Coordinated ciliary beating requires Odf2-mediated polarization of basal bodies via basal feet.

Coordinated beating of cilia in the trachea generates a directional flow of mucus required to clear the airways. Each cilium originates from a barrel-shaped basal body, from the side of which protrudes a structure known as the basal foot. We generated mice in which exons 6 and 7 of Odf2, encoding a basal body and centrosome-associated protein Odf2/cenexin, are disrupted. Although Odf2(ΔEx6,7/ΔEx6,7) mice form cilia, ciliary beating is uncoordinated, and the mice display a coughing/sneezing phenotype. Whereas residual expression of the C-terminal region of Odf2 in these mice is sufficient for ciliogenesis, the resulting basal bodies lack basal feet. Loss of basal feet in ciliated epithelia disrupted the polarized organization of apical microtubule lattice without affecting planar cell polarity. The requirement for Odf2 in basal foot formation, therefore, reveals a crucial role of this structure in the polarized alignment of basal bodies and coordinated ciliary beating.

Snakeskin, a membrane protein associated with smooth septate junctions, is required for intestinal barrier function in Drosophila.

Septate junctions (SJs) are the membrane specializations observed between epithelial cells in invertebrates. SJs play a crucial role in epithelial barrier function by restricting the free diffusion of solutes through the intercellular space. In arthropod species, two morphologically different types of SJs have been described: pleated septate junctions (pSJs) and smooth septate junctions (sSJs), which are specific to ectodermal and endodermal epithelia, respectively. In contrast to the recent identification of pSJ-related proteins, the molecular constituents of sSJs are mostly unknown. Here, we report the discovery of a new sSJ-specific membrane protein, designated 'Snakeskin' (Ssk). Ssk is highly concentrated in sSJs in the Drosophila midgut and Malpighian tubules. Lack of Ssk expression is embryonically lethal in Drosophila and results in defective sSJ formation accompanied by abnormal morphology of midgut epithelial cells. We also show that the barrier function of the midgut to a fluorescent tracer is impaired in ssk-knockdown larvae. These results suggest that Ssk is required for the intestinal barrier function in Drosophila.

Claudin-2-deficient mice are defective in the leaky and cation-selective paracellular permeability properties of renal proximal tubules.

Claudin-2 is highly expressed in tight junctions of mouse renal proximal tubules, which possess a leaky epithelium whose unique permeability properties underlie their high rate of NaCl reabsorption. To investigate the role of claudin-2 in paracellular NaCl transport in this nephron segment, we generated knockout mice lacking claudin-2 (Cldn2(-/-)). The Cldn2(-/-) mice displayed normal appearance, activity, growth, and behavior. Light microscopy revealed no gross histological abnormalities in the Cldn2(-/-) kidney. Ultrathin section and freeze-fracture replica electron microscopy revealed that, similar to those of wild types, the proximal tubules of Cldn2(-/-) mice were characterized by poorly developed tight junctions with one or two continuous tight junction strands. In contrast, studies in isolated, perfused S2 segments of proximal tubules showed that net transepithelial reabsorption of Na(+), Cl(-), and water was significantly decreased in Cldn2(-/-) mice and that there was an increase in paracellular shunt resistance without affecting the apical or basolateral membrane resistances. Moreover, deletion of claudin-2 caused a loss of cation (Na(+)) selectivity and therefore relative anion (Cl(-)) selectivity in the proximal tubule paracellular pathway. With free access to water and food, fractional Na(+) and Cl(-) excretions in Cldn2(-/-) mice were similar to those in wild types, but both were greater in Cldn2(-/-) mice after i.v. administration of 2% NaCl. We conclude that claudin-2 constitutes leaky and cation (Na(+))-selective paracellular channels within tight junctions of mouse proximal tubules.

Radixin deficiency causes conjugated hyperbilirubinemia with loss of Mrp2 from bile canalicular membranes.

The ezrin-radixin-moesin (ERM) family of proteins crosslink actin filaments and integral membrane proteins. Radixin (encoded by Rdx) is the dominant ERM protein in the liver of wildtype mice and is concentrated at bile canalicular membranes (BCMs). Here we show that Rdx(-/-) mice are normal at birth, but their serum concentrations of conjugated bilirubin begin to increase gradually around 4 weeks, and they show mild liver injury after 8 weeks. This phenotype is similar to human conjugated hyperbilirubinemia in Dubin-Johnson syndrome, which is caused by mutations in the multidrug resistance protein 2 (MRP2, gene symbol ABCC2), although this syndrome is not associated with overt liver injury. In wildtype mice, Mrp2 concentrates at BCMs to secrete conjugated bilirubin into bile. In the BCMs of Rdx(-/-) mice, Mrp2 is decreased compared with other BCM proteins such as dipeptidyl peptidase IV (CD26) and P-glycoproteins. In vitro binding studies show that radixin associates directly with the carboxy-terminal cytoplasmic domain of human MRP2. These findings indicate that radixin is required for secretion of conjugated bilirubin through its support of Mrp2 localization at BCMs.

Odf2-deficient mother centrioles lack distal/subdistal appendages and the ability to generate primary cilia.

Outer dense fibre 2 (Odf2; also known as cenexin) was initially identified as a main component of the sperm tail cytoskeleton, but was later shown to be a general scaffold protein that is specifically localized at the distal/subdistal appendages of mother centrioles. Here we show that Odf2 expression is suppressed in mouse F9 cells when both alleles of Odf2 genes are deleted. Unexpectedly, the cell cycle of Odf2(-/-) cells does not seem to be affected. Immunofluorescence and ultrathin-section electron microscopy reveals that in Odf2(-/-) cells, distal/subdistal appendages disappear from mother centrioles, making it difficult to distinguish mother from daughter centrioles. In Odf2(-/-) cells, however, the formation of primary cilia is completely suppressed, although approximately 25% of wild-type F9 cells are ciliated under the steady-state cell cycle. The loss of primary cilia in Odf2(-/-) F9 cells can be rescued by exogenous Odf2 expression. These findings indicate that Odf2 is indispensable for the formation of distal/subdistal appendages and the generation of primary cilia, but not for other cell-cycle-related centriolar functions.

Requirement of ZO-1 for the formation of belt-like adherens junctions during epithelial cell polarization.

The molecular mechanisms of how primordial adherens junctions (AJs) evolve into spatially separated belt-like AJs and tight junctions (TJs) during epithelial polarization are not well understood. Previously, we reported the establishment of ZO-1/ZO-2-deficient cultured epithelial cells (1[ko]/2[kd] cells), which lacked TJs completely. In the present study, we found that the formation of belt-like AJs was significantly delayed in 1(ko)/2(kd) cells during epithelial polarization. The activation of Rac1 upon primordial AJ formation is severely impaired in 1(ko)/2(kd) cells. Our data indicate that ZO-1 plays crucial roles not only in TJ formation, but also in the conversion from "fibroblastic" AJs to belt-like "polarized epithelial" AJs through Rac1 activation. Furthermore, to examine whether ZO-1 itself mediate belt-like AJ and TJ formation, respectively, we performed a mutational analysis of ZO-1. The requirement for ZO-1 differs between belt-like AJ and TJ formation. We propose that ZO-1 is directly involved in the establishment of two distinct junctional domains, belt-like AJs and TJs, during epithelial polarization.

Phosphorylation state regulates the localization of Scribble at adherens junctions and its association with E-cadherin-catenin complexes.

Mammalian ortholog of Scribble tumor suppressor has been reported to regulate cadherin-mediated epithelial cell adhesion by stabilizing the coupling of E-cadherin with catenins, but the molecular mechanism involved remains unknown. In this study, we investigated the relationship between the localization of mouse Scribble at cadherin-based adherens junctions (AJs) and its phosphorylation state. Immunofluorescence staining confirmed that Scribble was localized at AJs as well as at the basolateral plasma membrane in epithelial cells. We found that Scribble was detected as two bands by Western blotting analysis and that the band shift to the higher molecular weight was dependent on its phosphorylation at Ser 1601. Triton X-100 treatment extracted Scribble localized on the basolateral membrane but not Scribble localized at AJs in cultured epithelial cells, and the Triton X-100-resistant Scribble was the Ser 1601-unphosphorylated form. Conversely, an in-house-generated antibody that predominantly recognized Ser 1601-phosphorylated Scribble only detected Scribble protein on the lateral plasma membrane. Furthermore, Ser 1601-unphosphorylated Scribble was selectively coprecipitated with E-cadherin-catenin complexes in E-cadherin-expressing mouse L fibroblasts. Taken together, these results suggest that the phosphorylation state of Scribble regulates its complex formation with the E-cadherin-catenin system and may control cadherin-mediated cell-cell adhesion.

Claudins in occluding junctions of humans and flies.

The epithelial barrier is fundamental to the physiology of most metazoan organ systems. Occluding junctions, including vertebrate tight junctions and invertebrate septate junctions, contribute to the epithelial barrier function by restricting free diffusion of solutes through the paracellular route. The recent identification and characterization of claudins, which are tight junction-associated adhesion molecules, gives insight into the molecular architecture of tight junctions and their barrier-forming mechanism in vertebrates. Mice lacking the expression of various claudins, and human hereditary diseases with claudin mutations, have revealed that the claudin-based barrier function of tight junctions is indispensable in vivo. Interestingly, claudin-like molecules have recently been identified in septate junctions of Drosophila. Here, we present an overview of recent progress in claudin studies conducted in mammals and flies.

Claudin-based barrier in simple and stratified cellular sheets.

For homeostasis in multicellular organisms, isolation and compartmentalisation of the internal environment are essential, and are established by various cellular sheets. For these cellular sheets to function as barriers, the intercellular route must be sealed. Recent advances reveal that claudins - major cell adhesion molecules in tight junctions - are directly involved in this intercellular sealing, not only in simple but also in stratified cellular sheets in vertebrates.

Tricellulin constitutes a novel barrier at tricellular contacts of epithelial cells.

For epithelia to function as barriers, the intercellular space must be sealed. Sealing two adjacent cells at bicellular tight junctions (bTJs) is well described with the discovery of the claudins. Yet, there are still barrier weak points at tricellular contacts, where three cells join together. In this study, we identify tricellulin, the first integral membrane protein that is concentrated at the vertically oriented TJ strands of tricellular contacts. When tricellulin expression was suppressed with RNA interference, the epithelial barrier was compromised, and tricellular contacts and bTJs were disorganized. These findings indicate the critical function of tricellulin for formation of the epithelial barrier.

Achlorhydria by ezrin knockdown: defects in the formation/expansion of apical canaliculi in gastric parietal cells.

Loss of gastric acid secretion is pathologically known as achlorhydria. Acid-secreting parietal cells are characterized by abundant expression of ezrin (Vil2), one of ezrin/radixin/moesin proteins, which generally cross-link actin filaments with plasma membrane proteins. Here, we show the direct in vivo involvement of ezrin in gastric acid secretion. Ezrin knockout (Vil2(-/-)) mice did not survive >1.5 wk after birth, making difficult to examine gastric acid secretion. We then generated ezrin knockdown (Vil2(kd/kd)) mice by introducing a neomycin resistance cassette between exons 2 and 3. Vil2(kd/kd) mice born at the expected Mendelian ratio exhibited growth retardation and a high mortality. Approximately 7% of Vil2(kd/kd) mice survived to adulthood. Ezrin protein levels in Vil2(kd/kd) stomachs decreased to <5% of the wild-type levels without compensatory up-regulation of radixin or moesin. Adult Vil2(kd/kd) mice suffered from severe achlorhydria. Immunofluorescence and electron microscopy revealed that this achlorhydria was caused by defects in the formation/expansion of canalicular apical membranes in gastric parietal cells.

Tight junctions in Schwann cells of peripheral myelinated axons: a lesson from claudin-19-deficient mice.

Tight junction (TJ)-like structures have been reported in Schwann cells, but their molecular composition and physiological function remain elusive. We found that claudin-19, a novel member of the claudin family (TJ adhesion molecules in epithelia), constituted these structures. Claudin-19-deficient mice were generated, and they exhibited behavioral abnormalities that could be attributed to peripheral nervous system deficits. Electrophysiological analyses showed that the claudin-19 deficiency affected the nerve conduction of peripheral myelinated fibers. Interestingly, the overall morphology of Schwann cells lacking claudin-19 expression appeared to be normal not only in the internodal region but also at the node of Ranvier, except that TJs completely disappeared, at least from the outer/inner mesaxons. These findings have indicated that, similar to epithelial cells, Schwann cells also bear claudin-based TJs, and they have also suggested that these TJs are not involved in the polarized morphogenesis but are involved in the electrophysiological "sealing" function of Schwann cells.

CLASP1 and CLASP2 bind to EB1 and regulate microtubule plus-end dynamics at the cell cortex.

CLIP-associating protein (CLASP) 1 and CLASP2 are mammalian microtubule (MT) plus-end binding proteins, which associate with CLIP-170 and CLIP-115. Using RNA interference in HeLa cells, we show that the two CLASPs play redundant roles in regulating the density, length distribution and stability of interphase MTs. In HeLa cells, both CLASPs concentrate on the distal MT ends in a narrow region at the cell margin. CLASPs stabilize MTs by promoting pauses and restricting MT growth and shortening episodes to this peripheral cell region. We demonstrate that the middle part of CLASPs binds directly to EB1 and to MTs. Furthermore, we show that the association of CLASP2 with the cell cortex is MT independent and relies on its COOH-terminal domain. Both EB1- and cortex-binding domains of CLASP are required to promote MT stability. We propose that CLASPs can mediate interactions between MT plus ends and the cell cortex and act as local rescue factors, possibly through forming a complex with EB1 at MT tips.

Megaintestine in claudin-15-deficient mice.

BACKGROUND & AIMS: Claudins, the major components of tight junction (TJ) strands, which form paracellular barriers, consist of 24 family members, the combination of which determines the properties of TJ-based paracellular barriers. Here, we generated claudin-15-deficient (Cldn15(-/-)) mice to examine the ubiquitously expressed functions of claudin-15. METHODS: We generated Cldn15(-/-) mice by the conventional gene-targeting strategy. Because the upper small intestine was enlarged in Cldn15(-/-) mice, we analyzed the phenotype from various angles regarding histology, physiology, and cell biology. RESULTS: Cldn15(-/-) mice were born and grew normally with an enlarged upper small intestinal phenotype, megaintestine. Deficiency of claudin-15 did not cause a compensatory increase in the background expression of other types of claudins, claudin-1, -2, -3, -4, -7, -12, -18, -20, and -23, in the small intestine. Cldn15(-/-) mice showed enhanced proliferation of normal cryptic cells after weaning without diseased states such as polyps or cancer, resulting in megaintestine, in which the upper small intestine was approximately 2 times larger than normal in length and diameter. The number of transit-amplifying cells in crypts increased approximately 2-fold. Freeze-fracture electron microscopy revealed that deficiency of claudin-15 decreased the number of TJ strands, although the electric conductance was decreased in distal segments in Cldn15(-/-) jejunum, as compared with Cldn15(+/+) littermates. CONCLUSIONS: Based on the specific roles of claudins in paracellular barrier formation without any direct role in cell proliferation, as previously shown in cultured epithelial cells, we propose that claudin-15-based formation of TJs to organize the microenvironment including ion conductance is important for normal-sized morphogenesis of the small intestine.

Generation of transgenic medaka expressing claudin7-EGFP for imaging of tight junctions in living medaka embryos.

The tight junction (TJ) is a specialized cell-cell adhesion structure in epithelial and endothelial sheets unique to the chordates and functions as a barrier of fluidal diffusion across the cell sheets. In order to study the dynamics of TJ formation in vivo during embryogenesis, we have generated a transgenic medaka line that expresses claudin-7 protein fused to enhanced green fluorescent protein under the regulation of the red seabream beta-actin promoter in transparent medaka embryos. Claudins contain four transmembrane domains and have been identified as the key molecules that dictate the function of TJs. This transgenic medaka line will thus be useful for imaging of TJs in living embryos and hence in screening for mutations affecting cell-cell adhesion.

Size-selective loosening of the blood-brain barrier in claudin-5-deficient mice.

Tight junctions are well-developed between adjacent endothelial cells of blood vessels in the central nervous system, and play a central role in establishing the blood-brain barrier (BBB). Claudin-5 is a major cell adhesion molecule of tight junctions in brain endothelial cells. To examine its possible involvement in the BBB, claudin-5-deficient mice were generated. In the brains of these mice, the development and morphology of blood vessels were not altered, showing no bleeding or edema. However, tracer experiments and magnetic resonance imaging revealed that in these mice, the BBB against small molecules (<800 D), but not larger molecules, was selectively affected. This unexpected finding (i.e., the size-selective loosening of the BBB) not only provides new insight into the basic molecular physiology of BBB but also opens a new way to deliver potential drugs across the BBB into the central nervous system.

Claudin-based tight junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice.

The tight junction (TJ) and its adhesion molecules, claudins, are responsible for the barrier function of simple epithelia, but TJs have not been thought to play an important role in the barrier function of mammalian stratified epithelia, including the epidermis. Here we generated claudin-1-deficient mice and found that the animals died within 1 d of birth with wrinkled skin. Dehydration assay and transepidermal water loss measurements revealed that in these mice the epidermal barrier was severely affected, although the layered organization of keratinocytes appeared to be normal. These unexpected findings prompted us to reexamine TJs in the epidermis of wild-type mice. Close inspection by immunofluorescence microscopy with an antioccludin monoclonal antibody, a TJ-specific marker, identified continuous TJs in the stratum granulosum, where claudin-1 and -4 were concentrated. The occurrence of TJs was also confirmed by ultrathin section EM. In claudin-1-deficient mice, claudin-1 appeared to have simply been removed from these TJs, leaving occludin-positive (and also claudin-4-positive) TJs. Interestingly, in the wild-type epidermis these occludin-positive TJs efficiently prevented the diffusion of subcutaneously injected tracer (approximately 600 D) toward the skin surface, whereas in the claudin-1-deficient epidermis the tracer appeared to pass through these TJs. These findings provide the first evidence that continuous claudin-based TJs occur in the epidermis and that these TJs are crucial for the barrier function of the mammalian skin.

Radixin deficiency causes deafness associated with progressive degeneration of cochlear stereocilia.

Ezrin/radixin/moesin (ERM) proteins cross-link actin filaments to plasma membranes to integrate the function of cortical layers, especially microvilli. We found that in cochlear and vestibular sensory hair cells of adult wild-type mice, radixin was specifically enriched in stereocilia, specially developed giant microvilli, and that radixin-deficient (Rdx(-)(/)(-)) adult mice exhibited deafness but no obvious vestibular dysfunction. Before the age of hearing onset ( approximately 2 wk), in the cochlea and vestibule of Rdx(-)(/)(-) mice, stereocilia developed normally in which ezrin was concentrated. As these Rdx(-)(/)(-) mice grew, ezrin-based cochlear stereocilia progressively degenerated, causing deafness, whereas ezrin-based vestibular stereocilia were maintained normally in adult Rdx(-)(/)(-) mice. Thus, we concluded that radixin is indispensable for the hearing ability in mice through the maintenance of cochlear stereocilia, once developed. In Rdx(-)(/)(-) mice, ezrin appeared to compensate for radixin deficiency in terms of the development of cochlear stereocilia and the development/maintenance of vestibular stereocilia. These findings indicated the existence of complicate functional redundancy in situ among ERM proteins.

Normal establishment of epithelial tight junctions in mice and cultured cells lacking expression of ZO-3, a tight-junction MAGUK protein.

ZO-1, ZO-2, and ZO-3 are closely related MAGUK family proteins that localize at the cytoplasmic surface of tight junctions (TJs). ZO-1 and ZO-2 are expressed in both epithelia and endothelia, whereas ZO-3 is exclusively expressed in epithelia. In spite of intensive studies of these TJ MAGUKs, our knowledge of their functions in vivo, especially those of ZO-3, is still fragmentary. Here, we have generated mice, as well as F9 teratocarcinoma cell lines, that do not express ZO-3 by homologous recombination. Unexpectedly, ZO-3(-/-) mice were viable and fertile, and rigorous phenotypic analyses identified no significant abnormalities. Moreover, ZO-3-deficient F9 teratocarcinoma cells differentiated normally into visceral endoderm epithelium-like cells in the presence of retinoic acid. These cells had a normal epithelial appearance, and the molecular architecture of their TJs did not appear to be affected, except that TJ localization of ZO-2 was upregulated. Suppression of ZO-2 expression by RNA interference in ZO-3(-/-) cells, however, did not affect the architecture of TJs. Furthermore, the speed with which TJs formed after a Ca(2+) switch was indistinguishable between wild-type and ZO-3(-/-) cells. These findings indicate that ZO-3 is dispensable in vivo in terms of individual viability, epithelial differentiation, and the establishment of TJs, at least in the laboratory environment.

Publisher Correction: Claudin-3-deficient C57BL/6J mice display intact brain barriers.

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