Visualization of 'water secretion' by confocal microscopy in rat salivary glands: possible distinction of para- and transcellular pathway

Visualization of water transport in cells, tissues and organs is an important, yet still difficult, task in morphological science. By using confocal microscopy and the fluid-phase fluorescent tracer technique, we visualized water secretion and estimated the routes of water transport across the acinar epithelia in rat parotid and submandibular glands. Confocal microscopy of whole glands perfused arterially with Lucifer yellow revealed a bright fluorescence at the basolateral space of acini. Luminal space was devoid of fluorescence, but revealed it after isoproterenol pretreatment, ductal infusion of fluorescent dextrans into the lumen, or tissue dissociation by collagenase. Under these conditions, stimulation of fluid secretion with carbachol caused a rapid decline of the luminal fluorescence intensity, indicating that the secreted water washed out the fluorescent probes in the acinar lumen. In the stimulated dissociated acini, the luminal fluorescence disappeared by 15 sec, but reappeared at 30-45 sec to maintain a low plateau level. By assuming that the tight junction was 'paralyzed' by the collagenase digestion and that the paracellular fluid transport could not influence the dilution of Lucifer yellow, we estimated that the initial water secretion by CCh occurs via the transcellular pathway, while later than 30-45 sec the additional water permeates through the paracellular pathway.     

Tight junctions in the rat parotid gland

The aim of this study was to elucidate the distribution and morphological changes of tight junctions during secretion in parotid gland acinar cells. Localization of tight junction-associated polypeptide ZO-1, and of tight junction transmembrane protein Occludin, was examined in rat parotid gland by immunofluorescence and immunogold labelling of ultrathin sections. Adult male Sprague-Dawley rats were intraperitoneally injected with IPR and, after 10 and 30 minutes, parotid glands were extirpated. In control specimens, positive immunoreaction for ZO-1 and Occludin was observed on the adluminal side between adjacent cells in the form of narrow elongated profiles corresponding to intercellular canaliculi. After IPR injection, canaliculi became dilated and fluorescence was no longer seen as a continuous line but appeared as an aggregation of separate bright particles. ZO-1 was more widely distributed and was recognized in other areas of the cytoplasm as well. Concurrently, omega-shaped concavities, marked by actin fluorescence, appeared along the intercellular canaliculi. We concluded that, during exocytosis, the selective permeability barrier to the paracellular pathway, based on tight junctions, becomes more leaky, owing to segregation of Occludin caused by intracellular ZO-1 distributional changes associated with actin filaments.     

Structure and permeability of goblet cell tight junctions in rat small intestine

Summary Two major cell types, goblet and absorptive cells, dominate the epithelial lining of small intestinal villi. We used freezefracture replicas of rat ileal mucosa to examine the possibility that tight junction structure, known to relate to transepithelial resistance, might vary with cell type. Tight junctions between absorptive cells were uniform in structure while those associated with villus goblet cells displayed structural variability. In 23% of villus goblet cell tight junctions the strand count was less than 4 and in 30% the depth was less than 200 nm. In contrast, only 4% of absorptive cell tight junctions had less than 4 strands and only 9% had depth measurements less than 200 nm. Other structural features commonly associated with villus goblet cell tight junctions but less commonly with absorptive cell tight junctions were: deficient strand cross-linking, free-ending abluminal strands, and highly fragmented strands. Bothin vivo ileal segments and everted loops were exposed to ionic lanthanum. Dense lanthanum precipitates in tight junctions and paracellular spaces were restricted to a subpopulation of villus goblet cells and were not found between villus absorptive cells. After exposure of prefixed ileal loops to lanthanum for 1 hour, faint precipitates of lanthanum were found in 14% of tight junctions and paracellular spaces between absorptive cells compared to 42% of tight junctions and paracellular spaces adjacent to villus goblet cells. When tested in Ussing chambers, the methods used for lanthanum exposure did not lower transepithelial resistance. Everted loops exposed to ionic barium and examined by light microscopy showed dense barium precipitates in the junctional zone and region of the paracellular space of villus goblet cells but not in these regions between absorptive cells. However, the macromolecular tracers, microperoxidase, cytochromec and horseradish peroxidase, were excluded from both villus goblet cell and absorptive cell paracellular spaces inin vivo segments. These findings suggest that a subpopulation of villus goblet cells may serve as focal sites of high ionic permeability and contribute to the relatively low resistance to ionic flow which characterizes the small intestinal epithelium.     

Tight junction of sinus endothelial cells of the rat spleen

The fine structure of the tight junctions between sinus endothelial cells of the rat spleen and the permeability of such sinus endothelial cells were examined by transmission electron microscopy, using freeze-fracture, triton extraction, and lanthanum-tracer techniques. In freeze-fracture replicas, the segmented strands and grooves of the tight junctions were frequently observed on the basolateral surfaces of the sinus endothelial cells irrespective of the location of the ring fiber. There were one or two wavy-strands or grooves which were, for the most part, oriented parallel to the long cell axis thus forming networks at places. In addition, some strands or grooves were discontinuous while some networks of the junctional strands were not closed. These strands also occasionally lacked intramembranous particles in the tight junctions. The junctional strands run apicobasically at certain sites. In the vertical sections of the sinus endothelial cells treated with lanthanum nitrate, although no tight junctions were observed wherever the endothelial cells were apposed, most of them were situated on the basal part of the lateral surfaces of the adjacent endothelial cells. Several fusions of the junctional membranes were observed in a vertical section of the lateral surfaces of the adjacent endothelial cells. The intercellular spaces of the adjacent endothelial cells except for the fusion of the junctional membranes, were electron dense and the infiltration of lanthanum nitrate was found not to be interrupted by these tight junctions. Based on these observations, the molecular 'fence' and paracellular 'gate' functions of the tight junctions in the sinus endothelial cells are discussed.     

Freeze-fracture of capillary endothelium in rat brain

Summary The rat brain capillary was studied with freeze-fracture technique. The attached plasmalemmal vesicles were quite few in number on the luminal front and sometimes numerous on the contraluminal side. The fracture appearance of some tight junctions showed interconnecting ridges on face A and complementary furrows devoid of particles on face B, comparable to the common tight junction in the normal epithelia. Other tight junctions revealed a preferential disposition of quasicontinuous rows of particles on shallow furrows of face B, resembling the tight junctional strands of capillary endothelium in non-cerebral tissues. Either behavior is probably due to the difference in the fracture plane around the single fibril. In addition, the tight junctional strand could surround the perimeter of the endothelial cell completely although the exposed strand of tight junction was limited in length.     

Reciprocal influence of connexins and apical junction proteins on their expressions and functions

Membranes of adjacent cells form intercellular junctional complexes to mechanically anchor neighbour cells (anchoring junctions), to seal the paracellular space and to prevent diffusion of integral proteins within the plasma membrane (tight junctions) and to allow cell-to-cell diffusion of small ions and molecules (gap junctions). These different types of specialised plasma membrane microdomains, sharing common adaptor molecules, particularly zonula occludens proteins, frequently present intermingled relationships where the different proteins co-assemble into macromolecular complexes and their expressions are co-ordinately regulated. Proteins forming gap junction channels (connexins, particularly) and proteins fulfilling cell attachment or forming tight junction strands mutually influence expression and functions of one another.     

Claudins--key pieces in the tight junction puzzle

Tight junctions restrict the flow of ions and aqueous molecules between cells by forming a selective barrier to the paracellular pathway. Permeability of the tight junction barrier is determined by a class of transmembrane proteins known as claudins. The relationship between claudins and paracellular permeability is complex and determined not only by the profile of claudin expression but also by the arrangement of claudins and other proteins into tight junction strands. This review summarizes progress in understanding how claudins are assembled into tight junctions and how they interact with other tight junction proteins.     

The tight junctional protein occludin is found in the uterine epithelium of squamate reptiles

Occludin, an integral protein associated with the mammalian tight junction, has for the first time been identified in the uterus of squamate reptiles. The tight junction is made up of anastamosing strands and forms a selective barrier that regulates paracellular diffusion of solutes across uterine epithelium. Occludin exclusively labels tight junctional strands and is an excellent marker for tight junction permeability. Using western blotting and immunohistochemistry, occludin expression was examined in the uterine epithelium of five species of Australian skinks at different stages of gestation. More occludin was detected during late stage pregnancy/gravidity compared to the lower levels of occludin detected in vitellogenic and post-parturient females in three of the five species. We conclude that the paracellular permeability of the squamate uterine epithelium decreases as gestation progresses. As placental transport of ions and solutes to the embryo is highest during the last third of pregnancy in viviparous squamates, it is likely that a decrease in paracellular permeability is compensated by an upregulation of other transporting mechanisms such as histotrophy.     

The gastric mucosal barrier: structure of intercellular junctions in the dog

The canine gastric mucosa consists of two regions, the surface mucous cells and gland area cells including parietal, chief, and mucous-containing cells. We have used quantitative freeze-fracture methods in conjunction with thin-section extracellular tracers to document and correlate tight junction morphology with epithelial permeability. The number of strands in the tight junction complexes of the surface cells and gland cells is the same, but differences in strand arrangement exist. The surface cells have an interwoven tight junction configuration which is impermeable to extracellular tracers. The gland cell junctions are regularly arranged and often permeable to extracellular lanthanum. The possibility that the observed difference in permeability between the tight junctions of the surface mucous cells and those of the gland cells is related to their structural configuration is discussed.     

Blocking by 2,4,6-triaminopyrimidine of increased tight junction permeability induced by acetylcholine in the pancreas

1. The permeability of the paracellular pathway in the isolated rabbit pancreas has been studied with the aid of 2,4,6-triaminopyrimidine. 2. Addition of 2,4,6-triaminopyrimidine (1--10 mM) to the bathing medium has no effect on the rate of fluid secretion or on protein, Na+, K+, Ca2+ and sucrose concentrations in the secreted fluid. 3. When 1 x 10(-5) M carbachol is also added to the 2,4,6-triaminopyrimidine-containing bathing medium, there is a marked reduction in the increase of the paracellular permeability for sucrose and Ca2+ found upon addition of carbachol alone. The enzyme secretion, induced by carbachol, is not affected. 4. The minimal concentration of 2,4,6-triaminopyrimidine in the bathing medium required to reach its maximal effect on the paracellular permeability is approx. 0.55 mM at pH 7.4. 5. The effect of 2,4,6-triaminopyrimidine on the paracellular permeability after carbachol stimulation is also present when 2,4,6-triaminopyrimidine is added 5 min after the addition of 1 x 10(-5) M carbachol. 6. 2,4,6-Triaminopyrimidine has no effect on the increases in enzyme secretion and sucrose permeability caused by 1 x 10(-8) pancreozymin C octapeptide. 7. 2,4,6-Triaminopyrimidine appears in the secreted fluid at a concentration of 50% of that in the bathing medium. Upon addition of 1 x 10(5) M carbachol this concentration increases up to 80%. 8. These results indicate that: (a) the increased paracellular permeability upon stimulation with carbachol is not caused by the enzyme secretion as such and (b) addition of 2,4,6-triaminopyrimidine prevents the carbachol-induced increase in permeability of a channel in the tight junction complex.     

Claudin-8 expression in Madin-Darby canine kidney cells augments the paracellular barrier to cation permeation

Claudins are a family of integral membrane proteins of the tight junction that are thought to participate in the permeation of solutes across epithelia via the paracellular pathway. Claudin-8 is expressed in the distal renal tubule, which has a characteristically low passive permeability to monovalent cations. To test the hypothesis that claudin-8 plays a role in forming a tight paracellular barrier to cations, stably transfected Madin-Darby canine kidney II cell lines with inducible expression of claudin-8 were generated. Induction of claudin-8 expression was associated with down-regulation of endogenous claudin-2 protein. Other tight junction proteins were expressed and targeted normally, and the number of junctional strands was minimally altered. By Ussing chamber and radiotracer flux studies, claudin-8 expression was found to reduce paracellular permeability to monovalent inorganic and organic cations and to divalent cations but not to anions or neutral solutes. The size selectivity, charge dependence, and activation energy of paracellular cation permeation were all unchanged. These observations are consistent with a model in which claudin-2 encodes a highly cation-permeable channel, whereas claudin-8 acts primarily as a cation barrier. When exogenous claudin-8 is expressed, it replaces endogenous claudin-2, inserting in its place into existing tight junction strands, thereby reducing the apparent number of functional cation pores. Our findings suggest that claudin-8 plays an important role in the paracellular cation barrier of the distal renal tubule.     

A freeze-fracture and lanthanum tracer study of the complex junction between Sertoli cells of the canine testis

What appear to be true septate junctions by all techniques currently available for the cytological identification of intercellular junctions are part of a complex junction that interconnects the Sertoli cells of the canine testis. In the seminiferous epithelium, septate junctions are located basal to belts of tight junctions. In thin sections, septate junctions appear as double, parallel, transverse connections or septa spanning an approximately 90-A intercellular space between adjacent Sertoli cells. In en face sections of lanthanum-aldehyde-perfused specimens, the septa themselves exclude lanthanum and appear as electron-lucent lines arranged in a series of double, parallel rows on a background of electron-dense lanthanum. In freeze-fracture replicas this vertebrate septate junction appears as double, parallel rows of individual or fused particles which conform to the distribution of the intercellular septa. Septate junctions can be clearly distinguished from tight junctions as tight junctions prevent the movement of lanthanum tracer toward the lumen, appear as single rows of individual or fused particles in interlacing patterns within freeze-fracture replicas, and are seen as areas of close membrane apposition in thin sections. Both the septate junction and the tight junction are associated with specializations of the Sertoli cell cytoplasm. This is the first demonstration in a vertebrate tissue of a true septate junction.     

CNS myelin and sertoli cell tight junction strands are absent in Osp/claudin-11 null mice

Oligodendrocyte-specific protein (OSP)/claudin-11 is a recently identified transmembrane protein found in CNS myelin and testis with unknown function. Herein we demonstrate that Osp null mice exhibit both neurological and reproductive deficits: CNS nerve conduction is slowed, hindlimb weakness is conspicuous, and males are sterile. Freeze fracture reveals that tight junction intramembranous strands are absent in CNS myelin and between Sertoli cells of mutant mice. Our results demonstrate that OSP is the mediator of parallel-array tight junction strands and distinguishes this protein from other intrinsic membrane proteins in tight junctions. These novel results provide direct evidence of the pivotal role of the claudin family in generating the paracellular physical barrier of tight junctions necessary for spermatogenesis and normal CNS function.     

Microtubule Integrity Is Necessary for the Epithelial Barrier Function of Cultured Thyroid Cell Monolayers

Vectorial transport in the thyroid epithelium requires an efficient barrier against passive paracellular flux, a role which is principally performed by the tight junction (zonula occludens). There is increasing evidence that tight junction integrity is determined by integral and peripheral membrane proteins which interact with the cell cytoskeleton. Although the contribution of the actin cytoskeleton to tight junction physiology has been intensively studied, less is known about possible interactions with microtubules. In the present study we used electrophysiological and immunohistochemical approaches to investigate the contribution of microtubules to the paracellular barrier in cultured thyroid cell monolayers which displayed a high transepithelial electrical resistance (6000-9000 ohm · cm²). Colchicine (1 μM) caused a progressive fall in electrical resistance to <10% of baseline after 6 h and depolarization of the transepithelial electrical potential difference consistent with a significant increase in paracellular permeability. The effect of colchicine on TER was not affected by agents which inhibit the major apical conductances of thyroid cells but was reversed upon removal of the drug. Immunofluorescent staining for tubulin combined with confocal laser scanning microscopy demonstrated that thyroid cells possessed a dense microtubule network extending throughout the cytoplasm which was destroyed by colchicine. Colchicine also produced changes in the localization of the tight junction-associated protein, ZO-1: its normally continuous junctional distribution was disrupted by striking discontinuities and the appearance of many fine strands which extended into the cytoplasm. A similar disruption in E-cadherin staining was also observed, but colchicine did not affect the distribution of vinculin associated with adherens junctions nor the integrity of the perijunctional actin ring. We conclude that microtubules are necessary for the functional and structural integrity of tight junctions in this electrically tight, transporting epithelium.     

Tight junctional permeability of the resting and carbachol stimulated exocrine rabbit pancreas

The permeability of the pancreatic epithelium to horseradish peroxidase is investigated in the resting and carbachol stimulated rabbit pancreas. Horse radish peroxidase administered to the bathing medium of the isolated rabbit pancreas appears in the secreted fluid of the pancreas in a relatively low concentration. Carbachol stimulates both protein secretion and the passage of horse radish peroxidase into the secretory fluid. Histochemical assessment shows that horseradish peroxidase enters the interstitial spaces of the pancreatic tissue and is present along basal and lateral plasma membranes of acinar and ductular cells. In the absence of carbachol, horseradish peroxidase is seen more frequently in the tight junctions of ductular cells than in those of acinar cells. However, in the carbachol stimulated gland horseradish peroxidase is observed in the junctions between adjacent acinar cells more frequently than in the unstimulated gland. Freeze-fracture of acinar cells shows that the number of tight junctional strands and the tight junction depth are slightly decreased upon carbachol stimulation. The findings suggest that cholinergic stimulation of the exocrine pancreas increases the permeability of the acinar cell junctions to moderately large molecules such as horseradish peroxidase. This may result in an increase of the concentration of the molecule in the secreted fluid.     

Tight junctional permeability of the resting and carbachol stimulated exocrine rabbit pancreas

Summary The permeability of the pancreatic epithelium to horseradish peroxidase is investigated in the resting and carbachol stimulated rabbit pancreas. Horse radish peroxidase administered to the bathing medium of the isolated rabbit pancreas appears in the secreted fluid of the pancreas in a relatively low concentration. Carbachol stimulates both protein secretion and the passage of horse radish peroxidase into the secretory fluid. Histochemical assessment shows that horseradish peroxidase enters the interstitial spaces of the pancreatic tissue and is present along basal and lateral plasma membranes of acinar and ductular cells. In the absence of carbachol, horseradish peroxidase is seen more frequently in the tight junctions of ductular cells than in those of acinar cells. However, in the carbachol stimulated gland horseradish peroxidase is observed in the junctions between adjacent acinar cells more frequently than in the unstimulated gland. Freeze-fracture of acinar cells shows that the number of tight junctional strands and the tight junction depth are slightly decreased upon carbachol stimulation. The findings suggest that cholinergic stimulation of the exocrine pancreas increases the permeability of the acinar cell junctions to moderately large molecules such as horseradish peroxidase. This may result in an increase of the concentration of the molecule in the secreted fluid.     

Claudins upregulation in human colorectal cancer

In colorectal cancer tight junction molecular and morphological alterations are poorly understood. In this study, adenocarcinoma tissues and their paired normal mucosa (n = 12) were analyzed for tight junction alterations molecular. The expression of claudin-1, -3 and -4 was upregulated 5.7-, 1.5- and 2.4-fold, respectively, in colorectal tumor tissues in comparison to the normal ones. Although tight junction remains in the cancerous epithelium, its barrier function was altered. Despite claudins overexpression, paracellular permeability to ruthenium red was increased and a significant disorganization of tight junction strands was observed in freeze fracture replicas. Whereas the functional significance of claudin overexpression in colorectal cancer is unclear, these proteins can become potential markers and targets in colorectal cancer.     

Tight junctions and the modulation of barrier function in disease

Tight junctions create a paracellular barrier in epithelial and endothelial cells protecting them from the external environment. Two different classes of integral membrane proteins constitute the tight junction strands in epithelial cells and endothelial cells, occludin and members of the claudin protein family. In addition, cytoplasmic scaffolding molecules associated with these junctions regulate diverse physiological processes like proliferation, cell polarity and regulated diffusion. In many diseases, disruption of this regulated barrier occurs. This review will briefly describe the molecular composition of the tight junctions and then present evidence of the link between tight junction dysfunction and disease.     

Beta-adrenergic receptor stimulation induces inositol trisphosphate production and Ca2+ mobilization in rat parotid acinar cells

In dispersed rat parotid gland acinar cells, the beta-adrenergic agonist (-)-isoproterenol, but not its stereoisomer (+)-isoproterenol, induced a transient 1.6-fold (at maximum stimulation, 2 x 10(-4) M) increase in cytosolic free calcium ([Ca2+]i) within 9 s, which returned to resting levels (approximately 190 nM) by 60 s. This [Ca2+]i response was not altered by chelating extracellular Ca2+ with [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) and could be completely blocked by the beta-adrenergic antagonists propranolol (beta 1 + beta 2) and ICI 118,551 (beta 2) but not by atenolol (beta 1). The muscarinic-cholinergic agonist carbachol (at maximum stimulation, 10(-5) M) induced a 3-4-fold elevation in [Ca2+]i within 6 s, which slowly returned to resting levels by 8-10 min. The peak carbachol [Ca2+]i response was not substantially altered by the addition of EGTA to the extracellular medium. However, if the cells were first stimulated with isoproterenol in the EGTA-containing medium, the peak carbachol response was decreased approximately 54%. When carbachol was added to cells in the presence of high extracellular calcium, at the isoproterenol-stimulated [Ca2+]i peak, the resulting [Ca2+]i level was equal to that achieved when carbachol was either added alone or added after propranolol and isoproterenol. 8-Bromo-cyclic AMP induced a [Ca2+]i response similar to that elicited by isoproterenol, which was not additive to that by carbachol. Carbachol induced a approximately 3.5-fold increase in inositol trisphosphate (IP3) production in parotid cells within 30 s. 8-Bromo-cAMP, N6,O2'-dioctanoyl-cAMP, and isoproterenol consistently induced a significant stimulation in IP3 production. The half-maximal concentration of isoproterenol required for [Ca2+]i mobilization and IP3 production was comparable (approximately 10(-5) M). Isoproterenol-induced IP3 formation was blocked by propranolol. The data show that in rat parotid acinar cells, beta-adrenergic stimulation results in IP3 formation and mobilization of a carbachol-sensitive intracellular Ca2+ pool by a mechanism involving cAMP. This demonstrates an interaction between the cAMP and phosphoinositide second messenger systems in these cells.