Distribution of microtubules and microfilaments in exocrine (ventral prostatic epithelial cells and pancreatic exocrine cells) and endocrine cells (cells of the adenohypophysis and islets of Langerhans). The relationship between cytoskeletons and epithelial-cell polarity

Changes in organelle position and epithelial architecture associated with loss of CrebA

Drosophila CrebA facilitates high-level secretion by transcriptional upregulation of the protein components of the core secretory machinery. In CrebA mutant embryos, both salivary gland (SG) morphology and epidermal cuticle secretion are abnormal, phenotypes similar to those observed with mutations in core secretory pathway component genes. Here, we examine the cellular defects associated with CrebA loss in the SG epithelium. Apically localized secretory vesicles are smaller and less abundant, consistent with overall reductions in secretion. Unexpectedly, global mislocalization of cellular organelles and excess membrane accumulation in the septate junctions (SJs) are also observed. Whereas mutations in core secretory pathway genes lead to organelle localization defects similar to those of CrebA mutants, they have no effect on SJ-associated membrane. Mutations in tetraspanin genes, which are normally repressed by CrebA, have mild defects in SJ morphology that are rescued by simultaneous CrebA loss. Correspondingly, removal of several tetraspanins gives partial rescue of the CrebA SJ phenotype, supporting a role for tetraspanins in SJ organization.

Functional implications of the Golgi and microtubular network in gonadotropes

In contrast to the widely accepted images of the Golgi apparatus as a cup-like shape, the Golgi in pituitary gonadotropes is organized as a spherical shape in which the outer and inner faces are cis- and trans-Golgi elements, respectively. At the center of the spherical Golgi, a pair of centrioles is situated as a microtubule-organizing center from which radiating microtubules isotropically extend toward the cell periphery. This review focuses on the significance of the characteristic organization of the Golgi and microtubule network in gonadotropes, considering the roles of microtubule-dependent membrane transport in the formation and maintenance of the Golgi structure. Because the highly symmetrical organization of the Golgi is possibly perturbed in response to experimental treatments of gonadotropes, monitoring of the Golgi structure in gonadotropes under various experimental conditions will be a novel in vivo approach to elucidate the biogenesis of the Golgi apparatus.

Exocytosis and movement of zymogen granules observed by VEC-DIC microscopy in the pancreatic tissue en bloc

The dynamic aspects of exocytosis, especially in the normal acinar tissue en bloc, have remained unclear. We visualized exocytosis directly in the tissue of the exocrine pancreas of rodents by video-enhanced contrast-differential interference contrast (VEC-DIC) microscopy to investigate various exocytosis-related rates and the relationship between the movement of granules and exocytotic responses. Stimulation of the tissue with bethanechol or cholecystokinin caused many of the zymogen granules in the apical pole to disappear abruptly. The exocytotic transients of individual granules were completed in 0.48-0.65 s. Granules destined to participate in the exocytotic response moved randomly at velocities of approximately 0.06 microm/s or less during stimulation. In the tissue preparation, granules located far from the apical pole frequently moved back and forth for 1-7 microm without showing exocytosis. Colchicine suppressed this movement and the late phase of the secretory response. Real-time (VEC-DIC) observation of granule dynamics revealed that the initial step of exocytosis was not coupled directly with the microtubule-dependent translocation but with a continuous, slow Brownian fluctuation of granules.

Microtubules regulate local Ca2+ spiking in secretory epithelial cells

The role of the cytoskeleton in regulating Ca(2+) release has been explored in epithelial cells. Trains of local Ca(2+) spikes were elicited in pancreatic acinar cells by infusion of inositol trisphosphate through a whole cell patch pipette, and the Ca(2+)-dependent Cl(-) current spikes were recorded. The spikes were only transiently inhibited by cytochalasin B, an agent that acts on microfilaments. In contrast, nocodazole (5-100 micrometer), an agent that disrupts the microtubular network, dose-dependently reduced spike frequency and decreased spike amplitude leading to total blockade of the response. Consistent with an effect of microtubular disruption, colchicine also inhibited spiking but neither Me(2)SO nor beta-lumicolchicine, an inactive analogue of colchicine, had any effect. The microtubule-stabilizing agent, taxol, also inhibited spiking. The nocodazole effects were not due to complete loss of function of the Ca(2+) signaling apparatus, because supramaximal carbachol concentrations were still able to mobilize a Ca(2+) response. Finally, as visualized by 2-photon excitation microscopy of ER-Tracker, nocodazole promoted a loss of the endoplasmic reticulum in the secretory pole region. We conclude that microtubules specifically maintain localized Ca(2+) spikes at least in part because of the local positioning of the endoplasmic reticulum.

Preparation of a monoclonal antibody specific for the class I isotype of beta-tubulin: the beta isotypes of tubulin differ in their cellular distributions within human tissues

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta consist of various isotypes. Although the isotypes differ in their tissue distributions, the question of whether the isotypes perform different functions in vivo is unanswered. In mammals, the betaI and betaIV isotypes are quite widespread, and betaII is less so, while betaIII and betaVI have narrow distributions and betaV distribution is unknown. As a tool for localizing the isotypes, we report the preparation of a monoclonal antibody specific for betaI, to add to our previously described monoclonal antibodies specific for betaII, betaIII, and betaIV [Banerjee et al., J. Biol. Chem. 263:3029-3034, 1988; 265:1794-1799, 1990; 267:5625-5630, 1992]. In order to prepare this antibody, we have purified betaI-rich rat thymus tubulin. We have used our battery of antibodies to localize the beta isotypes in four human tissues: oviduct, skin, colon, and pancreas. We have found striking differences in their tissue distributions. There is little or no betaIII in these tissues, except for the columnar epithelial cells of the colon. BetaII is restricted to very few cells, except in the skin, where it is concentrated in the stratum granulosum. BetaI is widespread in all the epithelia. In the skin it is found in the entire stratum malpighii. In the oviduct, betaI is found largely in the nonciliated epithelial cells. In the exocrine pancreas, betaI occurs only in the centroacinar cells and not in the acinar cells; the latter do not stain with any of these antibodies. BetaIV is present at very low levels in skin and pancreas. By contrast, it is prominent in the colon and also in the oviduct, where it occurs in all the epithelial cells, especially in the ciliated cells, with the highest concentrations in the cilia themselves. These results suggest that the regulation of the expression and localization of isotypes in tissues is very complex.

Ultrastructural localisation and size distribution of collagen fibrils in Glisson's sheath of rat liver: implications for mechanical environment and possible producing cells

The ultrastructure and size distributions of collagen fibrils in Glisson's sheath were investigated in the rat liver to analyse the mechanical environment around the fibrils and their possible cells of origin. Glisson's sheath was found to contain 2 populations of collagen fibrils with different diameters and distinct localisations, namely fibroblast-associated and bile epithelium-associated. Fibroblast-associated collagen was composed of fibrils arranged in bundles and constituted the majority of the collagen in Glisson's sheath. Bile epithelium-associated collagen was represented by small dispersed groups of fibrils just beneath the basement membrane of the bile duct. The basement membrane of the bile duct was frequently reduplicated into a few or as many as 10 layers of laminae densae, with scattered collagen fibrils between these laminae. The diameters of the fibrils of both groups of collagen increased in relation to the calibre of the bile duct, whereas at any given place in Glisson's sheath bile epithelium-associated collagen fibrils had a smaller diameter compared with those of the fibroblast-associated fibrils. The increment in fibril diameter along the bile duct is considered to be correlated with the increase in mechanical stress acting on Glisson's sheath. The difference in diameter between the 2 populations as well as the incorporation of fibrils between the laminae densae of the basement membrane of the bile duct supports the view that the bile epithelium-associated collagen is produced by the epithelial cells of the bile duct, thus having a different origin from that of fibroblast-associated collagen. These findings provide the first evidence that the epithelial cells of the interlobular bile duct produce fibril-forming collagen. Furthermore, it is suggested that cholestasis stimulates the epithelial cells of interlobular bile duct to increased synthesis of fibril-forming collagen that is also produced by these cells under physiological conditions.

Changes in kinesin distribution and phosphorylation occur during regulated secretion in pancreatic acinar cells

In secretory cells, microtubule- (Mt-) based motor enzymes are thought to support transport of secretory vesicles to the cell surface for subsequent release. At present, the role of Mts and kinesin in secretory vesicle transport in exocrine epithelial cells has not been defined. Furthermore, it is unclear whether an agonist-induced secretory event modifies kinesin function and distribution, thus altering vesicle transport. To this end, we utilized isolated rat pancreatic acini and cultured rat pancreatic acinar cells to examine the role of Mts and kinesin in regulated secretion. Exposure of cells to cytoskeletal antagonistic drugs demonstrated that the observed movements of apically clustered zymogen granules (ZGs) are supported by Mts, but not actin. Morphological studies of Mt organization in polarized acini show that Mt plus ends extend outward from the apical membrane toward the cell center. Immunofluorescence microscopy in both cell models revealed a clear association of kinesin with apical ZGs, while quantitative immunoblot analysis of pancreatic subcellular fractions confirmed kinesin enrichment on ZG membranes. In addition, microinjection of kinesin antibodies into cultured acinar cells inhibited ZG movements. Indirect immunofluorescence staining of isolated cells and quantitative Western blotting of isolated ZGs revealed that kinesin association with granule membranes increased up to 3-fold in response to a secretory stimulus. Autoradiographic studies of 32P-labeled acini showed up to a 6-fold increase in kinesin heavy chain (KHC) phosphorylation during stimulated secretion. These studies provide the first direct evidence that Mts and kinesin support ZG movements and that physiological agonists induce a marked phosphorylation of KHC while increasing the association of kinesin with ZG membranes. These changes during agonist stimulation suggest that the participation of kinesin in zymogen secretion is regulated.

The cortical actin cytoskeleton of lactotropes as an intracellular target for the control of prolactin secretion

We investigated the role of cortical actin filaments (F-actin) in the regulation of PRL secretion in cultured normal anterior pituitary cells. F-actin dynamics were evaluated by fluorescence microscopy, and PRL secretion from attached cells was measured by the reverse hemolytic plaque assay. F-actin localized to the periphery of lactotropes. PRL-releasing factors such as TRH, vasoactive intestinal peptide (VIP), and forskolin, or removal of the PRL-inhibiting factor dopamine (DA) from cultures chronically exposed to DA, caused fragmentation, i.e. focal disassembly of cortical F-actin. Basal, VIP-, and DA withdrawal-induced cortical F-actin disassembly were dependent on extracellular Ca2+ whereas TRH- and forskolin-induced disassembly were not. Short-term (5 min) treatment of cells with the F-actin-disrupting agent cytochalasin D (CD) enhanced basal PRL secretion but did not further stimulate TRH- or VIP-induced PRL secretion. The results support the existence of a causal link between F-actin disassembly and increased PRL secretion. On the other hand, exposure of cultures to DA decreased the percentage of cells showing cortical F-actin disassembly within minutes. Longer treatments (2-4 h) caused stabilization of cortical actin filaments as revealed by the protection vis-a-vis the depolymerizing effect of CD. The protective effect was specific for lactotropes and was evident with DA concentrations as low as 50 nM. Chronic exposure of the cells to DA blocked CD- and TRH-evoked actin disassembly and PRL secretion while VIP-induced effects were partially inhibited. Stabilization of F-actin with the marine sponge venom, jasplakinolide, also decreased basal and stimulated PRL secretion. In conclusion, our results suggest that, first, the cortical actin cytoskeleton of lactotropes is an integrator of the multiple factors regulating PRL secretion directly on the lactotrope, and second, the tonic inhibition of PRL secretion is mediated, at least in part, by DA-induced stabilization of cortical F-actin.

Reperfusion after brief ischemia disrupts the microtubule network in canine hearts

Histological changes in the stunned myocardium are believed to be minimal. This study examined whether cytoskeletal structures of microtubules are disrupted in the stunned myocardium. In 38 dogs, the left anterior descending coronary artery was occluded for 15 minutes and reperfused to produce the stunned myocardium. Microtubules were stained immunohistochemically. In intact myocardium, microtubules appeared as a filamentous network throughout the cytoplasm and encircled the nucleus. This pattern was not affected by 15 minutes of ischemia. One hour of reperfusion, however, disrupted microtubular structure substantially (disruption score in the endocardium, 53.4 +/- 6.0%) although actin filaments remained intact. Microtubular structures were reconstituted 1-3 days after reperfusion, showing supernormal immunoreactivity. Five days after reperfusion, the pattern of microtubular staining was normal. In another protocol, the role of Ca2+ during reperfusion in microtubular disruption was examined. When intracoronary infusion of EDTA (1.67 mumol/kg body wt per minute) was performed during the initial 10 minutes of reperfusion, myocardial stunning was attenuated. The fractional shortening in the perfused area after 1 hour of reperfusion was 20.1 +/- 1.2% versus 11.5 +/- 0.5% in the control condition (p < 0.05), and the microtubular disruption score was lower (12.6 +/- 1.4%). Although intracoronary infusion of calcium chloride (9 mumol/kg body wt per minute) for 10 minutes in nonischemic hearts increased contractile function (fractional shortening, 25.3 +/- 2.0%), it severely disrupted microtubular networks (microtubular disruption score, 64.0 +/- 10.6%). We conclude that microtubules supporting the structural integrity of myofibrils and other organelles are reversibly disrupted by reperfusion after brief ischemia probably through calcium overload.

The cell biology of blastocyst development

Preimplantation development encompasses the "free"-living period of mammalian embryogenesis, which culminates in the formation of a fluid-filled structure, the blastocyst. Cavitation (blastocyst formation) is accompanied by the expression of a novel set of gene products that contribute directly to the attainment of cell polarity with the trophectoderm, which is both the first epithelium of development and the outer cell layer encircling the inner cell mass of the blastocyst. Several of these gene products have been identified and include the tight junction (ZO-1), Na/K-ATPase (alpha and beta subunits), uvomorulin, gap junction (connexin43), and growth factors such as transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF). This review will examine the role(s) of each of these gene products during the onset and progression of blastocyst formation. The trophectodermal tight junctional permeability seal regulates the leakage of blastocoel fluid and also assists in the maintenance of a polarized Na/K-ATPase distribution to the basolateral plasma membrane domain of the mural trophectoderm. The polarized distribution of the Na/K-ATPase plays an integral role in the establishment of a trans-trophectoderm Na+ gradient, which drives the osmotic accumulation of water across the epithelium into the nascent blastocoelic cavity. The cell adhesion provided by uvomorulin is necessary for the establishment of the tight junctional seal, as well as the maintenance of the polarized Na/K-ATPase distribution. Growth factors such as TGF-alpha and EGF stimulate an increase in the rate of blastocoel expansion, which could, in part, be mediated by secondary messengers that result in an increase in Na/K-ATPase activity. Insight into the mechanism of cavitation has, therefore, directly linked blastocyst formation to trophectoderm cell differentiation, which arises through fundamental cell biological processes that are directly involved in the attainment of epithelial cell polarity.

The endocrine pancreas of glucagon- and somatostatin-immunized rabbits. II. Electron microscopy

An active or passive immunization against hormones and the subsequent neutralization of hormones by circulating antibodies is a valuable tool for the identification of hormonal action. To recognize presumed local (autocrine, paracrine) effects exerted by pancreatic hormones, the endocrine pancreas of rabbits was investigated electron-microscopically after long-term immunization against glucagon or somatostatin. Glucagon immunization resulted in hyperplasia and hypertrophy of glucagon- (A-) cells and in their increased metabolic activities: They showed prominent nucleoli, increased amounts of endoplasmic reticulum, Golgi areas, and mitochondria. These changes were paralleled by alterations in secretion granules (increased size, decreased hormonal content), increased numbers of lysosomes (crinophagic bodies), and an increment of the filamentous system. Basically, these findings point to an autocrine regulation of A-cells. Following somatostatin immunization, somatostatin- (D-) cells were hyperplastic but unchanged in their metabolic state. Instead, insulin-(B-) cells and A-cells exhibited equivalents of increased cellular activities (parameters, see above). This stimulation most probably is caused by cancelled paracrine (inhibitory) effects of somatostatin. The changes observed after both immunizations were differently expressed in morphologically heterogeneous islet types (size, angioarchitecture, cellular composition, microtopology of the various cell types). It is concluded, therefore, that the regulation of islets is not uniform. Autocrine and paracrine effects exerted by islet hormones are of different significance in individual islets, or they interfere differently with other regulatory signals.

Disruption of microtubules as an early sign of irreversible ischemic injury. Immunohistochemical study of in situ canine hearts

Structural disruption of the cytoskeleton may be involved in irreversible ischemic injury. In the present study, ischemic changes in microtubules during various periods of myocardial ischemia were studied with an immunohistochemical technique in open-chest dogs. In intact myocardium, microtubules were stained as a filamentous network throughout cytoplasm and a circular network around the nucleus, which disappeared with colchicine treatment. In brief ischemia of less than 15 minutes, microtubule patterns were unaltered. After 20 minutes, however, characteristic microtubule stains were partly lost in patchy lesions. As an increase in ischemic period, lesions of loss of microtubule stains were increased in number and size. After 120 minutes of reperfusion following 60 minutes of ischemia, the lesions with intact actin filaments but with disrupted microtubules were replaced by the severely injured cells in which the regular myofibrillar registrations were distinctly disrupted. After 24 hours of reperfusion following 40 minutes of occlusion of the left circumflex artery, the percent area of disrupted microtubules at 40 minutes of ischemia was replaced by that of irreversibly injured lesions in the posterior papillary muscle. These results indicate that disruption of microtubules during ischemia heralds irreversible ischemic injury. However, in in vitro study, the myocardium incubated in hypoxic solution for 60-120 minutes demonstrated earlier disruption of the microtubules than the vinculin. Electron microscopic study also showed minimal irreversible changes in the lesions with disrupted microtubules. Thus, taken together, we conclude that microtubules that support the structural integration of myofibrils and other organelles are disrupted in severe myocardial ischemia before the irreversible injury, promoting the irreversible change after reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin cytoskeleton and calcium-ATPase in the process of abomasal mucus secretion in cattle

The distribution of actin filaments in pyloric gland cells of cattle was studied with respect to their functional significance in the process of exocrine secretion by use of rhodamine-phalloidin labelling and immunogold-electron microscopy based on the biotin-streptavidin bridge technique. Actin concentrates on the filamentous network of the luminal-cell cortex. Membranes of secretory vesicles accumulating in the cell cortex are also labelled for actin. The present results support the concept of a barrier function of cortical microfilaments entrapping vesicles and linking them to the cytoskeleton. In addition, intracellular localization of calcium-ATPase activity was determined. Enzyme activity associated with the microfilamentous cortical matrix is supposed to be of cytoskeletal nature indicating participation of myosin (-like) structures in the dynamic secretion event. Deposition on the interior aspect of secretory vesicle membranes points to an ATPase transporting calcium into these organelles and enabling them to participate via storage of the cation in intracellular calcium homeostasis, thereby influencing the functional architecture of the cortical cytoskeleton.

Distribution of microtubules and microfilaments in thyroid follicular epithelial cells of normal, TSH-treated, aged, and hypophysectomized rats

We investigated the distribution of microtubules and microfilaments in rat thyroid follicular epithelial cells by applying an immunofluorescence technique with monoclonal antibodies against tubulin and by staining sections with rhodamine-phalloidin. In normal thyroid cells, microtubules run longitudinally from the apical region to the basal region intersecting with each other. In addition, intense labelling with tubulin antibodies was observed in the apical part of the cell. The ultrastructural examinations showed that microtubules often run along the apical plasma membrane. Dot-like labelling with anti-tubulin antibodies was often observed in the perinuclear space, but not microtubules were recognized in the nucleus. Microfilaments bound to rhodamine-phalloidin were distributed mainly beneath the apical plasma membrane, and the portion along the basolateral membrane was scarcely positive. The apical pole of the follicle cell was also decorated by anti-microtubule-associated protein-2 (MAP-2). After TSH stimulation, the intensity of immunocytochemical staining against tubulin was remarkably increased in the cytoplasm. Simultaneously, at the apical region, the staining intensity of rhodamine-phalloidin was increased. Microtubules and microfilaments appeared in the pseudopods after TSH injection. In hypophysectomized or aged rats, thyroid follicular epithelial cells decreased in height, and both immunofluorescent labelling against tubulin and rhodamine-phalloidin labelling were markedly decreased. These results indicate that the distribution and polymerization of microtubules and microfilaments in thyroid follicular epithelial cells vary with the functional stage.