Colloidal gold post-embedding immunocytochemistry

Developmental appearance of dentin matrix protein 1 during the early dentinogenesis in rat molars as identified by high-resolution immunocytochemistry

Dentin matrix protein 1 (DMP 1) is an acidic phosphoprotein that has been postulated to play an important role in mineralized tissue formation. We have examined rat molar tooth germs by applying a high-resolution immunocytochemical approach with the purpose to identify the temporal and spatial localization of DMP 1 at the onset of dentinogenesis. Upper molar tooth germs of 2- to 3-day-old Wistar rats were fixed in a cacodylate-buffered 0.1% glutaraldehyde + 4% formaldehyde fixative, left unosmicated and embedded in LR White resin. The sections were incubated with a polyclonal DMP 1 antibody for postembedding colloidal gold immunolabeling and examined in a Jeol 1010 transmission electron microscope. The earliest localization of DMP 1 was in the Golgi region as well as in the nucleus of differentiating odontoblasts. When mineralization spread from matrix vesicles to the surrounding matrix, DMP 1 was extracellularly detected around the mineralizing globules. In the regions of fully mineralized mantle dentin, it was present in the mineralized regions, mainly around the peritubular dentin. The appearance of DMP 1 during early dentinogenesis implies a direct role for this protein in both odontoblast differentiation and matrix mineralization.

Sequence of protein expression of bone sialoprotein and osteopontin at the developing interface between repair cementum and dentin in human deciduous teeth

Experimental periodontal regeneration studies have revealed the weak binding of repair cementum to the root surface, whereas attachment of cementum to dentin preconditioned by odontoclasts appears to be superior. The aim of this study has been, therefore, to analyze the structural and partial biochemical nature of the interface that develops between resorbed dentin and repair cementum by using human deciduous teeth as a model. Aldehyde-fixed and decalcified tooth samples were embedded in acrylic or epoxy resins and sectioned for light and transmission electron microscopy. Antibodies against bone sialoprotein (BSP) and osteopontin (OPN), two noncollagenous proteins accumulating at hard tissue interfaces in bone and teeth, were used for protein A-gold immunocytochemistry. Light microscopy revealed a gradually increasing staining intensity of the external dentin matrix starting after the withdrawal of the odontoclast. Labeling for both BSP and OPN was first detected among the exposed collagen fibrils and in the intratubular dentin matrix when odontoclasts had withdrawn but mesenchymal cells were present. Subsequently, collagen fibrils of the repair cementum were deposited concomitantly with the appearance of labeling for BSP and OPN over the intratubular, intertubular, and peritubular dentin matrix. Labeled mineralization foci indicated the advancing mineralization front, and the collagenous repair matrix became integrated in an electron-dense organic material that showed labeling for BSP and OPN. Thus, no distinct planar interfacial matrix layer lies between the resorbed dentin and the repair cementum. The results suggest that odontoclasts precondition the dentin matrix such that the repair cementum becomes firmly attached.

Identification of microsomal triglyceride transfer protein in intestinal brush-border membrane

Microsomal triglyceride transfer protein (MTP) is a heterodimeric complex consisting of a unique large 97-kDa protein and the multifunctional 58-kDa protein disulfide isomerase (PDI). It plays an essential role in the assembly of lipoproteins by shuttling lipids between phospholipid membranes. Based on cell fractionation, early studies have suggested the endoplasmic reticulum (ER) as the exclusive site of MTP. Focusing on the plasma membrane in this study, our attempts with immunoelectron microscopy and specific antibodies surprisingly revealed that labeling was not exclusively confined to the microsomes of rat absorptive cells. Immunogold labeling was also detected over the microvillus membrane of enterocytes. Western blot analysis and biochemical activity measurement confirmed MTP protein expression in brush-border membrane vesicles (BBMV) isolated from the intestinal epithelial cells of various species. Furthermore, MTP was coexpressed in microvilli membrane with PDI that is crucial to maintain the structure and activity of the MTP complex. The treatment of Caco-2 cells with nocodazole and colchicine blocked the appearance of MTP in the apical membrane. Similarly, the addition of BMS-197636, a known inhibitor of MTP transfer activity, suppressed the latter. In conclusion, the present studies suggest that MTP is present in the brush-border membrane of the enterocyte. Understanding the possible physiological role of MTP in this location may reveal additional functions.

Rapid in vivo transport of proteins from digested allergen across pre-sensitized gut

Although the route of sensitization to food allergens is still the subject of debate, it is generally accepted the gut immune system plays a pivotal role. However, hitherto the transport of allergens across the normal, pre-sensitized gut epithelium remained largely unknown. Our aim was to identify the route through which protein bodies and soluble proteins from digested peanuts penetrated the pre-sensitized gut epithelium in vivo and the specific cell types involved in the transport. Digestion of peanuts released a large number of protein bodies that are exclusively transported across the epithelium by specialized antigen-sampling M cells and delivered to the lymphoid tissue of Peyer's patch. Intracellular transport of soluble protein also occurred almost exclusively via M cells and it was negligible across absorptive enterocytes. We hypothesize that these conditions which are known to favour strongly the induction of immune responses rather than oral tolerance may play a significant role in the genesis of allergic reactions.

Atomic force spectroscopy-based study of antibody pesticide interactions for characterization of immunosensor surface

Development of immunobiosensor detector surfaces involves the immobilization of active antibodies on the capture surface without any significant loss of antigen binding activity. An atomic force microscope (AFM) was used to directly evaluate specific interactions between pesticides and antibodies on a biosensor surface. Oriented immobilization of antibodies against two herbicide molecules 2,4-dichlorophenoxyacetic acid (2,4-D) and atrazine, on gold, was carried out to create the active immunobiosensor surfaces. The adhesive forces between immobilized antibodies and their respective antigens were measured by force spectroscopy using hapten-carrier protein functionalized AFM cantilevers. Relative functional affinity (avidity) measurements of the antibodies carried out prior to immobilization, well correlated with subsequent AFM force measurement observations. Analysis showed that immobilization had not compromised the reactivity of the surface immobilized antibody molecules for antigen nor was there any change in their relative quality with respect to each other. The utility of the immunoreactive surface was further confirmed using a Surface Plasmon Resonance (SPR) based detection system. Our study indicates that AFM can be utilized as a convenient immunobiosensing tool for confirming the presence and also assessing the strength of antibody-hapten interactions on biosensor surfaces under development.

Expression differences in mitochondrial and secretory chaperonin 60 (Cpn60) in pancreatic acinar cells

In pancreatic acinar cells, chaperonin Cpn60 is present in all the cellular compartments involved in protein secretion as well as in mitochondria. To better understand the role Cpn60 plays in pancreatic secretion, we have evaluated its changes under experimental conditions known to alter pancreatic secretion. Quantitative protein A-gold immunocytochemistry was used to reveal Cpn60 in pancreatic acinar cells. Cpn60 immunolabelings in cellular compartments involved in secretion were found to decrease in acute pancreatitis as well as upon stimulation of secretion and in starvation conditions. A major increase in Cpn60 was recorded in diabetic condition. This was normalized by insulin treatment. Although in certain situations changes in secretory enzymes and in Cpn60 correlate well, in others, nonparallel secretion seemed to take place. In contrast, expression of mitochondrial Cpn60 in acinar cells appeared to remain stable in all conditions except starvation, where its levels decreased. Expression of Cpn60 in the secretory pathway and in mitochondria thus appears to behave differently, and Cpn60 in the secretory pathway must be important for quality control and integrity of secretion.

Sorting of furin in polarized epithelial and endothelial cells: expression beyond the Golgi apparatus

The conversion of proteins into their mature forms underlies the functionality of many fundamental cellular pathways. One posttranslational modification leading to maturation of precursor proteins consists of the cleavage of their prodomain at pairs of basic amino acids by enzymes of the subtilisin-like mammalian proprotein convertase family. One of these enzymes, furin, acts in the constitutive secretory pathway of almost every cell type. However, in spite of furin's major roles in many pathophysiological processes, the exact subcellular sites of processing and activation of its substrates remain elusive. In this study, furin antigenic sites were tracked in subcellular compartments of various tissues and corresponding cell lines by high-resolution immunogold electron microscopy, Western blotting, cell transfection, and in vivo gene delivery of the furin cDNA. In addition to the Golgi apparatus, furin was assigned to endosomes and plasma membranes of polarized intestinal and renal epithelial cells and endothelial cells of the continuous, fenestrated, and discontinuous capillaries. Roles of furin in endothelial permeability, basement membrane turnover, and shedding of transmembrane proteins are supported by our data.

Immunogold detection of co-localized neuropeptides: methodological aspects

Whatever the protocol used, electron microscopic immunogold detection still suffers from a lack of sensitivity. In rat supraoptico-posthypophyseal neurons, unlabeled secretory granules are always detectable after electron microscopic immunocytochemistry, and their real status remains questionable. To improve the sensitivity of this approach, we assessed a protocol to visualize either one or the other of co-localized neuropeptides, i.e., vasopressin or galanin, after two successive rounds of immunogold with the same primary antibody performed on both faces of the grid. The use of different-sized gold particles enabled us to visualize the respective contribution of each face of the section to the final labeling. Our results showed a moderate but significant increase in both the proportion of labeled granules and the labeling intensity. Although limited, this improvement of immunogold detection strengthens the relevance of quantitative studies at the electron microscopic level, likely to reveal fine variations of the neuron peptidergic content. However, this enhancement depended on the peptide studied. The present data confirmed a progressive decrease of vasopressin immunoreactivity, already suggested by the single-staining procedure, all along the hypothalamo-posthypophyseal tract. In contrast, labeling intensity for galanin remained steady. Finally, our double-face labeling supported a preferential routing of galanin-containing secretory granules towards dendrites.

Ontogeny, immunolocalisation, distribution and function of SR-BI in the human intestine

Studies employing human fetal intestine have yielded remarkable information on the role of polarized enterocytes in fat absorption. In this report, we investigated the intestinal expression, spatiotemporal distributions, ontogeny and function of the scavenger receptor, Class B, Type I (SR-BI) that plays a crucial role in cholesterol homeostasis. SR-BI was detected as early as week 14 of gestation in all gut segments and was almost entirely confined to the absorptive epithelial cells. By using immunofluorescence staining, the distribution of SR-BI rarely appeared as a gradient, increasing from the developing crypt to the tip of the villus. Western blot showed high levels of immunodetectable SR-BI in the duodenum, which progressively decreased toward the distal colon. The high-resolution immunogold technique revealed labelling mainly over microvilli of the enterocyte. SR-BI was not associated with caveolin-1 and was not detectable in caveolae. In order to define the role of SR-BI in intestinal cholesterol absorption, Caco-2 cells were transfected with a constitutive expression vector (pZeoSV) containing human SR-BI cDNA inserted in an antisense orientation. As noted by immunoblotting and Protein A-gold techniques, stable transformants contained 40, 60 and 80% the SR-BI level of control Caco-2 cells and exhibited a proportional drop in free cholesterol uptake without altering the capture of phospholipids or cholesteryl ester. Confirmation of these data was obtained in intestinal organ culture where SR-BI antibodies lowered cholesterol uptake. These observations suggest that the human intestine possesses a developmental and regional SR-BI pattern of distribution, and extends our knowledge in SR-BI-mediated cholesterol transport.

Cellular localization and functional expression of P-glycoprotein in rat astrocyte cultures

We investigated the cellular/subcellular localization and functional expression of P-glycoprotein, an ATP-dependent membrane-associated efflux transporter, in astrocytes, a brain parenchyma compartment that is poorly characterized for the expression of membrane drug transporters. Analyses were carried out on primary cultures of astrocytes isolated from the cerebral cortex of neonatal Wistar rats and CTX TNA2, an immortalized rat astrocyte cell line. Both cell cultures display morphological features typical of type I astrocytes. RT-PCR analysis revealed mdr1a and mdr1b mRNA in primary cultures of astrocytes and in CTX TNA2 cells. Western blot analysis using the P-glycoprotein monoclonal C219 antibody detected a single band of appropriate size in both cell systems. Immunocytochemical analysis using the monoclonal antibodies C219 and MRK16 labeled P-glycoprotein along the plasma membrane, caveolae, coated vesicles and nuclear envelope. Immunoprecipitation studies using the caveolin-1 polyclonal H-97 antibody demonstrated that P-glycoprotein is physically associated with caveolin-1 in both cell culture systems. The accumulation of [(3)H]digoxin (an established P-glycoprotein substrate) by the astrocyte cultures was significantly enhanced in the presence of standard P-glycoprotein inhibitors and an ATP depleting agent. These results demonstrate the cellular/subcellular location and functional expression of P-glycoprotein in rat astrocytes and suggest that this glial compartment may play an important role in the regulation of drug transport in the CNS.

Hertwig's epithelial root sheath, enamel matrix proteins, and initiation of cementogenesis in porcine teeth

OBJECTIVES

The aim of this study was to analyze the association between Hertwig's epithelial root sheath (HERS) cells, enamel matrix proteins (EMPs), and cementogenesis.

MATERIAL AND METHODS

Porcine teeth were examined at the beginning of root formation by light and transmission electron microscopy. Colloidal gold immunocytochemistry was used to analyze the protein expression of amelogenin and ameloblastin.

RESULTS

Before and during disintegration of HERS, its cells displayed the cytologic features of protein synthesis and secretion. While some cells assumed an ameloblast-like phenotype, others extended their territory away from the root surface. A collagenous matrix filled the widening intercellular spaces, and tonofilaments and desmosomes were still present in cells featuring the morphologic characteristics of cementoblasts. Labeling for amelogenin was observed but ameloblastin was not immunodetected. Labeling was associated with organic matrix deposits that were sporadically and randomly distributed both along the root surface and away from it among the dissipated epithelial cells.

CONCLUSIONS

These findings suggest that HERS' cells occasionally assume a lingering ameloblastic activity at the beginning of root formation in the pig. While the results do not support the hypothesis of a causal relationship between EMPs and cementogenesis, they lend support to the concept of an epithelial origin of cementoblasts.

Nanotexturing of titanium-based surfaces upregulates expression of bone sialoprotein and osteopontin by cultured osteogenic cells

Bone formation around implants is influenced by surface geometry. Since cell/matrix/substrate interactions associated with cell signaling occur in the nanoscale dimension, we have evaluated the influence of nanotexturing of titanium-based surfaces on the expression of matrix proteins by cultured osteogenic cells at initial time points. Cells were obtained by enzymatic digestion of newborn rat calvaria and grown on titanium and titanium alloy discs with nanotextured or machined surfaces, and on glass coverslips for periods of 6 h, 1 day, and 3 days, under standard culture conditions. Cultures were processed for single or dual immunolabeling with monoclonal and/or polyclonal antibodies against bone sialoprotein (BSP), fibronectin (FN), osteopontin (OPN), type-I pro-collagen, or tubulin, followed by corresponding fluorophore-conjugated secondary antibodies. Some samples were processed for scanning electron microscope analysis of morphology and immunogold labeling. After 6 h, nanotextured surfaces exhibited up to a nine-fold increase in the proportion of cells with peripheral OPN labeling. At day 3, the proportion of OPN and BSP labeled cells was higher, and the intensity of immunoreactivity dramatically increased. No significant differences were observed in the expression pattern and the proportion of cells immunoreactive for FN or type-I pro-collagen. Our results demonstrate that nanotexturing of titanium-based surfaces upregulates the early expression of BSP and OPN in osteogenic cell cultures.

Morphofunctional studies of the glomerular wall in mice lacking entactin-1

The architecture of the basement membranes is essential for proper function. This architecture is based on interactions among its components, which assemble in a complex network. Entactin-1 appears to be the mastermind of this assembling. In entactin-1-null transgenic mice, immunocytochemistry established the absence of entactin-1 in the glomerular basement membrane, and morphological thickening of this membrane was demonstrated. This prompted us to investigate the organization of other components of the glomerular basement membrane in the transgenic animals. The distribution of type IV collagen and laminin remained unchanged, whereas that of anionic charges was significantly altered. We also evaluated the impact of the absence of entactin-1 on cell relays by studying the alpha(3)- and the alpha(v)-integrins along the endothelial and epithelial glomerular cell plasma membranes. Only the density of alpha(v) was found to be increased. Finally, the filtration properties of the glomerular wall were evaluated by revealing endogenous albumin distribution across the basement membrane. This was altered in transgenic animals, suggesting changes in permselectivity properties. Entactin-1 appears to be an essential component in basement membranes because its absence appears to modify the molecular organization leading to alterations in functional properties.

A solid-phase method for evaluation of gold conjugate used in quantitative detection of antigen by immunogold-labeling electron microscopy

Rapid and sensitive screening for confirming the reactivity of reagents, before proceeding for electron microscopy, is highly desirable. ELISA-based methods have been shown to be highly efficient and successful for rapid prescreening and optimization of immunological as well as sample-processing reagents for the sensitive detection and quantitation of antigen by electron microscopy. The drawback of these methods lies in their inability to provide any information regarding the gold conjugate used for the final observed and measured signal. In this work, we demonstrate a simple and rapid, solid-phase method in ELISA format that is also suitable for evaluation and optimization of the gold conjugate. We have demonstrated the utility of this technique by screening for Vitreoscilla hemoglobin (VHb) antigen in cell lysates and confirming the results directly with immunogold-labeling transmission electron microscopy (TEM) of cell sections. The sensitivity of detection and quantitation of antigens by immuno-electron microscopy depends upon the assay procedure being optimized to obtain the best possible signal. Our study indicates that evaluation of gold conjugate by the solid-phase assay could help in the rapid optimization of this reagent for immunogold localization and quantification of antigens by TEM.

Apoptosis of tubular epithelial cells in glycogen nephrosis during diabetes

The important problem of the fate of glycogen-accumulating clear cells in glycogen nephrosis is still unsettled. In this study, we examine whether apoptosis plays a relevant role in the development of diabetic glycogen nephrosis and explore the involvement of the Fas/Fas-L system and the activation of the caspase cascade. Diabetes was induced in rats by streptozotocin injection. Glycogen-accumulating clear cells were identified in renal tissues of hyperglycemic rats. They were found to be concentrated in the thick ascending limbs and distal tubules. Large cellular glycogen accumulations were confirmed by biochemical assays and enzyme-gold cytochemistry. Clear cells displayed apoptotic features such as Annexin V binding, nuclear TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling), and the simultaneous occurrence of Fas, Annexin V, and TUNEL positivity. Western blot analysis demonstrated enhanced expression of Fas receptor/ligand and the activation of the caspase cascade in these cells because cleaved forms of the caspase-3, -8, and -9 were detected. Furthermore, active caspase-3 was located in nuclei by immunoelectron microscopy. Our results indicate that epithelial cells in thick ascending limbs and distal tubules that develop glycogen nephrosis in response to hyperglycemia undergo Fas/Fas-L mediated cell death. Thus, apoptosis could be playing a significant role in renal epithelial cell deletion during diabetes.

Furin interacts with proMT1-MMP and integrin alphaV at specialized domains of renal cell plasma membrane

Matrix metalloproteinases (MMPs) and integrins are essential for cell and extracellular matrix homeostasis. Both membrane type-1 MMP (MT1-MMP) and the integrin alphaV subunit are fully activated upon cleavage at a furin recognition site. Furin is shuttled to the cell surface through the trans-Golgi network and endosomal system, and its only known role on plasma membrane consists in activation of opportunistic pathogenic entities. Here, we report findings about the interaction of furin with MT1-MMP and the integrin alphaV at the cell surface. By using in vivo gene delivery, western blotting and immunogold electron microscopy, we provide evidence of significant pools of furin and proMT1-MMP along the surface of cells lining basement membranes. Moreover, furin and integrin alphaV are frequently found associated with the slit diaphragm of renal podocytes and around endothelial fenestrations. ProMT1-MMP, by contrast, is concentrated at the slit diaphragm. Coimmunoprecipitations and double immunogold labelings indicate that furin interacts with proMT1-MMP and alphaV at points of insertion of the slit diaphragm. Our results suggest that these focalized complexes could trigger basement membrane proteolysis either directly by activation of proMT1-MMP or indirectly by promoting activation of proMMP2.

The normal liver harbors the vitamin D nuclear receptor in nonparenchymal and biliary epithelial cells

The liver is generally considered negative for the vitamin D nuclear receptor (VDR(n)), even though several studies have shown significant effects of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) on liver cell physiology. The low abundance of VDR(n) in the liver led us to propose that hepatocytes (the largest hepatic cell population) were most likely negative for the receptor, whereas the small hepatic sinusoidal and ductular cell populations that contain cell types known to express VDR(n) in other tissues should express the receptor. Using freshly isolated cells from normal livers as well as biliary and epithelial hepatic cell lines, our data show that the human, rat, and mouse hepatocytes express very low VDR(n) messenger RNA (mRNA) and protein levels. In contrast, sinusoidal endothelial, Kupffer, and stellate cells of normal rat livers as well as the mouse biliary cell line BDC and rat hepatic neonatal epithelial SD6 cells clearly expressed both VDR(n) mRNA and protein. In addition, specimens of human hepatocarcinoma as well as intrahepatic colon adenocarcinoma metastases were also found to express the VDR(n) gene transcript. Kupffer, stellate, and endothelial cells responded to 1,25(OH)(2)D(3) by a significant increase in the CYP24, indicating that the VDR(n) is fully functional in these cells. In conclusion, selective hepatic cell populations are targets for the vitamin D endocrine/paracrine/intracrine system.

Immunocytochemical characterization of ectopic enamel deposits and cementicles in human teeth

Despite the relative frequency and clinical relevance of radicular enamel deposits and cementicles, their etiology and nature are unknown. The purpose of the present study was therefore to evaluate the presence and distribution of mineralization-associated non-collagenous matrix proteins (NCPs) in various types of root-associated ectopic mineralizations. Human teeth were processed for embedding in epoxy or acrylic resins. Tissue sections were incubated with antibodies to amelogenins (AMEL), bone sialoprotein (BSP), and osteopontin (OPN). Radicular enamel deposits contained residual organic matrix that labeled for AMEL. In contrast, BSP and OPN were not detected in the residual enamel matrix, they were found in the cementum deposited on its surface as well as in collagen-free cementicle-like structures in the adjacent periodontal ligament. True cementicles consisted of a collagenous matrix intermixed with a non-collagenous ground substance. Labeling for BSP and OPN was mainly associated with the interfibrillar ground substance. No immunoreactivity for AMEL was detected in cementicles. These data indicate that ectopic enamel deposits on the root retain a high amount of AMEL, whereas cementicles contain BSP and OPN, two NCPs typically found in bone and cementum. These NCPs may, like in their normal tissue counterparts, play a role in the mineralization process.

Quantitative immunogold labeling of bone sialoprotein and osteopontin in methylmethacrylate-embedded rat bone

Methylmethacrylate (MMA) embedding of undecalcified bone is routinely employed for histomorphometric analyses. Although MMA-embedded bone has been used for immunolabeling at the light microscopic level after removal of the resin, there are no such reports for electron microscopy. The aim of the present study was to determine whether MMA embedding can be used for ultrastructural immunolabeling and how it compares to LR White (LRW), an acrylic resin frequently used for immunocytochemistry of bone. Rat tibiae were fixed by vascular perfusion with aldehyde and embedded either in MMA or LRW resin. Thin sections were processed for postembedding protein A-gold immunolabeling with antibodies to rat bone sialoprotein (BSP) and osteopontin (OPN). The density of gold particles over bone was quantified. The density and distribution of immunolabeling for BSP and OPN respectively, were comparable between MMA and LRW. These results indicate that MMA performs as well as LRW for the ultrastructural immunolabeling of noncollagenous bone matrix proteins.

Expression differences in mitochondrial and secretory chaperonin 60 (Cpn60) in pancreatic acinar cells

In pancreatic acinar cells, chaperonin Cpn60 is present in all the cellular compartments involved in protein secretion as well as in mitochondria. To better understand the role Cpn60 plays in pancreatic secretion, we have evaluated its changes under experimental conditions known to alter pancreatic secretion. Quantitative protein A-gold immunocytochemistry was used to reveal Cpn60 in pancreatic acinar cells. Cpn60 immunolabelings in cellular compartments involved in secretion were found to decrease in acute pancreatitis as well as upon stimulation of secretion and in starvation conditions. A major increase in Cpn60 was recorded in diabetic condition. This was normalized by insulin treatment. Although in certain situations changes in secretory enzymes and in Cpn60 correlate well, in others, nonparallel secretion seemed to take place. In contrast, expression of mitochondrial Cpn60 in acinar cells appeared to remain stable in all conditions except starvation, where its levels decreased. Expression of Cpn60 in the secretory pathway and in mitochondria thus appears to behave differently, and Cpn60 in the secretory pathway must be important for quality control and integrity of secretion.

Immunodetection of osteopontin at sites of resorption in the pulp of rat molars

Osteopontin (OPN) has been proposed to act as a substrate for osteoclast adhesion during bone resorption. The aim of the present study was to examine the presence and distribution of OPN at sites of resorption in traumatized radicular pulp. The upper first molars of 6-week-old male Sprague-Dawley rats were luxated and then repositioned in the original sockets. The animals were sacrificed by intracardiac perfusion at 10 and 14 days after tooth reimplantation. The teeth were decalcified in EDTA and then processed for embedding in paraffin for histochemistry or LR White resin for immunocytochemistry. Odontoclasts were identified by their multinucleated morphology and expression of tartrate-resistant acid phosphatase (TRAP). Osteopontin was immunolocalized using postembedding colloidal gold labeling with a chicken egg yolk anti-rat OPN antibody. After reimplantation of the teeth, TRAP-positive cells were present along the pulp dentin wall. Osteopontin was not consistently detected at exposed predentin/dentin surfaces. However, gold particles were often found at the margin of resorption lacunae. Labeling was also seen over the Golgi region and cytoplasmic vesicles of odontoclasts and of neutrophils and fibroblast-like cells. The results suggest that accumulation of OPN at the predentin/dentin surface is not a prerequisite for adhesion of odontoclasts to the wall substance and that recruited odontoclasts produce OPN locally to mediate cell and/or matrix events within the resorption lacuna.

Functional expression and localization of P-glycoprotein at the blood brain barrier

Until recently, the blood-brain barrier was viewed as a static lipid membrane barrier. Physical attributes of the cerebral endothelial cells such as the presence of tight junctions, paucity of vesicles or caveolae, and high electrical resistance were believed to be the primary components that provide the membrane selectivity of the blood-brain barrier to a variety of circulating compounds from the periphery. However, results from molecular biology, immunocytochemistry, biochemistry, and transport studies show that the cerebral endothelial cells possess an asymmetrical array of metabolic enzymes (i.e., alkaline phosphatase, cytochrome P450 enzymes, glutathione transferases) and energy-dependent efflux transport proteins (i.e., P-glycoprotein and Multidrug-resistance proteins) that are instrumental to the barrier function. P-glycoprotein, a membrane-associated, energy-dependent, efflux transporter, is expressed in brain parenchyma (i.e., astrocytes and microglia) as well as in blood-brain and blood-cerebrospinal fluid barriers. Its function along the blood-brain barrier is believed to prevent the accumulation of potentially harmful compounds in the brain by actively removing them from the brain into the peripheral circulation. This is a brief review on the expression and activity of P-glycoprotein at the blood-brain barrier, which reports on the localization of the protein in rat brain capillaries in situ as well as in a well-characterized in vitro model of the blood-brain barrier, an immortalized rat brain endothelial cell line, the RBE4. Immunocytochemical analysis employing various P-glycoprotein monoclonal antibodies, demonstrated the presence of the protein along the plasma membrane, in plasmalemmal vesicles and nuclear envelope of rat cerebral endothelial cells, both in situ and in vitro. Western blot analysis revealed a single band with a molecular weight of 170-180 kDa, a size previously reported for P-glycoprotein, in RBE4 cells. In addition, results from functional studies show that the accumulation of the P-glycoprotein substrate digoxin by RBE4 monolayer cells is significantly enhanced in the presence of standard P-glycoprotein inhibitors (verapamil, cyclosporin A, PSC 833), protease inhibitors (saquinavir, ritonavir, indinavir), and the metabolic inhibitor, sodium azide. These results demonstrate the functional expression of P-glycoprotein in the immortalized rat brain endothelial cell line, RBE4. Novel in situ and in vitro intracellular locations of P-glycoprotein in cerebral endothelial cells have been identified suggesting that this transporter may play a significant role in the subcellular distribution of substrates in the brain.

Optimization of immunogold labeling TEM: an ELISA-based method for evaluation of blocking agents for quantitative detection of antigen

We developed an ELISA-based method for rapid selection of optimal blocking agents to be used in antigen quantification by immunogold labeling electron microscopy. Casein, skim milk, BSA from two sources, acetylated BSA, fish skin gelatin, horse serum, and goat serum were tested for their ability to block nonspecific binding of antibody to recombinant Vitreoscilla hemoglobin (VHb) antigen expressed in Escherichia coli cells by ELISA and the results were confirmed by quantitative immunogold labeling transmission electron microscopy (TEM). Ability to minimize NSB was also evaluated by dot-blot and Western blotting methods. The results demonstrated that ELISA was most accurate in predicting the most efficient blocking agent for TEM. Existing methods could not provide an accurate picture of the ability of various reagents to suppress background labeling. The sensitivity of detection of antigens by immunoelectron microscopy depends on the assay procedure being optimized to obtain the highest possible signal along with as low a background (noise) as possible. Our study indicated that an ELISA-based evaluation of various blocking agents could help in the rapid selection and optimization of a suitable protocol for immunogold localization and quantification of antigens by TEM.

Morphological and cytochemical aspects of capillary permeability

Transport of plasma soluble constituents across the capillary wall is of primordial importance in cardiovascular physiology. While physiological experiments have concluded with the existence of two sets of pores, a large one responsible for the transport of proteins and a small one designed for the diffusion of small solutes, the morphological counterparts have yet to get general agreement. In this review, we present the different proposed paths within and between the endothelial cells that do allow passage of plasma constituents and may respond to the definitions established by physiological means. The vesicular system existing in endothelial cells has been the first transendothelial path to be proposed. Several data have demonstrated the involvement of this system in transport, although others have systematically brought controversy. One alternative to the vesicles has been the demonstration of membrane-bound tubules creating, in certain cases, transendothelial channels that would allow diffusion of plasma proteins and other constituents across the capillary wall. Access to this tubulo-vesicular system could be restrained by the stomatal diaphragm and facilitated by specific membrane receptors. Further, we have demonstrated for the first time with morpho-cytochemical tools, that the intercellular clefts are the site of diffusion for small molecules such as peptides having a molecular weight inferior to 3,000. For the fenestrated capillary bed, we have shown that fenestrae are the site through which plasma constituents cross the capillary wall. However, and in spite of the existence of these large open pores, the endothelial cells still display the tubulo-vesicular system involved in transport of large molecules and their intercellular clefts are also the site of diffusion of small molecules. Making consensus on the existence of an intracellular tubulo-vesicular system in non-fenestrated capillaries, responsible for the transport of large molecules by the endothelial cells, and understanding the rational for the fenestrated capillary to have three paths for transport--the fenestrae, the tubulo-vesicular system, and the inter-endothelial clefts--require further investigation.

Localization of microsomal triglyceride transfer protein in the Golgi: possible role in the assembly of chylomicrons

Although a critical role of microsomal transfer protein (MTP) has been recognized in the assembly of nascent apolipoprotein B (apoB)-containing lipoproteins, it remains unclear where and how MTP transfers lipids in the secretory pathway during the maturational process of apoB lipidation. The aims of this study were to determine whether MTP functions in the secretory pathway as well as in the endoplasmic reticulum and whether its large 97-kDa subunit interacts with the small 58-kDa protein disulfide isomerase (PDI) subunit and apoB, particularly in the Golgi apparatus. Using a high resolution immunogold approach combined with specific polyclonal antibodies, the large and small subunits of MTP were observed over the rough endoplasmic reticulum and the Golgi. Double immunocytochemical detection unraveled the colocalization of MTP and PDI as well as MTP and apoB in these same subcellular compartments. To confirm the spatial contact of these proteins, Golgi fractions were isolated, homogenized, and incubated with an anti-MTP large subunit antibody. Immunoprecipitates were applied on SDS-PAGE and then transferred on to nitrocellulose. Immunoblotting the membrane with PDI and apoB antibodies confirmed the colocalization of these proteins with MTP. Furthermore, MTP activity assay disclosed a substantial triglyceride transfer in the Golgi fractions. The occurrence of membrane-associated apoB in the Golgi, coupled with its interaction with active MTP, suggests an important role for the Golgi in the biogenesis of apoB-containing lipoproteins.

Alterations of vitronectin and its receptor alpha(v) integrin in the rat renal glomerular wall during diabetes

Vitronectin, a multifunctional glycoprotein present in blood and extracellular matrix, is not only a member of the cell adhesion molecules, but also a regulator of proteolytic enzyme cascades, thereby providing a unique regulatory factor for proteolytic degradation of extracellular matrix and tissue remodeling. Vitronectin interacts with the cell surface through integrins of the alpha(v)-related system. Because vitronectin and its receptor may have a role in various renal physiological and pathological processes, we evaluated their expression in renal tissues of streptozotocin-induced short- and long-term hyperglycemic rats by applying quantitative immunoelectron microscopy and Western blot analysis. Vitronectin was shown over the glomerular basement membrane (GBM) and mesangial matrix (MM), whereas alpha(v) was located along the plasma membrane of endothelial, epithelial, and mesangial cells. Although distribution patterns of vitronectin and alpha(v) integrin labeling in renal tissues from short- and long-term hyperglycemic rats, as well age-matched normoglycemic rats, were similar, increases in their immunoreactive sites were detected in hyperglycemic conditions. Changes also were present in old compared with young normoglycemic animals. The diabetes-related increase in vitronectin was more significant in the GBM than MM, whereas the increase in alpha(v) integrin was as significant in podocytes as mesangial cells. Western blot analysis, performed on isolated glomerular material from normoglycemic and hyperglycemic animals, confirmed those changes. Our results suggest that vitronectin and its receptor, alpha(v) integrin, must have defined roles in molecular mechanisms involved in the pathogenesis of both diabetic and aging nephropathy.

Transcytosis of pancreatic bile salt-dependent lipase through human Int407 intestinal cells

In previous studies, we have shown that the bile-salt-dependent-lipase (BSDL), secreted by pancreatic acinar cells and secreted into the duodenal lumen, can be transcytosed through intestinal cells up to the lamina propria. In this study, we used an in vitro system to provide insights into the apical to basolateral transport of BSDL, across the intestinal barrier. The Int407 human epithelial cell line, grown under conditions that optimize polarity, was used as a tight epithelium model. We attempted to delineate uptake mechanisms and the transcytotic pathway followed by this pancreatic enzyme within the intestinal Int407 cells, which do not produce BSDL. When added to the apical reservoir of Transwell-grown Int407 cells, BSDL was shown to first interact with the apical membrane. Further, BSDL forms clusters that are internalized via clathrin-coated pits. Following endocytosis, BSDL is directed to a nocodazole- and colchicin-sensitive multivesicular compartment. Interestingly, this protein transits through the Golgi apparatus, where it was found to colocalize with the KDEL retrieval-receptor. Finally, enzymatically active intact BSDL was released at the basolateral membrane level. This is the first demonstration for an apical-to-basolateral transcytotic pathway of a secreted pancreatic digestive enzyme through polarized intestinal cells.

Effect of human recombinant leptin on lipid handling by fully differentiated Caco-2 cells

It has been established that leptin displays a number of effects on peripheral tissues. We have investigated the effect of the hormone on lipid synthesis, apolipoprotein biogenesis and lipoprotein secretion in Caco-2 cells. Immunocytochemistry revealed the presence of leptin receptors (Ob-Rb) on the basolateral membrane. Incubation of cells with 200 nM leptin resulted in a decreased export of triglycerides in the basolateral medium without affecting monoglyceride, diglyceride and cholesterol ester lipid classes. It also significantly reduced the output of de novo-synthesized apolipoprotein (Apo)B-100 and ApoB-48 as well as that of newly formed chylomicrons and of low-density lipoproteins. It also enhanced that of ApoA-I, ApoA-IV and ApoE. Our results support the hypothesis that leptin can affect energy balance at the gut level by reducing lipid release into the circulation.

High-resolution immunocytochemistry of noncollagenous matrix proteins in rat mandibles processed with microwave irradiation

The mineral phase in calcified tissues represents an additional factor to be considered during their preservation for ultrastructural analyses. Microwave (MW) irradiation has been shown to facilitate fixative penetration and to improve structural preservation and immunolabeling in a variety of soft tissues. The aim of the present study was to determine whether MW processing could offer similar advantages for hard tissues. Rat hemimandibles were immersed in 4% formaldehyde + 0.1% glutaraldehyde buffered with 0.1 M sodium cacodylate, pH 7.2, and exposed to MWs for three periods of 5 min at temperatures not exceeding 37C. They were then decalcified in 4.13% EDTA, pH 7.2, for 15 hr, also under MW irradiation. Osmicated and non-osmicated samples were dehydrated in graded concentrations of ethanol and embedded in LR White resin. Sections of incisor, molars, and alveolar bone were processed for postembedding colloidal gold immunolabeling using antibodies against ameloblastin, amelogenin, bone sialoprotein, or osteopontin. Ultrastructural preservation of tissues was in most cases comparable to that obtained by perfusion-fixation, and there was no difference in distribution of labeling with those previously reported for the antibodies used. However, the immunoreactivities obtained were generally more intense, particularly at early stages of tooth formation. Amelogenin was abundant between differentiating ameloblasts and labeling for osteopontin appeared over the Golgi apparatus of odontoblasts after initiation of dentine mineralization. We conclude that MW irradiation represents a simple method that can accelerate the processing of calcified tissues while yielding good structural preservation and antigen retention. (J Histochem Cytochem 49:1099-1109, 2001)

Colorimetric silver detection of DNA microarrays

Development of microarrays has revolutionized gene expression analysis and molecular diagnosis through miniaturization and the multiparametric features. Critical factors affecting detection efficiency of targets hybridization on microarray are the design of capture probes, the way they are attached to the support, and the sensitivity of the detection method. Microarrays are currently detected in fluorescence using a sophisticated confocal laser-based scanner. In this work, we present a new colorimetric detection method which is intented to make the use of microarray a powerful procedure and a low-cost tool in research and clinical settings. The signal generated with this method results from the precipitation of silver onto nanogold particles bound to streptavidin, the latter being used for detecting biotinylated DNA. This colorimetric method has been compared to the Cy-3 fluorescence method. The detection limit of both methods was equivalent and corresponds to 1 amol of biotinylated DNA attached on an array. Scanning and data analysis of the array were obtained with a colorimetric-based workstation.

Amplification methods for the immunolocalization of rare molecules in cells and tissues

The needs to precisely assign macromolecules to specific locations and domains within tissues and cells and to reveal antigens which are present in low or even in trace amounts, led to the elaboration of a wide spectrum of immunocytochemical amplification procedures. These arise from the successive improvements of tissue preparation techniques, of antigen retrieval procedures and of immunological or non-immunological detection systems. Improvement of detection systems may be the most active in the development of amplification techniques. Since the early work of Coons, in which by the introduction of the indirect technique has started amplifying the signal, different systems have succeeded in increasing the sensitivity of antigens detection. Indeed, amplification techniques such as the multiple antibody layers, the multiple bridges, the enzyme complexes, the avidin-biotin, the silver intensification, and the numerous variations and combinations among these have increased the sensitivity for the detection of scarce tissue antigens. However, as shown by the recent progress carried out with new approaches such as the catalyzed reporter deposition (CARD) and the enhanced polymer one-step staining (EPOS), more efficient methods are still needed. In electron microscopy, few techniques have reached the resolution afforded by the post-embedding immunogold approach. In spite of this and in order to further increase its sensitivity, new probes and novel approaches are allowing combination of the gold marker with the amplification capacity of enzymes afforded by the CARD technique. Immunogold amplification strategies, such as the multiple incubations with the primary antibody and the use of an anti-protein A antibody have also led to enhanced signals displaying the advantages in terms of resolution and possibilities of quantification inherent to the colloidal gold marker.

Optimization of immunogold labeling TEM. An ELISA-based method for rapid and convenient simulation of processing conditions for quantitative detection of antigen

We developed an ELISA-based method for rapid optimization of various tissue processing parameters in immunogold labeling for electron microscopy. The effects of aldehyde fixation, tannic acid, postfixation, dehydration, temperature, and antigen retrieval on antibody binding activity of Vitreoscilla hemoglobin (VHb) expressed in E. coli cells were assayed by ELISA and the results confirmed by quantitative immunogold labeling transmission electron microscopy (TEM). Our results demonstrated that low concentrations (0.2%) of glutaraldehyde fixation caused minimal loss in total binding compared to higher concentrations. Dehydration in up to 70% ethanol resulted in some distortion of cellular ultrastructure but better antibody binding activity compared to dehydration up to 100%. Postfixation or incorporation of tannic acid in the primary fixative caused almost total loss of activity, whereas antigen retrieval of osmium-postfixed material resulted in approximately 90-100% recovery. The sensitivity of detection of proteins by immunogold labeling electron microscopy depends on the retention of antibody binding activity during tissue processing steps, e.g., fixation and dehydration. Our study indicated that an ELISA-based screening method of various tissue processing procedures could help in rapid selection and optimization of a suitable protocol for immunogold localization and quantification of antigen by TEM.

Immunochemical characterization of a chicken egg yolk antibody to secretory forms of rat incisor amelogenin

Amelogenins represent the major component of the organic matrix of enamel, and consist of several intact and degraded forms. A precise knowledge of their respective distributions throughout the enamel layer could provide some insight into their functions. To date, no antibody exists that can selectively detect the secretory forms of amelogenin. In this study we used the chicken egg yolk system to generate an antibody to recombinant mouse amelogenin. Immunoblots of whole homogenates from rat incisor enamel organs and enamel showed that the resulting antibody (M179y) recognized proteins corresponding to the five known secretory forms of rat amelogenin. Immunogold cytochemistry demonstrated that reactivity was restricted to ameloblasts and enamel. Secretory forms of amelogenin persisted in significant amounts throughout the enamel layer. The density of labeling was highest over the surface portion of the enamel layer, but enamel growth sites in this region showed a localized paucity of gold particles. Immunoreactivity was lowest over the mid-portion of the layer and increased moderately near the dentino-enamel junction. These results indicate that intact forms of amelogenin probably have a more complex distribution in the enamel layer than was heretofore suspected.

Localized infusion of tunicamycin in rat hemimandibles: alteration of the basal lamina associated with maturation stage ameloblasts

At the beginning of the maturation stage of amelogenesis, ameloblasts deposit a basal lamina (BL) at the interface between their apical surface and maturing enamel. This structure is rich in glycoconjugates and is proposed to exhibit adhesive and/or filtering functions. To clarify its role, we have applied a recently developed surgical window model to locally administer tunicamycin (TM), an antibiotic that interferes with N-glycosylation, in the rat hemimandible using an osmotic minipump. Male Wistar rats were infused with either TM or saline as a control. Lectin-gold cytochemistry was performed to reveal glycoconjugates in the BL. Immunogold labeling of enamel proteins and albumin was carried out to verify whether depletion of N-linked sugars in the BL affects the content and distribution of endogenous and exogenous proteins in the enamel layer. Under the influence of the drug, the BL became irregular and exhibited alterations in structural organization and composition. The number of Helix pomatia agglutinin binding sites was not significantly affected but their distribution was altered. The labeling density of wheat germ agglutinin over the BL was slightly reduced. Immunoreactivity for enamel proteins showed only a small decrease, but that of albumin, both between ameloblasts and within the enamel layer, increased significantly. No structural alterations were observed in the contralateral incisor and in other sampled tissues and organs. These results demonstrate that it is possible to achieve a localized administration of TM without systemic side effects and lend support to the proposal that the BL represents a specialized structure with filtering functions.(J Histochem Cytochem 49:165-176, 2001)

Demonstration of acetylcholinesterase molecular forms in a continuous tubular lysosomal system of rat pancreatic acinar cells

By applying the highly sensitive cytochemical Gautron's technique, we were able to reveal AChE activity in rat pancreatic acinar cells, particularly at the level of a complex membrane-bound network formed by tubules with varicosities located around the nuclei and close to the basolateral membrane. The Golgi apparatus was devoid of cytochemical reaction beside the trans-Golgi network cisternae, which showed a positive reaction. The RER of some acinar cells also presented a signal, demonstrating their capability of synthesizing AChE. Immunogold using a specific anti-AChE antibody yielded similar results. Double-labeling experiments corroborated the presence of enzyme cytochemical and immunocytochemical signals in the same lysosomal tubular network. Biochemical sedimentation assays confirmed the presence of AChE in acinar cells, which exists as two globular molecular forms, G(1) and G(4). These results were obtained with pancreatic tissue in situ as well as with isolated acinar cells maintained in culture and devoid of neural elements. The existence of a continuous tubular lysosomal network containing AChE is in agreement with previous reports on acinar and other cell types, and supports a more general hypothesis on dynamic continuities among cell structures. Whether AChE is being secreted by the acinar cells or internalized through this endo-lysosomal system was not defined. However, the capability of the acinar cells to synthesize AChE and to channel it through a tubular system is a good indication that the cells can modulate their cholinergic stimulation for optimal secretion of digestive enzymes.

Morphological and immunocytochemical analyses on the effects of diet-induced hypocalcemia on enamel maturation in the rat incisor

During the maturation stage of amelogenesis, the loss of matrix proteins combined with an accentuated but regulated influx of calcium and phosphate ions into the enamel layer results in the "hardest" tissue of the body. The aim of the present investigation was to examine the effects of chronic hypocalcemia on the maturation of enamel. Twenty-one-day old male Wistar rats were given a calcium-free diet and deionized water for 28 days, while control animals received a normal chow. The rats were perfused with aldehyde and the mandibular incisors were processed for histochemical and ultrastructural analyses and for postembedding colloidal gold immunolabeling with antibodies to amelogenin, ameloblastin, and albumin. The maturation stage enamel organ in hypocalcemic rats exhibited areas with an apparent increase in cell number and the presence of cyst-like structures. In both cases the cells expressed signals for ameloblastin and amelogenin. The content of the cysts was periodic acid-Schiff- and periodic acid-silver nitrate-methanamine-positive and immunolabeled for amelogenin, ameloblastin, and albumin. Masses of a similar material were also found at the enamel surface in depressions of the ameloblast layer. In addition, there were accumulations of glycoproteinaceous matrix at the interface between ameloblasts and enamel. In decalcified specimens, the superficial portion of the enamel matrix sometimes exhibited the presence of tubular crystal "ghosts." The basal lamina, normally separating ameloblasts and enamel during the maturation stage, was missing in some areas. Enamel crystals extended within membrane invaginations at the apical surface of ameloblasts in these areas. Immunolabeling for amelogenin, ameloblastin, and albumin over enamel was variable and showed a heterogeneous distribution. In contrast, enamel in control rats exhibited a homogeneous labeling for amelogenin, a concentration of ameloblastin at the surface, and weak reactivity for albumin. These results suggest that diet-induced chronic hypocalcemia interferes with both cellular and extracellular events during enamel maturation.

A histomorphometric, structural, and immunocytochemical study of the effects of diet-induced hypocalcemia on bone in growing rats

Despite several studies on the effect of calcium deficiency on bone status, there is relatively little information on the ensuing histological alterations. To investigate bone changes during chronic hypocalcemia, weanling rats were kept on a calcium-free diet and deionized water for 28 days while control animals were fed normal chow. The epiphyseal-metaphyseal region of the tibiae were processed for histomorphometric, histochemical, and structural analyses. The distribution of bone sialoprotein (BSP), osteocalcin (OC), and osteopontin (OPN), three noncollagenous bone matrix proteins implicated in cell-matrix interactions and regulation of mineral deposition, was examined using postembedding colloidal gold immunocytochemistry. The experimental regimen resulted in serum calcium levels almost half those of control rats. Trabecular bone volume showed no change but osteoid exhibited a significant increase in all its variables. There were a multitude of mineralization foci in the widened osteoid seam, and intact matrix vesicles were observed in the forming bone. Many of the osteoblasts apposed to osteoid were tartrate-resistant acid phosphatase (TRAP)- and alkaline phosphatase-positive, whereas controls showed few such TRAP-reactive cells. Osteoclasts in hypocalcemic rats generally exhibited poorly developed ruffled borders and were inconsistently apposed to bony surfaces showing a lamina limitans. Sometimes osteoclasts were in contact with osteoid, suggesting that they may resorb uncalcified matrix. Cement lines at the bone-calcified cartilage interface in some cases were thickened but generally did not appear affected at bone-bone interfaces. As in controls, electron-dense portions of the mineralized matrix showed labeling for BSP, OC, and OPN but, in contrast, there was an abundance of immunoreactive mineralization foci in osteoid of hypocalcemic rats. These data suggest that chronic hypocalcemia affects both bone formation and resorption.

Colocalization of chaperone Cpn60, proinsulin and convertase PC1 within immature secretory granules of insulin-secreting cells suggests a role for Cpn60 in insulin processing

Many of the mechanisms that control insulin processing and packaging by interaction with different elements along the secretory pathway remain poorly understood. We have investigated the possibility that Cpn60, a member of the heat shock protein family, may be present in rat insulin-secreting cells, participating in the proinsulin-insulin maturation process. Immunofluorescence and high resolution immunocytochemical studies revealed the presence of the Cpn60 protein all along the insulin secretory pathway, being particularly abundant over the proinsulin-containing immature secretory granules. Double-labeling experiments showed associations between Cpn60 and proinsulin, as well as between Cpn60 and PC1 convertase, with a preferential binding to proinsulin. These findings paralleled those of coimmunoprecipitation studies showing the Cpn60 chaperone and the mature form of the PC1 convertase in proinsulin immunoprecipitates, as well as the PC1 in Cpn60 immunoprecipitates from total islet cell extracts. In vitro binding of Cpn60 to proinsulin, insulin and glucagon was also documented. Cpn60, significantly abundant in proinsulin-containing secretory granules where conversion of proinsulin to insulin takes place, and the colocalization of the chaperone with proinsulin and PC1 convertase suggest that the Cpn60 protein may play a role directing precise molecular interactions during insulin processing and/or packaging.

Use of immunoelectron microscopy and intestinal models to explore the elaboration of apolipoproteins required for intraenterocyte lipid transport

The intestine is the organ that contributes the majority of circulating alimentary lipoproteins. Intestinal epithelial cells have the unique ability to elaborate chylomicrons, the largest triglyceride-rich lipoproteins and the main vehicle for the transport of dietary lipids. The final intracellular assembly and exocytosis of chylomicrons require enterocyte-derived apolipoproteins (apo). As research on lipoprotein metabolism evolved, it has become increasingly evident that apo B is a crucial protein for the normal packaging of triglyceride-rich lipoproteins. Immunocytochemical techniques have successfully been used to demonstrate the presence of two types of apo B, the B-100 and the B-48, in different subcellular compartments of the human enterocyte. Confirmation was obtained by biochemically analyzing human lymph and intestine from pediatric patients. In addition, the immunoelectron microscopic approach revealed the location of apo A-I in the rough endoplasmic reticulum (ER) and predominantly in the Golgi apparatus and the basolateral membrane, which confirms the rapid transport of apo A-I documented by other studies. Proven utility and experimental conditions were defined to demonstrate the ability of Caco-2 cells, a colon carcinoma cell line, to esterify lipids, synthesize apo, and assemble lipoproteins. Thus, immunocytochemical and biochemical techniques can be combined with in vivo and in vitro intestinal models for the study of the intestinal lipid transport.

Roles of molecular chaperones in pancreatic secretion and their involvement in intestinal absorption

This review focuses on the contribution of molecular chaperones in the secretory process of digestive enzymes and their interaction with enterocytes. By using biochemistry and immunocytochemistry, we have shown that Grp94, Cpn10, Cpn60, and protein disulfide isomerase (PDI) are present all along the rough endoplasmic reticulum-Golgi-granule secretory pathway of the pancreatic acinar cells and are secreted into the acinar lumen. Two other molecular chaperones, Grp78 and the Hsp70, appear to be restricted to the rough endoplasmic reticulum and the trans-Golgi apparatus, respectively. We have found that chaperones can be associated with pancreatic enzymes along the secretory pathway. Indeed, double immunogold and immunocoprecipitation revealed an association between Cpn60 and the colipase-dependent lipase (CDL) and between Grp94 and the bile salt-dependent lipase (BSDL). These complexes are secreted into the acinar lumen and diverted to the duodenal lumen. These findings led us to investigate these enzyme-chaperone complexes in intestinal tissue. Grp94, Cpn60, and PDI are present on microvilli and on the endosomal compartment of enterocytes. Furthermore, we have shown that the Grp94-BSDL complexes are internalized by enterocytes through classical endocytosis. Upon dissociation of the BSDL-Grp94 complex in the late endosome, BSDL is transferred to the basolateral membrane. We propose that Grp94 interacts with specific receptors and/or could force the associated protein to adopt a specific conformation that allows its binding to corresponding membrane receptors and its internalization by enterocytes. These two hypotheses need not to be exclusive. The existence of such a pancreatic secretion-intestinal absorption link speaks in favor of a coordinated functional connection between these two entities, through molecular chaperones, in order to optimize intestinal activities.

Introduction of a novel HRP substrate-Nanogold probe for signal amplification in immunocytochemistry

Amplification of immunological signals with catalyzed reporter deposition (CARD) allows improved detection of scarce tissue antigens in light and electron microscopy. The technique takes advantage of the oxidation ability of horseradish peroxidase (HRP), in the presence of hydrogen peroxide, to yield the accumulation of one of its specific reporter-tagged substrates. This immunocytochemical approach continues to be improved by the introduction of new reporter molecules tagged to tyramine or to other HRP substrates. In this study we introduced a novel HRP substrate tagged to Nanogold particles. The amplification protocol is based on the application of a specific primary antibody, a biotinylated secondary antibody, streptavidin-HRP, and an HRP substrate coupled to Nanogold, followed by silver intensification. In addition to amplification of immunological signals of high resolution, direct accumulation of Nanogold particles at target sites by enzymatic activity of HRP improves the efficiency of the technique compared to other amplification protocols. Moreover, this approach combines the CARD amplification potentials with the ultrasmall gold probe and the silver intensification method. Immunolabeling obtained by light and electron microscopy, as well as immunodot assay using this new amplification strategy, appear to be highly sensitive, specific, and of enhanced intensity.

Immunocytochemistry of the AfaE adhesin and AfaD invasin produced by pathogenic Escherichia coli strains during interaction of the bacteria with HeLa cells by high-resolution scanning electron microscopy

We used a recent scanning electron microscope equipped with field emission gun and highly sensitive detectors to develop a fast and simple protocol for double immunogold staining using 10- and 15-nm gold particles. We used this approach to analyse the afimbrial adhesive sheath produced by pathogenic Escherichia coli interacting with the surface of epithelial cells. We demonstrated that AfaE adhesin and AfaD invasin were exposed at the bacterial surface during the interaction. This method could be easily and widely extended to the study of the early invasion process of many bacterial and viral pathogens, by immunocytochemical probing.

Demonstration of a fusion mechanism between a fluid bactericidal liposomal formulation and bacterial cells

It was previously demonstrated that fluid liposomal-encapsulated tobramycin, named Fluidosomes, was successful in eradicating mucoid Pseudomonas aeruginosa in an animal model of chronic pulmonary infection, whereas free antibiotic did not reduce colony-forming unit (CFU) counts (C. Beaulac et al., Antimicrob. Agents Chemother. 40 (1996) 665-669; C. Beaulac et al., J. Antimicrob. Chemother. 41 (1998) 35-41). These liposomes were also shown to be bactericidal in in vitro tests against strong resistant P. aeruginosa 64 microg/ml). The time needed to reach the maximal fusion rate was about 5 h for the resistant strain comparatively to much shorter time for the sensitive strain. The specific characteristics of Fluidosomes could help overcome bacterial resistance related to permeability barrier and even enzymatic hydrolysis considering the importance of synergy in the whole process of antibiotic resistance.

The affinity binding sites of pancreatic bile salt-dependent lipase in pancreatic and intestinal tissues

In previous studies, we have shown that the bile salt-dependent lipase (BSDL) associates with the Grp94 molecular chaperone, an association that appears to play essential roles in the folding of BSDL. More recently, combined biochemical and immunocytochemical investigations were carried out to show that the transport of BSDL occurs via an association with the Grp94 all along the pancreatic secretory route (ER-Golgi-granules). The Grp94-BSDL complex is secreted with the pancreatic juice into the acinar lumen and reaches the duodenal lumen, where it is internalized by enterocytes. The dissociation of the complex could take place within the endosomal compartment because BSDL continues further on its way to the basolateral membrane of the enterocyte. To localize the affinity binding sites of pancreatic BSDL in pancreatic and duodenal tissues, we have used an affinity-gold ultrastructural technique. BSDL coupled to gold particles appears to interact with specific sites in tissue sections. This was confirmed by another indirect morphological approach using biotin-labeled BSDL and streptavidin-gold complexes on tissue sections. We have shown that BSDL associates with sites in the pancreatic secretory pathway compartments and in the microvilli, the endosomal compartment, and the basolateral membrane of enterocytes. By biochemical approaches, biotin-labeled BSDL displayed affinities with proteins of 180-190 kD in both pancreatic and duodenal tissues. We have also shown that the Grp94-BSDL complexes, which are insensitive to denaturing conditions, are present in pancreatic homogenate but not in duodenal lysate. Thus, BSDL is able to bind protein complexes formed by either BSDL-Grp94 or Grp94 dimers. (J Histochem Cytochem 48:267-276, 2000)

Effects of soybean diet on the beta cells in the streptozotocin treated rats for induction of diabetes

Certain dietary components have been reported to potentially suppress the initiation of experimental insulin-dependent diabetes mellitus (IDDM) in animal models. In the present study, we showed that dietary soybean prevents induction of experimental hyperglycemia by retaining beta cell activity. In rats fed raw soybean, expression of insulin mRNA in pancreatic beta cells was significantly increased compared to those fed with normal diet. In those rats and upon injection of streptozotocin, only few beta cells underwent cell death, most of them demonstrating active viability with enhanced mRNA expression and insulin content. This is consistent with the fact that blood glucose level was normalized (72.51 +/- 1.54 mg/dl) after a transitory hyperglycemic state (> 300 mg/dl). It implies the dietary soybean can prevent beta cell injury by streptozotocin. Moreover, a prolonged hyperglycemia was not observed in rats fed raw soybean even when this was substituted by the normal diet after streptozotocin injection. In contrast, in streptozotocin-treated rats fed normal diet, most beta cells were destroyed and severe hyperglycemia was observed. Although the protective effect was not recorded in the rats fed with heated soybean, some beta cells were found to retain their cell organelles for insulin secretion indicating that some heat-stable components of soybean might prevent the cytotoxic insult. Therefore, we suggest that dietary soybean protected the streptozotocin-induced beta cell damage and restrained the development of hyperglycemia in rats.

Selective but nonspecific immunolabeling of enamel protein-associated compartments by a monoclonal antibody against vimentin

Vimentin, an intermediate filament component, has been identified in many mesenchymal cells by a variety of LM and EM immunolabeling techniques. In our study, several tissue-processing conditions and monoclonal and polyclonal antibodies against vimentin were screened for immunostaining of rat incisor odontoblasts. Using postembedding colloidal gold immunocytochemistry, we were unable to detect any convincing vimentin antigenicity in these cells, but one of the monoclonal antibodies (V9-S) unexpectedly resulted in intense labeling over intra- and extracellular compartments that normally are strongly immunoreactive with anti-amelogenin antibodies. Blocking experiments showed that V9-S binding was competed by anti-amelogenin antibody. Immunoblots indicated that enamel proteins reacted with this anti-vimentin antibody after fixation with glutaraldehyde. These data suggest that the observed immunoreaction is directed against an epitope apparently created by crosslinking of enamel proteins during fixation. Although the labeling cannot be considered specific, it is nevertheless selective because it is very precisely localized over compartments containing enamel proteins and shows no binding to other calcified dental tissues, including dentin and bone. The V9-S antibody can therefore be used as a reliable probe to identify the presence and distribution of amelogenins in fixed tissues. (J Histochem Cytochem 47:1237-1245, 1999)

Content and distribution of noncollagenous matrix proteins in bone and cementum: relationship to speed of formation and collagen packing density

The organic matrix of collagen-based calcified tissues consists of a supporting collagen meshwork and various noncollagenous matrix proteins (NCPs). Together, they contribute to determining the structure and biomechanical properties of the tissue. Their respective organization and interrelation can advantageously be examined by immunocytochemistry, an approach which allows correlation of composition with structure. The aim of this article is to review postembedding immuno- and lectin-gold-labeling data on the characterization of the noncollagenous compartment in rat and human bone and cementum, and on its relationship to collagen. The two major NCPs, bone sialoprotein and osteopontin, generally codistribute and accumulate in cement lines and in the spaces among the mineralized collagen fibrils. However, there are variations in their distribution and density of labeling throughout the tissue. Indeed, bone and cementum can form in environments that are either poor or enriched in NCPs. The amount of NCPs generally correlates with bone and cementum types and with speed of formation of the tissue and packing density of collagen fibrils. Taken together, the data suggest that production of both collagenous and noncollagenous constituents can be "modulated" during formation of collagen-based calcified tissues. It is concluded that, in addition to structural and compositional parameters, tissue dynamics must be taken into consideration in order to understand the significance of the apparent accumulation of NCPs at some sites and to determine the mechanisms of normal and pathological calcified tissue formation.

Brush cells of the mouse intestine possess a specialized glycocalyx as revealed by quantitative lectin histochemistry. Further evidence for a sensory function

Brush cells occur in the epithelium of the small intestine and in various other epithelia of endodermal origin. Ultrastructural and histochemical characteristics suggest that they represent sensory cells. Because the apical membrane of brush cells might be involved in and specialized for (chemo-)receptive functions, we investigated the composition of the glycocalyx and compared it with that of enterocytes. Ultrathin sections of murine small intestine were labeled with a panel of eight lectins. Their binding sites in the brush border and on vesicles of the apical cytoplasm were detected by colloidal gold and quantified using image analysis. The glycocalyx of brush cells contained significantly higher amounts of l-fucose residues than that of enterocytes, as detected by the lectins UEA-I and LTA. In contrast, most of the other lectins bound more avidly to the glycocalyx of enterocytes. The cytoplasmic vesicles closely resembled the apical membrane in their labeling pattern. Quantitation of the brush cells' distribution revealed that the epithelia of the Peyer's patches contained 10-fold higher numbers of brush cells than the small intestinal mucosa distant from lymphoid tissue. We conclude that brush cells possess a glycocalyx with a specialized composition and differ significantly from enterocytes. Because similar peculiarities of the apical membrane have previously been described for sensory cells of the olfactory and gustatory organs, this study provides further evidence in favor of a sensory function of brush cells.