Mechanism of concanavalin A-induced anchorage of the major cell surface glycoproteins to the submembrane cytoskeleton in 13762 ascites mammary adenocarcinoma cells

Concanavalin A (Con A)-induced anchorage of the major cell surface sialoglycoprotein component complex (ASGP-1/ASGP-2) was studied in 13762 rat mammary adenocarcinoma sublines with mobile (MAT-B1 subline) and immobile (MAT-C1 subline) cell surface Con A receptors. Treatment of cells, isolated microvilli, or microvillar membranes with Con A resulted in marked retention of ASGP-1 and ASGP-2, a Con A-binding protein, in cytoskeletal residues of both sublines obtained by extraction with Triton X-100 in PBS. When Con A-treated microvillar membranes were extracted with a buffer containing Triton X-100, the sialoglycoprotein complex was found associated in the residues with a transmembrane complex composed of actin, a 58,000-dalton polypeptide, and a cytoskeleton-associated glycoprotein (CAG), also a Con A-binding protein, in MAT-C1 membranes, and of actin and CAG in MAT-B1 membranes. Untreated membrane Triton residues retained very little ASGP-1/ASGP-2 complex. Association of the sialoglycomembrane complex and the transmembrane complex was also demonstrated in Con A-treated, but not untreated, microvilli by their comigration on CsCl gradients. Association of both complexes with the cytoskeleton of microvilli was shown by sucrose density gradient centrifugation. A fraction of the polymerized actin comigrated with the transmembrane complex alone in the absence of Con A and with both the transmembrane complex and the sialoglycoprotein complex in the presence of Con A. From these results we propose that anchorage of the sialoglycoprotein complex to the cytoskeleton on Con A treatment occurs by cross-linking ASGP-2, the major cell surface Con A-binding component, to CAG of the transmembrane complex, which is natively linked to the cytoskeleton via its actin component. Since Con A-induced anchorage occurs in sublines with mobile and immobile receptors, the anchorage process cannot be responsible for the differences in receptor mobility between the sublines.     

Polar effects of concanavalin A on the cortical cytoskeleton of a molluscan egg (Nassarius reticulatus,Gastropoda)

Summary The organization of the surface of fertilizedNassarius reticulatus eggs was probed by investigating the effects of treatment with concanavalin A (Con A). This lectin causes abnormal polar lobe formation as well as inhibition of cleavage. At low concentrations of Con A (0.3–1.0 μg/ml) the polar lobe constriction becomes considerably elongated, whereas at higher concentrations (2.5–50 μ/ml) the position of the constriction undergoes an extreme shift towards the animal pole. In the latter case, the surface of the animal part of the egg forms large protrusions and folds. Con A also causes resorption of microvilli and disappearance of the extracellular layer covering these villi; this process starts at the vegetal pole and propagates towards the animal pole. These changes in surface architecture are associated with profound changes in the organization of filamentous (F-) actin as assessed by confocal laser scanning microscopy of NBD-phallacidin-labelled eggs. Divalent succinyl-Con A has the same effects on polar lobe formation and surface architecture as tetravalent Con A, but only at very high concentrations (100–200 μg/ml), indicating that Con A exerts its effects by cross-linkage of its binding sites. Experiments with cytoskeleton inhibitors (cytochalasin D, colchicine, and nocodazole) reveal that in Con A-treated eggs — as in untreated eggs — microfilaments, but not microtubules, are involved in the formation of the polar lobe constriction. The calcium ion channel blocker D600 affects neither normal nor Con A-induced abnormal polar lobe formation, which suggests that influx of external calcium is not required. In contrast, treatment with TMB-8, an antagonist of internal calcium release, prevents the formation of a polar lobe in both normal and Con A-treated eggs. Finally, eggs from which the polar lobe has been removed prior to Con A treatment show none of the effects described, whereas isolated polar lobes react similarly to intact eggs. These results suggest that binding of Con A to sites present at the vegetal pole of the egg is responsible for the observed effects of the lectin.     

Microvillar elongation following parthenogenetic activation of sea urchin eggs

Parthenogenetic activation of unfertilized sea urchin eggs with ammonium chloride at pH 8.0 resulted in a slow, but dramatic, reorganization of surface microvilli in four species of sea urchin eggs. Following NH4Cl treatment, elongation of microvilli on the egg surface was observed concomitant with the formation of microfilament bundles within the microvillar cores. A minimum of 2 h of treatment was required for elongation and microfilament bundle formation to occur. The maintenance of elongated microvilli was pH-sensitive; removal of the activating agent resulted in the retraction of extended microvilli while readdition of NH4Cl caused microvilli to elongate again. Accompanying microvillar elongation in activated eggs, there was an increased calcium uptake as measured by 45Ca uptake. Blocking calcium uptake by incubation in lanthanum chloride or zero-calcium seawater containing 2 mM EGTA prevented microvillar elongation. These results suggested that elongation of microvilli following parthenogenetic activation by NH4Cl is pH- and calcium-dependent and is similar to that observed during normal fertilization.     

Surface polarization in loach eggs and two-cell embryos: correlations between surface relief, endocytosis and cortex contractility

The aim of this study was to examine the reorganization of the microfilamentous cortical layer (MC) accompanying ooplasmic segregation in loach eggs. Using scanning (SEM) and transmission electron microscopy (TEM), we found that the MC is thicker in folded areas. Prior to fertilization, surface microvilli are distributed more or less uniformly throughout the egg. A similar, more or less uniform, distribution of endocytotic events was observed in the eggs 5-15 min after insemination using fluorescence microscopy of Lucifer yellow CH uptake. During ooplasmic segregation, the surface is progressively polarized so that before the first cleavage onset (50-60 min after insemination) only the blastodisc surface is folded and undergoes endocytosis, whereas the vegetal surface is smooth and does not show internalization. In two-cell embryos, the blastomeric surface is also regionalized according to its relief and endocytosis. When surface tension was lowered by sucking most yolk granules out of the egg, we observed contractile responses only in the animal folded surface. These data suggest that a polar distribution of contractile structures is established in the loach egg undergoing ooplasmic segregation.     

Microfilament association of ASGP-2, the concanavalin A-binding glycoprotein of the cell-surface sialomucin complex of 13,762 rat mammary ascites tumor cells

Microfilament-associated proteins and membrane-microfilament interactions are being investigated in microvilli isolated from 13,762 rat mammary ascites tumor cells. "Phalloidin shift" analyses on velocity sedimentation gradients of Triton X-100 extracts of [3H]-glucosamine-labeled microvilli identified a 120-kDa cell-surface glycoprotein associated with the microvillar microfilament core. The identification was verified by concanavalin A (Con A) blots of one- and two-dimensional (2D) electrophoresis gels of sedimented microfilament cores. By 2D-electrophoresis and lectin analyses the 120-kDa protein appeared to be a fraction of ASGP-2, the major Con A-binding glycoprotein of the sialomucin complex of the 13,762 cells. This identity was confirmed by immunoblot analyses using immunoblot-purified anti-ASGP-2 from anti-membrane serum prepared against microvillar membranes. Proteolysis of the microvilli with subtilisin or trypsin resulted in an increase in the amount of ASGP-2 associated with the microfilament cores. An increase was also observed with sialidase treatment of the microvilli, suggesting that negative charges, probably present on the highly sialated sialomucin ASGP-1 of the ASGP-1/ASGP-2 sialomucin complex, reduce ASGP-2 association with the microfilament core. Proteolysis of isolated microvillar membranes, which contain actin but not microfilaments, also increased the association of ASGP-2 with a Triton-insoluble, actin-containing membrane fraction. Purified ASGP-2 does not bind to microfilaments in sedimentation assays. Since the Triton-insoluble membrane residue is enriched in an actin-containing transmembrane complex, which contains a different glycoprotein, we suggest that the ASGP-2 is binding indirectly via this complex to the microfilament core in the intact microvilli.     

Relation of cytoplasmic alkalinization to microvillar elongation and microfilament formation in the sea urchin egg

Experiments have been carried out to test the proposal that the pH increase at fertilization in sea urchin eggs promotes microvillar elongation. Results presented herein show that microvillar elongation and microfilament formation occurred when sea urchin eggs were incubated in sodium-free seawater containing the calcium ionophore A23187, a treatment which initiates activation, i.e., induces a transient increase in intracellular free calcium, but prevents subsequent cytoplasmic alkalinization. Within elongated microvilli and cortices of these eggs, microfilaments were arranged in a loose meshwork. However, if the pH of the egg cytoplasm was increased experimentally, microfilament bundles appeared within individual microvilli. These findings suggest that: (1) microvillar elongation and microfilament formation in the sea urchin egg at fertilization may occur when cytoplasmic alkalinization is inhibited, and (2) formation of the microvillus bundle of microfilaments at egg activation is pH sensitive. Additionally, if the cytoplasmic pH of unfertilized eggs was experimentally elevated by NH₄Cl, microvilli failed to elongate. These data indicate that elevation of intracellular pH by this method is not sufficient to induce microvillar elongation.     

Changes in the topography of the sea urchin egg after fertilization

Changes in the topography of the sea urchin egg after fertilization were studied by scanning and transmission electron microscopy. Strongylocentrotus purpuratus eggs were treated with dithiothreitol to modify the vitelline layer and to prevent formation of a fertilization membrane. Dithiothreitol treatment caused the microvilli to become more irregular in shape, length, and diameter than those of untreated eggs. The microvilli were similarly modified by trypsin treatment. This effect did not appear to be due to disruption of cytoskeletal elements beneath the plasma membrane, for neither colchicine nor cytochalasin B altered microvillar morphology. Thus, it appears that the vitelline layer may act in the maintenance of surface form of unfertilized eggs. Since dithiothreitol-treated eggs did not elevate a fertilization membrane, scanning electron microscopy could be used to directly observe modifications in the egg plasma membrane after fertilization. The wave of cortical granule exocytosis initiated at the point of attachment of the fertilizing sperm was characterized by the appearance of pits that subsequently opened, releasing the cortical granule contents and leaving depressions upon the egg surface. The perigranular membranes inserted during exocytosis were seen as smooth patches between the microvillous patches remaining from the original egg surface. This produced a mosaic surface with more than double the amount of membrane of unfertilized eggs. The mosaic surface subsequently reorganized to accommodate the inserted membrane material by elongation of microvilli. Blebs and membranous whorls present before reorganization suggested the existence of an unstable intermediate state of plasma membrane reorganization. Exocytosis and mosaic membrane formation were not blocked by colchicine or cytochalasin B, but microvillar elongation was blocked by cytochalasin B treatment.     

Movements of Mitochondria Associated with Isolated Egg Cortex

Cortical tragments of Tubifex eggs undergoing ooplasmic segregation were isolated in plastic dishes with an oil droplet separating part of them from the dish. Except at the edges of fragments and at the base of the oil droplet, the nearly entire area of the cortex was firmly attached to the plastic surface. Many mitochondria were associated with the cortical actin lattice. On addition of ATP to the preparations, only nonadhering portions of the cortex exhibited contractions. At the base of oil droplets, some mitochondria moved in the same direction concurrently with cortical contractions. Contractile activities of isolated cortices were completely inhibited by their preincubation with N-ethylmaleimide-modified heavy meromyosin, suggesting the involvement of an actomyosin force-generating mechanism in the cortical contractions during ooplasmic segregation.     

Effects of concanavalin A on the fertilization of sea urchin eggs

Concanavalin A (Con A) affected sperm-egg interactions of Arbacia punctulata and Strongylocentrotus purpuratus by inhibiting insemination at minimally saturating sperm concentrations. However, this inhibition was overcome by increasing the sperm density. Sperm concentrations (10⁶/ml) yielding 100% fertilization of control preparations resulted in only 72% insemination of Con A-treated ova (10⁴/ml). Although a cortical granule reaction occurred in fertilized, Con A-treated eggs, the distance the fertilization membrane separated from the zygote's surface was not as great as observed in controls. These results may be the basis for previous reports of Con A inhibiting fertilization in sea urchins.     

Microvilli in sea urchin eggs : Differences in their formation and type

The elongation of microvilli, which normally occurs upon fertilization in sea urchin eggs, was also observed in unfertilized eggs treated with the enzyme, papain. Cortical granule exocytosis, which is thought to be the source of membrane used in microvillar elongation, does not occur in the papain-treated eggs. It appears, therefore, that there is more than one way in which the egg plasma membrane can increase very quickly and to a great extent. In addition, the kinds of microvilli formed in the two instances appear to be different. Previous work with reaggregating sea urchin cells is also cited to support the suggestion that microvilli can form in different ways and are of different types.     

Dynamics of the actin microfilament system in the Tubifex egg during ooplasmic segregation

Following the second polar body formation (PBF), the Tubifex egg undergoes ooplasmic segregation consisting of two steps, i.e., centrifugal migration of membranous organelles forming a subcortical ooplasmic layer and then movements of these organelles along the egg surface. The present investigation was undertaken to examine the microfilament organization in eggs during these ooplasmic rearrangements. Microfilaments throughout the egg are identified as actin by their reversible heavy meromyosin binding. Before the second PBF, a distinct network of actin filaments is present in the endoplasmic region. It is disorganized during the second PBF; short actin filaments are caused to aggregate with membranous organelles. Following the second PBF, similar short filaments become localized in the subcortical layer but not in the underlying yolky region. However, it is not until 50-60 min after the second PBF that an elaborate actin network is established in the subcortical layer. The cortex contains a sheet-like lattice of actin filaments. It is thickest around the animal pole, and tapes toward the equator of the egg. At about 90 min after the second PBF, this polarized distribution of cortical filaments becomes more pronounced as the result of their movements. Chronologically, subcortical actin network formation and cortical reorganization correspond to the later portion of the first step and the earlier portion of the second step of ooplasmic segregation, respectively. These findings are discussed in terms of ooplasmic movements and rearrangements.     

Divergent effects of cyclosporine A on interferon- and mitogen-induced Ly-6A antigen suggest autocrine regulation of Ly-6A expression in activated T cells

The murine Ly-6A cell surface antigen is normally present on a minor subset of mature T cells. This marker has been shown to become highly expressed on mitogen-activated T cells. We found that expression of Ly-6A is also markedly increased in resting T cells by incubation with IFN-alpha/beta or IFN-gamma. Here, we compared the effect of the immunosuppressant cyclosporine A (CsA) on Ly-6A induction by IFN and concanavalin A (Con A). The augmentation of Ly-6A expression produced by treatment of T cells with IFN-alpha/beta or IFN-gamma was found not to be affected by CsA concentrations up to 2 micrograms/ml. In contrast, at doses as low as 50 ng/ml, CsA prevented the enhancement of Ly-6A expression in Con A-treated T-cell cultures. Culture supernatant transfer experiments were performed to further explore this effect of CsA. It was found that supernatants from Con A-activated T cells enhanced Ly-6A expression in resting T cells. This activity could be neutralized with an anti-IFN-gamma monoclonal antibody. Supernatants from T cells treated with Con A in presence of CsA lacked Ly-6A-enhancing activity. Taken together, these data suggest that the inhibition by CsA of Ly-6A induction in Con A-treated T cells reflects the known inhibitory effects of the drug on IFN-gamma secretion. This may imply the existence in T cells of an autocrine circuit involving IFN-gamma and regulating Ly-6A expression.     

Concanavalin A inhibition of ecto-5'-nucleotidase of intact cultured C6 glioma cells.

A pronounced effect of concanavalin A (Con A) upon activity of ecto-5'-nucleotidase of intact C6 glioma cells in culture has been demonstrated. A near linear rate of decrease in 5'-nucleotidase activity was observed upon treatment with concentrations of Con A up to 0.25 muM. Nonspecific phosphatase activity and Ca2+-dependent ATPase activity were not inhibited by Con A treatment of the cells. Of the total 5'-nucleotidase activity of C6 cells (Vmax = 5.0 mumol of Pi liberated/mg of cell protein/hour), approximately 20% still remained after treatment with high concentrations of Con A. The inhibitory effect of Con A operated to reduce substantially Vmax for ecto-5'-nucleotidase. Inhibition was reversed by briefly incubating the Con A-treated cells with alpha-methyl-D-glucoside, or alpha-methyl-D-mannoside, the later being more effective. These findings suggest that a relatively specific, reversible, inhibition of ecto-5'-nucleotidase results from Con A binding to the surface of the intact cultured mammalian cells.     

Concanavalin A inhibition of alpha-bungarotoxin binding to a nonfusing muscle cell line.

Incubation of a nonfusing muscle cell line, BC3H1, with concanavalin A (Con A) results in a maximum decrease of 35% in the cell's ability to bind alpha-bungarotoxin (alpha-BuTx). The Con A-induced inhibition of 125I-alpha-BuTx binding is reversible and the degree of inhibition parallels the degree of saturation of Con A binding sites on the cell surface. The maximum level of Con A-induced inhibition of 125I-alpha-BuTx binding is not affected by increasing the time of incubation in Con A, using higher concentrations of Con A or by increasing the time of incubation in the presence of 125I-alpha-BuTx. In addition, all BC3H1 cells in culture are sensitive to the Con A-induced inhibition of 125I-alpha-BuTx binding. A comparison of the pseudo-first order rate constants for 125I-alpha-BuTx binding to untreated (8.6 x 10(4) M-1 S-1) and Con A-treated (5.4 x 10(4) M-1 S-1) BC3H1 cells, however, shows that those acetylcholine receptors in Con A-treated cells which bind 125I-alpha-BuTx do so with a lowered apparent affinity. Partial inhibition of toxin-binding capacity is not a consequence of two classes of acetylcholine receptors on the cell surface. Furthermore, individual receptors experience partial inhibition of their binding capacity by Con A, resulting in receptors with at least one binding site blocked and at least one site available for alpha-BuTx binding.     

A Cytological Analysis of Differentiation Without Cleavage in Cytochalasin B- and Colchicine-Treated Embryos of Chaetopterus pergamentaceus

Chaetopterus eggs undergo characteristic ooplasmic rearrangements during development. Ooplasmic rearrangement in the absence of cell division is called differentiation without cleavage. Treatment of fertilized eggs with cytochalasin B allowed the continuation of nuclear divisions in the absence of cytoplasmic division. The ooplasmic rearrangements in uncleaved cytochalasin B-treated fertilized eggs closely paralleled those of normal development. Colchicine treatment, which blocks mitosis, arrested ooplasmic movements at a stage comparable to that of normal embryos at first cleavage. Neither drug eliminated the segregation between hyaloplasm and endoplasm, even though colchicine prevented the later rearrangements. Localizing movements are therefore dependent upon normal microtubule function, but not on microfilament function. The maintenance of localized materials does not seem to depend exclusively on either of these organelles.     

Suppression of the polyclonal B-cell responses by concanavalin A-treated bone marrow B cells in vitro

A suppressor cell that inhibits the development of a polyclonal antibody response of splenic B cells to lipopolysaccharide is generated in the bone marrow cell culture in response to a mitotic dose (10 micrograms/ml) of concanavalin A (Con A). The Con A-responding suppressor cell is radioresistant and found in a bone marrow B (BM-B) cell population of normal as well as athymic mice. The suppressor activity of Con A-treated BM-B cells was consistently higher (P less than 0.01-0.0001) than those of untreated BM-B and fresh BM cells. The BM-B cell population recovered from short-term (3-day) cultures with Con A contained about 65% surface immunoglobulin (Ig)-positive cells, about 6% T cells, and less than 0.5% plastic-adherent cells, the latter two of which did not contribute to the suppressive activity. Thus, cytolytic treatment with various anti-T-cell antibodies could not eliminate the suppressive activity of the Con A-treated BM-B cells, and the Con A-treated macrophage population provided no significant suppression. The Con A-treated BM-B cells adherent to anti-Ig or anti-Con A dishes exhibited highly enriched suppressive activity. It was therefore concluded that an immature B-cell population of bone marrow could develop in response to stimulation with Con A into surface Ig-positive suppressor cells, contributing to the regulation of nonspecific B-cell responses.     

Analysis of sea urchin egg cortical transformation in the absence of cortical granule exocytosis

A burst of endocytosis accompanying microvillar elongation follows cortical granule exocytosis in normal sea urchin development. By 5 min postfertilization the burst is over and a lower level of endocytosis ensues (constitutive phase). To determine whether microvillar elongation and initiation of endocytosis are necessary concommitants of cortical granule exocytosis we utilized Chase's (1967, Ph.D. thesis, University of Washington, Seattle) high-hydrostatic pressure technique to block the latter and then examined developing eggs for endocytosis and microvillar elongation. To accomplish this, eggs were fertilized, after which hydrostatic pressure was quickly raised to 6000-7000 psi at the start of cortical granule exocytosis and maintained for 5 min. Only the cortical granules immediately surrounding the sperm penetration site were secreted (about 3% or less of the egg's total number of cortical granules). Blockage of major cortical granule exocytosis had the following consequences on surface events during first division: (1) The endocytosis burst normally associated with cortical granule exocytosis was effectively eliminated as was early microvillar elongation and elevation. Both occurred to a limited extent around the sperm penetration site which resulted in a highly localized surface transformation. (2) By 20 min after fertilization endocytosis began over the rest of the egg surface in the absence of any further cortical granule exocytosis. (3) Subsequently, during a 30-min period starting midway between fertilization and first cleavage microvilli more than doubled in length and endocytosis levels increased severalfold. These events brought about a complete surface transformation similar to that which normally occurs in early development but in the absence of cortical granule exocytosis. By first cleavage surfaces and cortices of high-pressure-treated and control eggs were nearly indistinguishable except for the presence of cortical granules in cortices of the former. Pressure-treated eggs cleaved normally and developed to larval forms overnight. The period of late surface transformation in high-pressure-treated Strongylocentrotus purpuratus eggs corresponds in timing and some of its characteristics to second phase microvillar elongation observed in normal development in this species and also in S. droebachiensis development. These observations suggest, therefore, that microvillar elongation and endocytosis are necessary membrane remodelling events which must occur for normal development even in the absence of membrane addition from the cortical granules.     

Topography and microfilament core association of a cell surface glycoprotein of ascites tumor cell microvilli

Membrane-microfilament interactions are being investigated in microvilli isolated from 13762 rat mammary ascites tumor cells. These microvilli are covered by a sialomucin complex, composed of the sialomucin ascites sialoglycoprotein-1 (ASGP-1) and the associated concanavalin A (Con A)-binding glycoprotein ASGP-2. Limited proteolysis of the microvilli releases large, highly glycosylated fragments of ASGP-1 from the microvilli and increases the association of ASGP-2 with the Triton-insoluble microvillar microfilament core (Vanderpuye OA, Carraway CAC, Carraway, KL: Exp Cell Res 178:211, 1988). To analyze the topography of ASGP-2 in the membrane and its association with the microfilament core, microvilli were treated with proteinase K for timed intervals and centrifuged. The pelleted microvilli were extracted with Triton X-100 for the preparation of microfilament cores and Triton-soluble proteins or with 0.1 M carbonate, pH 11, for the preparation of microvillar membranes depleted of peripheral membrane proteins. These microvilli fractions were analyzed by dodecyl sulfate gel electrophoresis, lectin blotting with Con A and L-phytohemagglutinin, and immunoblotting with anti-ASGP-2. The earliest major proteolysis product from this procedure was a 70 kDa membrane-bound fragment. At longer times a 60 kDa released fragment, 30-40 kDa Triton-soluble fragments, and 25-30 kDa membrane- and microfilament-associated fragments were observed. Phalloidin shift analysis of microfilament-associated proteins on velocity sedimentation gradients indicated that the 25-30 kDa fragments were strongly associated with the microfilament core. From these studies we propose that ASGP-2 has a site for indirect association with the microfilament core near the membrane on a 15-20 kDa segment.     

The scaffolding protein EBP50 regulates microvillar assembly in a phosphorylation-dependent manner

The mechanisms by which epithelial cells regulate the presence of microvilli on their apical surface are largely unknown. A potential regulator is EBP50/NHERF1 (ERM-binding phosphoprotein of 50 kD/Na(+)-H(+) exchanger regulatory factor), a microvillar scaffolding protein with two PDZ domains followed by a C-terminal ezrin-binding domain. Using RNAi and expression of RNAi-resistant EBP50 mutants we systematically show that EBP50 is necessary for microvillar assembly and requires that EBP50 has both a functional first PDZ domain and an ezrin-binding site. Expression of mutants mimicking Cdc2 or PKC phosphorylation are nonfunctional in microvillar assembly. Biochemical analysis reveals that these mutants are defective in PDZ1 accessibility when PDZ2 is occupied, and can be rendered functional in vivo by additional mutation of PDZ2. EBP50 is not necessary for mitotic cell microvilli, and PKC activation causes a rearrangement of microvilli on cells due to phosphorylation-dependent loss of EBP50 function. Thus, EBP50 is a critical factor that regulates microvilli assembly and whose activity is regulated by signaling pathways and occupation of its PDZ2 domain.     

Fundamental role of microvilli in the main functions of differentiated cells: Outline of an universal regulating and signaling system at the cell periphery

Until now, the general importance of microvilli present on the surface of almost all differentiated cells has been strongly underestimated and essential functions of these abundant surface organelles remained unrecognized. Commonly, the role of microvilli has been reduced to their putative function of cell‐surface enlargement. In spite of a large body of detailed knowledge about the specific functions of microvilli in sensory receptor cells for sound, light, and odor perception, their functional importance for regulation of basic cell functions remained obscure. Here, a number of microvillar mechanisms involved in fundamental cell functions are discussed. Two structural features enable the extensive functional competence of microvilli: First, the exclusive location of almost all functional important membrane proteins on microvilli of differentiated cells and second, the function of the F‐actin‐based cytoskeletal core of microvilli as a structural diffusion barrier modulating the flow of low molecular substrates and ions into and out of the cell. The specific localization on microvilli of important functional membrane proteins such as glucose transporters, ion channels, ion pumps, and ion exchangers indicate the importance and diversity of microvillar functions. In this review, the microvillar mechanisms of audioreceptor, photoreceptor, and olfactory/taste receptor cells are discussed as highly specialized adaptations of a general type of microvillar mechanisms involved in regulation of important basic cell functions such as glucose transport/energy metabolism, ion channel regulation, generation and modulation of the membrane potential, volume regulation, and Ca signaling. Even the constitutive cellular defence against cytotoxic compounds, also called “multidrug resistance (MDR),” is discussed as a microvillar mechanism. A comprehensive examination of the specific properties of “cable‐like” ion conduction along the microvillar core structure of F‐actin allows the proposal that microvilli are specifically designed for using ionic currents as cellular signals. In view of the multifaceted gating and signaling properties of TRP channels, the possible role of microvilli as a universal gating device for TRP channel regulation is discussed. Combined with the role of the microvillar core bundle of actin filaments as high‐affinity Ca store, microvilli may turn out as highly specialized Ca signaling organelle involved in store‐operated Ca entry (SOCE) and initiation of nonlinear Ca signals such as waves and oscillations. J. Cell. Physiol. 226: 896–927, 2011. © 2010 Wiley‐Liss, Inc.