Secretion of mucous granules and other membrane-bound structures: a look beyond exocytosis

Membrane mediated development of the vertebrate blood-gas-barrier

During embryonic lung development, establishment of the gas-exchanging units is guided by epithelial tubes lined by columnar cells. Ultimately, a thin blood-gas barrier (BGB) is established and forms the interface for efficient gas exchange. This thin BGB is achieved through processes, which entail lowering of tight junctions, stretching, and thinning in mammals. In birds the processes are termed peremerecytosis, if they involve cell squeezing and constriction, or secarecytosis, if they entail cutting cells to size. In peremerecytosis, cells constrict at a point below the protruding apical part, resulting in fusion of the opposing membranes and discharge of the aposome, or the cell may be squeezed by the more endowed cognate neighbors. Secarecytosis may entail formation of double membranes below the aposome, subsequent unzipping and discharge of the aposome, or vesicles form below the aposome, fuse in a bilateral manner, and release the aposome. These processes occur within limited developmental windows, and are mediated through cell membranes that appear to be of intracellular in origin. In addition, basement membranes (BM) play pivotal roles in differentiation of the epithelial and endothelial layers of the BGB. Laminins found in the BM are particularly important in the signaling pathways that result in formation of squamous pneumocytes and pulmonary capillaries, the two major components of the BGB. Some information exists on the contribution by BM to BGB formation, but little is known regarding the molecules that drive peremerecytosis, or even the origins and composition of the double and vesicular membranes involved in secarecytosis.

Development and remodeling of the vertebrate blood-gas barrier

During vertebrate development, the lung inaugurates as an endodermal bud from the primitive foregut. Dichotomous subdivision of the bud results in arborizing airways that form the prospective gas exchanging chambers, where a thin blood-gas barrier (BGB) is established. In the mammalian lung, this proceeds through conversion of type II cells to type I cells, thinning, and elongation of the cells as well as extrusion of the lamellar bodies. Subsequent diminution of interstitial tissue and apposition of capillaries to the alveolar epithelium establish a thin BGB. In the noncompliant avian lung, attenuation proceeds through cell-cutting processes that result in remarkable thinning of the epithelial layer. A host of morphoregulatory molecules, including transcription factors such as Nkx2.1, GATA, HNF-3, and WNT5a; signaling molecules including FGF, BMP-4, Shh, and TFG- β and extracellular proteins and their receptors have been implicated. During normal physiological function, the BGB may be remodeled in response to alterations in transmural pressures in both blood capillaries and airspaces. Such changes are mitigated through rapid expression of the relevant genes for extracellular matrix proteins and growth factors. While an appreciable amount of information regarding molecular control has been documented in the mammalian lung, very little is available on the avian lung.

The pulmonary blood-gas barrier in the avian embryo: inauguration, development and refinement

In vertebrates, efficient gas exchange depends primarily on establishment of a thin blood-gas barrier (BGB). The primordial air conduits of the developing avian lung are lined with a cuboidal epithelium that is ultimately converted to a squamous one that participates in the formation of the BGB. In the early stages, cells form intraluminal protrusions (aposomes) then transcellular double membranes separating the aposome from the basal part of the cell establish, unzip and sever the aposome from the cell. Additionally, better endowed cells squeeze out adjacent cells or such cells constrict spontaneously thus extruding the squeezed out aposome. Formation of vesicles or vacuoles below the aposome and fusion of such cavities with their neighboring cognates results in severing of the aposome. Augmentation of cavities and their subsequent fusion with the apical plasma membranes results in formation of numerous microfolds separating concavities on the apical part of the cell. Abscission of such microfolds results in a smooth squamous epithelium just before hatching.

Autofluorescence as a tool to study mucus secretion in Eisenia foetida

Autofluorescence in living cells is due to the presence of endogenous substances that emit fluorescence upon excitation by incidental light. A type of fluorescence, bioluminescence, has been suggested to be linked to mucus secretion in earthworms; however, the origin and the physiological function of this fluorescence are not clear. The aims of this work were to describe autofluorescence in the earthworm Eisenia foetida by SEM, CLSM, and fluorescence microscopy and to examine the possible mechanism of mucus secretion by video microscopy. Earthworms were stimulated either chemically or electrically to induce the secretion of yellow mucus, which was subsequently studied by video microscopy. Mucus was released from the body wall and near the mouth. This phenomenon was associated with autofluorescence and involved at least four distinct stages: release of vesicles, formation of granules, muscular contraction, and organization of strands. The fluorescent molecules were stored in vesicles bound to the membranes. These vesicles were intact when shed from the body. The vesicles were stable but also changed to a granular material or formed strands. Video analyses demonstrated that secretion was dependent on the type of stimulus.

Slug species- and population-specific effects on the end points of the Slug Mucosal Irritation test

The Slug Mucosal Irritation test seems to be a promising method for the evaluation of the local tolerance of products applied to the mucosa. Furthermore, the Slug Mucosal Irritation test is a reliable method to classify chemicals accurately into three eye irritation categories based on the mucus production and the score for tissue damage. Until now the slug Arion lusitanicus collected in Belgium was always used as test organism. The present study investigated the effects of the slug population and species on the end points of the test and on the eye irritation classification. For this purpose, the eye irritation/damage test procedure and eye reference chemicals were used. Comparison of the results of one Belgian and two Swiss A. lusitanicus populations indicated that the geographic and ecological origins of slug populations did neither influence the mucus production, nor the score for tissue damage. Slug species-specific effects on the test end points were investigated by comparing the data of Belgian Limax flavus with corresponding data of Belgian A. lusitanicus. L. flavus produced more mucus than A. lusitanicus, so that the mucus production cut-off values had to be increased. Therefore, the results indicated that the test procedure and prediction model have to be optimised and validated, if other slug species are used instead of A. lusitanicus.

Epithelial transformations in the establishment of the blood–gas barrier in the developing chick embryo lung

The tall epithelium of the developing chick embryo lung is converted to a squamous one, which participates in formation of the thin blood-gas barrier. We show that this conversion occurred through processes resembling exocrine secretion. Initially, cells formed intraluminal protrusions (aposomes), and then transcellular double membranes were established. Gaps between the membranes opened, thus, severing the aposome from the cell. Alternatively, aposomes were squeezed out by adjacent cells or were spontaneously constricted and extruded. As a third mechanism, formation and fusion of severed vesicles or vacuoles below the aposome and their fusion with the apicolateral plasma membrane resulted in severing of the aposome. The atria started to form by progressive epithelial attenuation and subsequent invasion of the surrounding mesenchyme at regions delineated by subepithelial alpha-smooth muscle actin-positive cells. Further epithelial attenuation was achieved by vacuolation; rupture of such vacuoles with resultant numerous microfolds and microvilli, which were abscised to accomplish a smooth squamous epithelium just before hatching.

Aldose reductase and macrophage migration inhibitory factor are associated with epididymosomes and spermatozoa in the bovine epididymis

During the epididymal transit, mammalian spermatozoa acquire new surface proteins necessary for male gamete function. We have previously shown that membranous vesicles, called epididymosomes, interact with spermatozoa allowing the transfer of some proteins to sperm surface within the epididymal lumen. The protein composition of those vesicles has been investigated to document the mechanisms of protein transfer from epididymosomes to spermatozoa. Electrophoretic analysis revealed that protein composition is different from the epididymal soluble compartment as well as from similar vesicles present in the semen. Protein association with epididymosome is very strong as revealed by resistance to extraction with detergent. Matrix-assisted laser desorption ionization time-of-flight as well as immunodetection techniques have been used to identify some proteins associated to epididymosomes and spermatozoa. An aldose reductase known for its 20alpha-hydroxysteroid dehydrogenase activity and the cytokine (macrophage migration inhibitory factor) have been identified. These two proteins have been immunolocalized in principal cells of the epididymal epithelium, a more intense signal being detected in the distal epididymal segment as well as in the vas deferens. Database search revealed that these two proteins are characterized by the lack of a signal peptide. These results are discussed with regard to a possible apocrine mode of secretion of these proteins acquired by spermatozoa during the epididymal transit.

Feeding behavior, epidermal structure and mucus cytochemistry of the scleractinian Mycetophyllia reesi, a coral without tentacles

The scleractinian Mycetophyllia reesi lacks even the vestiges of tentacles, but quickly captures particulate food by mucus entanglement. Mesenterial filaments emerge through the oral opening, collect the mucus-embedded particulates, and withdraw to the gastrovascular system within 15 min. Mucocytes dominate the outer epidermis with about 3,000 cells/mm(2) and are capable of apocrine discharge en masse. Mucocytes are spumous, typically with web-like inclusions, which for the most part lack electron opacity with ordinary staining, and are only weakly PAS positive. In contrast, the mucus reacts strongly to diamine and other reagents that suggest an appreciable acidic mucopolysaccharide component. The strongest staining reaction occurs in the presence of high iron diamine, suggesting with other tests that the mucus contains significant quantities of sulfated polysaccharides. Cells with cilia anchored by spiriliform microvilli flank the mucocytes and possess small, spumous inclusions that contain acidic, sulfated, and neutral polysaccharides that do not appear to discharge during feeding. These support cells are closely intertwined with narrow, sinuous, secretory cells containing an electron-opaque cytoplasm of unknown composition that is discharged along with mucus during feeding. The outer epidermis also contains scattered cnidae, rather than the clusters or batteries typical of tentacles. The overwhelming abundance of mucocytes is consistent with their importance in feeding. Likewise, the small number of epidermal cnidae suggest they play a minor role in acquiring food. An inner epidermal layer associated with the mesoglea contains epitheliomuscular cells, nerve cells and pigment cells. The two epidermal layers form an essentially pseudostratified, architecturally simple epithelium.

Selected proteins of "prostasome-like particles" from epididymal cauda fluid are transferred to epididymal caput spermatozoa in bull

During epididymal transit, spermatozoa acquire selected proteins secreted by epithelial cells. We recently showed that P25b, a protein with predictive properties for bull fertility, is transferred from prostasome-like particles present in the cauda epididymal fluid (PLPCd) to the sperm surface. To further characterize the interactions between PLPCd and epididymal spermatozoa, PLPCd were prepared by ultracentrifugation of bull epididymal fluid, then surface-exposed proteins were biotinylated and coincubated in different conditions with caput epididymal spermatozoa. Western blot analysis revealed that only selected proteins are transferred from PLPCd to spermatozoa. MALDI-TOF analysis revealed that these transferred proteins are closely related. The pattern of distribution of the PLPCd transferred varied from one sperm cell to the other, with a bias toward the acrosomal cap. This transfer appeared to be temperature sensitive, being more efficient at 32-37 degrees C than at 22 degrees C. Transfer of PLPCd proteins to spermatozoa was also pH dependant, the optimal pH for transfer being 6.0-6.5. The effect of divalent cations on PLPCd protein transfer to caput spermatozoa was investigated. Whereas Mg(2+) and Ca(2+) have no effect on the amount of proteins remaining associated with spermatozoa following coincubation, Zn(2+) had a beneficial effect. These results are discussed with regard to the function of PLPCd in epididymal sperm maturation.

Secretion of a peripheral membrane protein, MFG-E8, as a complex with membrane vesicles

MFG-E8 (milk fat globule-EGF factor 8) is a peripheral membrane glycoprotein, which is expressed abundantly in lactating mammary glands and is secreted in association with fat globules. This protein consists of two-repeated EGF-like domains, a mucin-like domain and two-repeated discoidin-like domains (C-domains), and contains an integrin-binding motif (RGD sequence) in the EGF-like domain. To clarify the role of each domain on the peripheral association with the cell membrane, several domain-deletion mutants of MFG-E8 were expressed in COS-7 cells. The immunofluorescent staining of intracellular and cell-surface proteins and biochemical analyses of cell-surface-biotinylated and secreted proteins demonstrated that both of the two C-domains were required for the membrane association. During the course of these studies for domain functions, MFG-E8, but not C-domain deletion mutants, was shown to be secreted as membrane vesicle complexes. By size-exclusion chromatography and ultracentrifugation analyses, the complexes were characterized to have a high-molecular mass, low density and higher sedimentation velocity and to be detergent-sensitive. Not only such a exogenously expressed MFG-E8 but also that endogenously expressed in a mammary epithelial cell line, COMMA-1D, was secreted as the membrane vesicle-like complex. Scanning electron microscopic analyses revealed that MFG-E8 was secreted into the culture medium in association with small membrane vesicles with a size from 100 to 200 nm in diameter. Furthermore, the expression of MFG-E8 increased the number of these membrane vesicle secreted into the culture medium. These results suggest a possible role of MFG-E8 in the membrane vesicle secretion, such as budding or shedding of plasma membrane (microvesicles) and exocytosis of endocytic multivesicular bodies (exosomes).

Infectivity of cysts of the ME-49 Toxoplasma gondii strain in bovine milk and homemade cheese

OBJECTIVE

Analyze the infectivity and storage resistance of cysts of the ME-49 strain of Toxoplasma gondii in artificially infected bovine milk and homemade fresh cheese.

METHODS

Pasteurized bovine milk was infected with 10 cysts/ml of the ME-49 strain of T.gondii and inoculated in different groups of mice, immediately or after storage at 4 degrees C for 5, 10 and 20 days. Homemade fresh cheese was prepared with artificially infected milk, and also tested in groups of mice, using the same storage process. Infection was identified by the presence of cysts in the brain or serological testing in challenged mice after 5 weeks, confirmed by Western Blot and histology.

RESULTS

The infectivity of cysts of the ME-49 strain of T.gondii was maintained in the milk even after storage for 20 days at refrigerator temperatures. Cysts were also able to survive the production process of homemade fresh cheese and storage for a period of 10 days in the same conditions.

CONCLUSIONS

These data demonstrated that milk and dairy products could be an important source of T.gondii in human contamination, reinforcing the importance of milk pasteurization before any processing or ingestion.

CFTR, MDR1, and MRP1 immunolocalization in normal human nasal respiratory mucosa

CFTR (cystic fibrosis transmembrane conductance regulator), MDR1 (multidrug resistance), and MRP1 (multidrug resistance-associated protein), members of the ABC transporter superfamily, possess multiple functions, particularly Cl(-), anion, and glutathione conjugate transport and cell detoxification. They are also hypothesized to have a number of complementary functions. It is generally accepted that data obtained from nasal mucosa can be extrapolated to lower airway cell physiology. The aim of the present study was to investigate by immunohistochemistry the differential localization of CFTR, MDR1, and MRP1 in the normal mucosa of 10 human nasal turbinates. In ciliated epithelial cells, CFTR was inconstantly expressed at the apical cell surface, intense membranous labeling was observed for MDR1, and intense cytoplasmic labeling was observed for MRP1. In the glands, a higher level of expression was observed on serous cells, at the apical surface (for CFTR), on lateral membranes (for MDR1), and with an intracytoplasmic distribution (for MRP1). In conclusion, CFTR, MDR1 and MRP1 are expressed in the epithelium and glands of the nasal respiratory mucosa, but with different patterns of expression. These results suggest major roles for CFTR, MDR1, and MRP1 in serous glandular cells and a protective function for MDR1 and MRP1 in respiratory ciliated cells. (J Histochem Cytochem 48:1215-1222, 2000)

The prevalence and avidity of Toxoplasma gondii IgG antibodies in pigs from Brazil and Peru

Raw or inadequately cooked pork is an important source of Toxoplasma gondii infection, and the infection rate in animals used as human food, is an important risk predictor. The prevalence of this infection was estimated in 396 sera from 5-month old pigs obtained at abattoirs in São Paulo, Brazil (300) and Lima, Peru (96). The seroprevalence was higher in pigs from Peru (32.3%) as compared to Brazil (9.6%), as detected by ELISA and Western blot. Hemagglutination gave poor resolution which was not useful for the diagnosis of T. gondii infection. Specific antibody avidity is correlated with infection time, as shown in experimentally infected piglets. Using an arbitrary cut-off of 50% avidity index, Brazilian pigs were found to be more recently infected than Peruvian pigs. Pork should be considered a significant source of human T. gondii infection both in Brazil and Peru. Avidity assays could help in the detection of the time of T. gondii infection in pigs, allowing preventive management.

Surface-accessible GABA supports tonic and quantal synaptic transmission

Exocytosis is commonly viewed as the only secretory process able to account for quantal forms of fast synaptic transmission. However, the demonstrated variability and composite properties of miniature postsynaptic signals are not easily explained by all-or-none exocytotic discharge of transmitter in solution from inside vesicles. Recent studies of endocrine secretion have shown that hormone release does not coincide with exocytosis due to its trapping in the core matrix of the granule. Thus, we tested whether the synaptic transmitter GABA could also be held in a matrix before being released. Using confocal microscopy and flow cytometry of embryonic rat hippocampal neurons, we found a GABA immunoreaction at the surface of live cell bodies and growth cones that coincided spatially and quantitatively with the binding of tetanus toxin fragment C (TTFC). TTFC binds predominantly at membrane sites containing the trisialoglycosphingolipid GT1b. Using flow cytometry, GT1b-containing liposomes preincubated in 100 nM GABA exhibited the same relationship between GABA and TTFC surface binding as found on neurons and growth cones. Embryonic neurons differentiated in culture expressed initially a tonic, and after 3-5 days, transient, postsynaptic signals mediated by GABA acting at GABA(A) receptor/Cl(-) channels. A stream of saline applied to the neuronal surface rapidly and reversibly suppressed both tonic and transient signals. A brief application of the GABAmimetic isoguvacine immediately transformed both tonic and transient GABAergic signals into tonic and transient isoguvacinergic signals. These results and those in the literature are consistent with an immediately releasable compartment of transmitter accessible from the presynaptic surface.