁺ exchanger NHE3 in the opossum kidney(OK) polarized renal proximal tubule cell line. NHE3 epitope taggedwith the vesicular stomatitis virus glycoprotein epitope(NHE3V) was stably expressed and called OK-E3V cells. On thebasis of cell surface biotinylation studies, these cells had10-15% of total NHE3 on the BB. Intracellular NHE3V largelycolocalized with Rab11 and to a lesser extent with EEA1. The BBlocation of NHE3V was examined by confocal microscopy relative to thelectins wheat germ aggluttinin (WGA) and phytohemagluttin E (PHA-E), aswell as the B subunit of cholera toxin (CTB). The cells were pyramidal,and NHE3 was located in microvilli in the center of the apical surface.In contrast, PHA-E, WGA, and CTB were diffusely distributed on the BB.Detergent extraction showed that total NHE3V was largely soluble inTriton X-100, whereas virtually all surface NHE3V was insoluble.Sucrose density gradient centrifugation demonstrated that total NHE3Vmigrated at the same size as ~400- and ~900-kDa standards, whereassurface NHE3V was enriched in the ~900-kDa form. Under basalconditions, NHE3 cycled between the cell surface and the recyclingpathway through a phosphatidylinositol (PI) 3-kinase-dependentmechanism. Measurements of surface and intracellular pH were obtainedby using FITC-WGA. Internalization of FITC-WGA occurred largely intothe juxtanuclear compartment that contained Rab11 and NHE3V. pH valueson the apical surface and in endosomes in the presence of the NHE3blocker, S3226, were elevated, showing that NHE3 functioned to acidifyboth compartments. In conclusion, NHE3V in OK cells exists in distinctdomains both in the center of the apical surface and in a juxtanuclearcompartment. In the BB fraction, NHE3 is largely in thedetergent-insoluble fraction in lipid rafts and/or in largeheterogenous complexes ranging from ~400 to ~900 kDa.

     

Purification of microvilli and an analysis of the protein components of the microfilament core bundle

A fast and convenient method for the purification of microvilli from chicken intestinal brush borders is described. The microvilli appear morphologically very similar to those found on intact brush borders. Removal of the microvillus membrane from the microvilli by Triton X-100 treatment reveals compact bundles of microfilaments with small regularly spaced projections along their length. SDS-polyacrylamide gel analysis of the protein components of the brush border, the microvilli and the microvillus core bundles shows that little or no tropomyosin, myosin or filamin is found in the microvillus, whereas polypeptide chains with mobilities characteristic for these proteins are present in the whole brush border. The majority of the microvillus core protein is actin, and the other major protein present has a polypeptide molecular weight of 95 000. Total actin from both brush borders and microvilli, characterized by isoelectric focussing analysis, contained about 40% β actin and 60% γ actin. The presence of both the β and γ cytoplasmic actins in the highly ordered parallel arrays of microfilaments of the microvilli is discussed in light of hypotheses for different functional roles of these two actin species.     

Immunolocalization of transferrin and transferrin receptor in mouse small intestinal absorptive cells

The mechanisms by which the duodenal mucosa absorbs iron are unknown. Insorption into absorptive cells of luminal iron bound to transferrin via receptor-mediated endocytosis has been hypothesized, but transferrin and transferrin receptor are absent in apical microvillous brush borders of small bowel biopsies taken from fasted patients and normal volunteers. We hypothesized that a normal iron-containing diet might induce the transient appearance of transferrin and transferrin receptor in apical brush borders of small intestinal absorptive cells in a normal mouse that was provided iron-containing chow until the moment of sacrifice. Light and electron microscopic immunolocalization of transferrin and transferrin receptor in proximal small intestinal absorptive cells was limited to basolateral membranes and coated pits of cells predominantly in the crypts and basal regions of the villi. Transferrin and transferrin receptor were not detected in apical microvillous brush border membranes of these enterocytes. In parallel immunolocalization protocols designed to show the ability to immunodetect other antigens at these locations, maltase and proteoglycan were demonstrated in apical microvillous brush border membranes and in basolateral membranes, respectively, in absorptive cells of small intestinal villous tip, base, and crypt regions. Furthermore, transferrin and transferrin receptor were immunolocalized in hepatocyte sinusoidal microvillus membranes. We conclude that food does not induce the appearance of immunodetectable transferrin and transferrin receptor in the apical microvilli of small intestinal absorptive cells and, therefore, that these iron transport proteins are not involved in the apical microvillous membrane transport of luminal dietary iron.     

Opossum kidney contains a functional receptor for the Escherichia coli heat-stable enterotoxin

The Escherichia coli heat-stable enterotoxin (ST1 or STa) binds to specific receptors on mammalian intestinal brush border membranes, and stimulates guanylate cyclase in those membranes. We have found a similar signal transduction system in brush border membranes prepared from kidney cortex of the American opossum (Didelphis virginiana, and in a cell line (OK cell) derived from that tissue. Activation of guanylate cyclase by ST1 is therefore not limited to intestinal cells. Furthermore, since it is unlikely that ST1 which is produced in the intestinal lumen, would have access to kidney receptors, this suggests the existence of an endogenous peptide resembling ST1, at least in marsupials.     

Identification of the microvillar 110-kDa calmodulin complex (myosin-1) in kidney.

The epithelial layer lining the proximal convoluted tubule of mammalian kidney contains a brush border of numerous microvilli. These microvilli appear in structure to be very similar to the microvilli on epithelial cells of the small intestine. Microvilli found in both the small intestine and the proximal convoluted tubules in kidney have a core bundle of actin filaments bundled by the accessory proteins villin and fimbrin. Along the length of intestinal microvilli, lateral links can be observed to connect the core bundle of actin filaments to the membrane. These cross-bridges are comprised of a 110-kDa calmodulin complex which belongs to a class of single-headed myosin molecules, collectively referred to as myosin-1. We now report that an analogous calmodulin-binding polypeptide of 105 kDa has been identified in rat kidney cortex. The 105-kDa polypeptide is preferentially found in purified kidney brush borders, can be extracted with ATP, and co-elutes with calmodulin on gel filtration and anion exchange chromatography. Fractions containing the 105-kDa polypeptide exhibit a modest ATPase activity in buffer containing CaCl2. The partially purified 105-kDa polypeptide will bind iodinated calmodulin and will sediment with F-actin in buffer containing ethylene glycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or Ca2+. The addition of ATP partially reverses this association with F-actin. These results indicate that myosin-1, in addition to its presence in intestinal brush borders, is present in the brush border of kidney. We also provide preliminary evidence to indicate that the 105-kDa polypeptide is not restricted to tissues possessing a brush border.     

Inhibitory effect of KW-3902, an adenosine A(1) receptor antagonist, on p-aminohippurate transport in OK cells.

KW-3902 (8-(noradamantan-3-yl)-1,3-dipropylxanthine) is a novel potent and selective adenosine A(1) receptor antagonist. We examined the effect of KW-3902 on p-aminohippurate (PAH) transport in opossum kidney (OK) epithelial cells. Pretreatment for 3 h with KW-3902 inhibited the transcellular transport of PAH across OK cell monolayers from the basal to the apical side. The uptake of PAH across the basolateral membrane of OK cells was inhibited by KW-3902 pretreatment in a time- and concentration-dependent manner. A kinetic analysis revealed that the inhibitory effect of KW-3902 on the basolateral PAH uptake was due to an increase in the Michaelis constant (K(m)) as well as a decrease in the maximum uptake rate (V(max)), showing that the inhibition was a mixed type. Pretreatment with adenosine deaminase or 8-cyclopentyl-1,3-dipropylxanthine, another selective adenosine A(1) receptor antagonist, also decreased the basolateral PAH uptake. KW-3902 pretreatment had no effect on the concentration of intracellular alpha-ketoglutarate which exchanges for PAH across the basolateral membrane of OK cells. These results suggest that KW-3902 has an inhibitory effect on PAH transport in OK epithelial cells.