Cadherin flexibility provides a key difference between desmosomes and adherens junctions

Desmosomes and adherens junctions are intercellular adhesive structures essential for the development and integrity of vertebrate tissue, including the epidermis and heart. Their cell adhesion molecules are cadherins: type 1 cadherins in adherens junctions and desmosomal cadherins in desmosomes. A fundamental difference is that desmosomes have a highly ordered structure in their extracellular region and exhibit calcium-independent hyperadhesion, whereas adherens junctions appear to lack such ordered arrays, and their adhesion is always calcium-dependent. We present here the structure of the entire ectodomain of desmosomal cadherin desmoglein 2 (Dsg2), using a combination of small-angle X-ray scattering, electron microscopy, and solution-based biophysical techniques. This structure reveals that the ectodomain of Dsg2 is flexible even in the calcium-bound state and, on average, is shorter than the type 1 cadherin crystal structures. The Dsg2 structure has an excellent fit with the electron tomography reconstructions of human desmosomes. This fit suggests an arrangement in which desmosomal cadherins form trans interactions but are too far apart to interact in cis, in agreement with previously reported observations. Cadherin flexibility may be key to explaining the plasticity of desmosomes that maintain tissue integrity in their hyperadhesive form, but can adopt a weaker, calcium-dependent adhesion during wound healing and early development.

The Rho GTPase RhoB regulates cadherin expression and epithelial cell-cell interaction

BACKGROUND

The Rho GTPase RhoB has been proposed to be a tumor suppressor in cancer and is downregulated in various tumors including prostate. RhoB has different effects on cell migration depending on the cell type and conditions, but the molecular basis for this variability is unclear. RhoB regulates trafficking of membrane receptors and integrins. We have previously shown that RhoB depletion alters focal adhesion dynamics and reduces surface levels of β1 integrin in PC3 prostate cancer cells, correlating with increased migration speed.

RESULTS

Here we show that RhoB depletion reduces cell-cell adhesion and downregulates E-cadherin levels as well as increasing internalized E-cadherin in DU145 prostate cancer cells. This is accompanied by increased migration speed. RhoB localizes to cell-cell junctions together with E-cadherin in DU145 cells. RhoB depletion also reduces N-cadherin levels in PC3 cells, which do not express E-cadherin.

CONCLUSIONS

These results indicate that RhoB alters migration of cells with cell-cell adhesions by regulating cadherin levels. We propose that the relative contribution of integrins and cadherins to cell migration underlies the variable involvement for RhoB in this process and that the downregulation of RhoB in some epithelial cancers could contribute to the weakening of epithelial cell-cell junction during tumor progression.

HIF2α signaling inhibits adherens junctional disruption in acute lung injury

Vascular endothelial barrier dysfunction underlies diseases such as acute respiratory distress syndrome (ARDS), characterized by edema and inflammatory cell infiltration. The transcription factor HIF2α is highly expressed in vascular endothelial cells (ECs) and may regulate endothelial barrier function. Here, we analyzed promoter sequences of genes encoding proteins that regulate adherens junction (AJ) integrity and determined that vascular endothelial protein tyrosine phosphatase (VE-PTP) is a HIF2α target. HIF2α-induced VE-PTP expression enhanced dephosphorylation of VE-cadherin, which reduced VE-cadherin endocytosis and thereby augmented AJ integrity and endothelial barrier function. Mice harboring an EC-specific deletion of Hif2a exhibited decreased VE-PTP expression and increased VE-cadherin phosphorylation, resulting in defective AJs. Mice lacking HIF2α in ECs had increased lung vascular permeability and water content, both of which were further exacerbated by endotoxin-mediated injury. Treatment of these mice with Fg4497, a prolyl hydroxylase domain 2 (PHD2) inhibitor, activated HIF2α-mediated transcription in a hypoxia-independent manner. HIF2α activation increased VE-PTP expression, decreased VE-cadherin phosphorylation, promoted AJ integrity, and prevented the loss of endothelial barrier function. These findings demonstrate that HIF2α enhances endothelial barrier integrity, in part through VE-PTP expression and the resultant VE-cadherin dephosphorylation-mediated assembly of AJs. Moreover, activation of HIF2α/VE-PTP signaling via PHD2 inhibition has the potential to prevent the formation of leaky vessels and edema in inflammatory diseases such as ARDS.

Modeling a genetic risk for schizophrenia in iPSCs and mice reveals neural stem cell deficits associated with adherens junctions and polarity

Defects in brain development are believed to contribute toward the onset of neuropsychiatric disorders, but identifying specific underlying mechanisms has proven difficult. Here, we took a multifaceted approach to investigate why 15q11.2 copy number variants are prominent risk factors for schizophrenia and autism. First, we show that human iPSC-derived neural progenitors carrying 15q11.2 microdeletion exhibit deficits in adherens junctions and apical polarity. This results from haploinsufficiency of CYFIP1, a gene within 15q11.2 that encodes a subunit of the WAVE complex, which regulates cytoskeletal dynamics. In developing mouse cortex, deficiency in CYFIP1 and WAVE signaling similarly affects radial glial cells, leading to their ectopic localization outside of the ventricular zone. Finally, targeted human genetic association analyses revealed an epistatic interaction between CYFIP1 and WAVE signaling mediator ACTR2 and risk for schizophrenia. Our findings provide insight into how CYFIP1 regulates neural stem cell function and may contribute to the susceptibility of neuropsychiatric disorders.

The β-actin mRNA zipcode regulates epithelial adherens junction assembly but not maintenance

Epithelial cell-cell contact stimulates actin cytoskeleton remodeling to down-regulate branched filament polymerization-driven lamellar protrusion and subsequently to assemble linear actin filaments required for E-cadherin anchoring during adherens junction complex assembly. In this manuscript, we demonstrate that de novo protein synthesis, the β-actin 3' UTR, and the β-actin mRNA zipcode are required for epithelial adherens junction complex assembly but not maintenance. Specifically, we demonstrate that perturbing cell-cell contact-localized β-actin monomer synthesis causes epithelial adherens junction assembly defects. Consequently, inhibiting β-actin mRNA zipcode/ZBP1 interactions with β-actin mRNA zipcode antisense oligonucleotides, to intentionally delocalize β-actin monomer synthesis, is sufficient to perturb adherens junction assembly following epithelial cell-cell contact. Additionally, we demonstrate active RhoA, the signal required to drive zipcode-mediated β-actin mRNA targeting, is localized at epithelial cell-cell contact sites in a β-actin mRNA zipcode-dependent manner. Moreover, chemically inhibiting Src kinase activity prevents the local stimulation of β-actin monomer synthesis at cell-cell contact sites while inhibiting epithelial adherens junction assembly. Together, these data demonstrate that epithelial cell-cell contact stimulates β-actin mRNA zipcode-mediated monomer synthesis to spatially regulate actin filament remodeling, thereby controlling adherens junction assembly to modulate cell and tissue adhesion.

Down-regulated desmocollin-2 promotes cell aggressiveness through redistributing adherens junctions and activating beta-catenin signalling in oesophageal squamous cell carcinoma

In contrast to the well-recognized loss of adherens junctions in cancer progression, the role of desmosomal components in cancer development has not been well explored. We previously demonstrated that desmocollin-2 (DSC2), a desmosomal cadherin protein, is reduced in oesophageal squamous cell carcinoma (ESCC), and is associated with enhanced tumour metastasis and poor prognosis. Here, we report that restoration of DSC2 in ESCC cells impeded cell migration and invasion both in vitro and in vivo, whereas siRNA-mediated suppression of DSC2 expression increased cell motility. In E-cadherin-expressing ESCC cells, DSC2 restoration strengthened E-cadherin-mediated adherens junctions and promoted the localization of β-catenin at these junctions, which indirectly inhibited β-catenin-dependent transcription. These effects of DSC2 were not present in EC109 cells that lacked E-cadherin expression. ESCC patients with tumours that had reduced E-cadherin and negative DSC2 had poorer clinical outcomes than patients with tumours that lacked either E-cadherin or DSC2, implying that the invasive potential of ESCC cells was restricted by both DSC2 and E-cadherin-dependent junctions. Further studies revealed that DSC2 was a downstream target of miR-25. Enhanced miR-25 promoted ESCC cell invasiveness, whereas restoration of DSC2 abolished these effects. Collectively, our work suggests that miR-25-mediated down-regulation of DSC2 promotes ESCC cell aggressiveness through redistributing adherens junctions and activating beta-catenin signalling.

A labile pool of IQGAP1 disassembles endothelial adherens junctions

Adhesion molecules are known to play an important role in endothelial activation and angiogenesis. Here we determined the functional role of IQGAP1 in the regulation of endothelial adherens junctions. VE-cadherin is found to be associated with actin filaments and thus stable, but IQGAP1 at intercellular junctions is not bound to actin filaments and thus labile. Expression of GFP labeled VE-α-catenin is shown to increase the electrical resistance across HUVEC monolayers and diminishes endogenous labile IQGAP1 at the intercellular junctions. Knockdown of endogenous IQGAP1 enhances intercellular adhesion in HUVECs by increasing the association of VE-cadherin with P120 and β-catenin. IQGAP1 knockdown also decreases the interaction of N-cadherin with P120 and β-catenin. Together, these results suggest that a labile pool of IQGAP1 at intercellular junctions disassembles adherens junctions and thus impairs endothelial cell-cell adhesion.

An autoinhibited structure of α-catenin and its implications for vinculin recruitment to adherens junctions

α-Catenin is an actin- and vinculin-binding protein that regulates cell-cell adhesion by interacting with cadherin adhesion receptors through β-catenin, but the mechanisms by which it anchors the cadherin-catenin complex to the actin cytoskeleton at adherens junctions remain unclear. Here we determined crystal structures of αE-catenin in the autoinhibited state and the actin-binding domain of αN-catenin. Together with the small-angle x-ray scattering analysis of full-length αN-catenin, we deduced an elongated multidomain assembly of monomeric α-catenin that structurally and functionally couples the vinculin- and actin-binding mechanisms. Cellular and biochemical studies of αE- and αN-catenins show that αE-catenin recruits vinculin to adherens junctions more effectively than αN-catenin, partly because of its higher affinity for actin filaments. We propose a molecular switch mechanism involving multistate conformational changes of α-catenin. This would be driven by actomyosin-generated tension to dynamically regulate the vinculin-assisted linkage between adherens junctions and the actin cytoskeleton.

Septins regulate the contractility of the actomyosin ring to enable adherens junction remodeling during cytokinesis of epithelial cells

How adhesive contacts with neighbors may affect epithelial cell cytokinesis is unknown. We report that in Drosophila, septins are specifically required for planar (but not orthogonal) cytokinesis. During planar division, cytokinetic furrowing initiates basally, resulting in a contractile ring displaced toward the adherens junction (AJ). The formation of new AJ between daughter cells requires the disengagement of E-Cadherin complexes between mitotic and neighboring cells at the cleavage furrow, followed by the assembly of E-Cadherin complexes on the daughter-daughter interface. The strength of adhesion with neighbors directly impacts both the kinetics of AJ disengagement and the length of the new AJ. Loss of septins causes a reduction in the contractility of the actomyosin ring and prevents local disengagement of AJ in the cleavage furrow. By modulating the strength of tension induced by neighbors, we uncover a mechanical function for septins to overcome the extrinsic tension induced by neighboring interphasic cells.

Presence of metastasis-associated protein 1 in Sertoli cells is required for proper contact between Sertoli cells and adjacent germ cells

OBJECTIVE

To investigate whether the normal expression of metastasis-associated protein 1 (MTA1) in Sertoli cells (SCs) is associated with adjacent germ cells (GCs) and to provide the functional relevance of MTA1 in this somatic cell.

METHODS

The expression pattern of MTA1 in the SCs of impaired human spermatogenesis was determined using immunohistochemistry. The effect of the depletion of GCs on the expression of MTA1 in isolated SCs was evaluated using reverse transcriptase polymerase chain reaction in murine testes treated with busulphan. Finally, using multiple assays, the functional investigation of MTA1 by its specific knockdown was performed in SC-GC co-cultures.

RESULTS

SCs were negatively immunolabeled in the tubules with impaired spermatogenesis. Depletion of murine GCs by treatment with busulphan resulted in a dramatic decrease of the MTA1 transcripts level in the isolated SCs on the 15th day of treatment and thereafter had totally abolished MTA1 expression by the 30th day of treatment, respectively. The addition of isolated round spermatids into SC culture could partially elevate MTA1 expression in the latter. Furthermore, MTA1 is crucial to maintain the GC nursery function and normal anchoring junction formation in SCs because ablation of MTA1 by siRNA induced extensive defects of genes related to SC homeostasis.

CONCLUSION

We propose a novel role for SC-expressing MTA1, which is determined by the presence of surrounding GCs, in mediating the crosstalk between SCs and GCs by influencing a broad spectrum of gene changes.

ALCAM regulates motility, invasiveness, and adherens junction formation in uveal melanoma cells

ALCAM, a member of the immunoglobulin superfamily, has been implicated in numerous developmental events and has been repeatedly identified as a marker for cancer metastasis. Previous studies addressing ALCAM's role in cancer have, however, yielded conflicting results. Depending on the tumor cell type, ALCAM expression has been reported to be both positively and negatively correlated with cancer progression and metastasis in the literature. To better understand how ALCAM might regulate cancer cell behavior, we utilized a panel of defined uveal melanoma cell lines with high or low ALCAM levels, and directly tested the effects of manipulating these levels on cell motility, invasiveness, and adhesion using multiple assays. ALCAM expression was stably silenced by shRNA knockdown in a high-ALCAM cell line (MUM-2B); the resulting cells displayed reduced motility in gap-closure assays and a reduction in invasiveness as measured by a transwell migration assay. Immunostaining revealed that the silenced cells were defective in the formation of adherens junctions, at which ALCAM colocalizes with N-cadherin and ß-catenin in native cells. Additionally, we stably overexpressed ALCAM in a low-ALCAM cell line (MUM-2C); intriguingly, these cells did not exhibit any increase in motility or invasiveness, indicating that ALCAM is necessary but not sufficient to promote metastasis-associated cell behaviors. In these ALCAM-overexpressing cells, however, recruitment of ß-catenin and N-cadherin to adherens junctions was enhanced. These data confirm a previously suggested role for ALCAM in the regulation of adherens junctions, and also suggest a mechanism by which ALCAM might differentially enhance or decrease invasiveness, depending on the type of cadherin adhesion complexes present in tissues surrounding the primary tumor, and on the cadherin status of the tumor cells themselves.

p120-catenin modulates airway epithelial cell migration induced by cigarette smoke

Cigarette smoking has been linked to almost all major types of cancer. Emerging evidence suggests that smoking initiates transformed cell growth and migration by disrupting cell-cell interactions in the polarized mucosal epithelium. Together with other adherens junction proteins, p120-catenin (p120ctn) maintains cell-cell adhesion through its direct interaction with E-cadherin (E-cad). Mislocalization and/or loss of p120ctn have been reported in all lung cancer subtypes and are related to poor prognosis. Here, we showed that p120ctn modulates smoke-induced cell migration via the EGFR/Src-P pathway. Chemical blockade of EGFR/Src signaling inhibited smoke-induced activation of cofilin (an actin severing protein) and promoted cell migration in the presence of p120ctn but had little effect on blocking migration in the absence of p120ctn. These data suggested that smoke-induced cell migration was mediated via an EGFR/Src-dependent signaling pathway in cells that expressed p120ctn, but upon loss of p120ctn, migration continued to occur via an alternative, EGFR/Src-independent pathway. Thus, gradual loss of membrane p120ctn with lung cancer progression may contribute to reduced effectiveness of conventional chemotherapies, such as those directed against EGFR.

A contractile actomyosin network linked to adherens junctions by Canoe/afadin helps drive convergent extension

Integrating individual cell movements to create tissue-level shape change is essential to building an animal. We explored mechanisms of adherens junction (AJ):cytoskeleton linkage and roles of the linkage regulator Canoe/afadin during Drosophila germband extension (GBE), a convergent-extension process elongating the body axis. We found surprising parallels between GBE and a quite different morphogenetic movement, mesoderm apical constriction. Germband cells have an apical actomyosin network undergoing cyclical contractions. These coincide with a novel cell shape change--cell extension along the anterior-posterior (AP) axis. In Canoe's absence, GBE is disrupted. The apical actomyosin network detaches from AJs at AP cell borders, reducing coordination of actomyosin contractility and cell shape change. Normal GBE requires planar polarization of AJs and the cytoskeleton. Canoe loss subtly enhances AJ planar polarity and dramatically increases planar polarity of the apical polarity proteins Bazooka/Par3 and atypical protein kinase C. Changes in Bazooka localization parallel retraction of the actomyosin network. Globally reducing AJ function does not mimic Canoe loss, but many effects are replicated by global actin disruption. Strong dose-sensitive genetic interactions between canoe and bazooka are consistent with them affecting a common process. We propose a model in which an actomyosin network linked at AP AJs by Canoe and coupled to apical polarity proteins regulates convergent extension.

A role for the retinoblastoma protein as a regulator of mouse osteoblast cell adhesion: implications for osteogenesis and osteosarcoma formation

The retinoblastoma protein (pRb) is a cell cycle regulator inactivated in most human cancers. Loss of pRb function results from mutations in the gene coding for pRb or for any of its upstream regulators. Although pRb is predominantly known as a cell cycle repressor, our data point to additional pRb functions in cell adhesion. Our data show that pRb regulates the expression of a wide repertoire of cell adhesion genes and regulates the assembly of the adherens junctions required for cell adhesion. We conducted our studies in osteoblasts, which depend on both pRb and on cell-to-cell contacts for their differentiation and function. We generated knockout mice in which the RB gene was excised specifically in osteoblasts using the cre-lox P system and found that osteoblasts from pRb knockout mice did not assemble adherens junction at their membranes. pRb depletion in wild type osteoblasts using RNAi also disrupted adherens junctions. Microarrays comparing pRb-expressing and pRb-deficient osteoblasts showed that pRb controls the expression of a number of cell adhesion genes, including cadherins. Furthermore, pRb knockout mice showed bone abnormalities consistent with osteoblast adhesion defects. We also found that pRb controls the function of merlin, a well-known regulator of adherens junction assembly, by repressing Rac1 and its effector Pak1. Using qRT-PCR, immunoblots, co-immunoprecipitation assays, and immunofluorescent labeling, we observed that pRb loss resulted in Rac1 and Pak1 overexpression concomitant with merlin inactivation by Pak1, merlin detachment from the membrane, and adherens junction loss. Our data support a pRb function in cell adhesion while elucidating the mechanism for this function. Our work suggests that in some tumor types pRb inactivation results in both a loss of cell cycle control that promotes initial tumor growth as well as in a loss of cell-to-cell contacts, which contributes to later stages of metastasis.

Implication of metastasis suppressor NM23-H1 in maintaining adherens junctions and limiting the invasive potential of human cancer cells

Loss of NM23-H1 expression correlates with the degree of metastasis and with unfavorable clinical prognosis in several types of human carcinoma. However, the mechanistic basis for the metastasis suppressor function of NM23-H1 is obscure. We silenced NM23-H1 expression in human hepatoma and colon carcinoma cells and methodologically investigated effects on cell-cell adhesion, migration, invasion, and signaling linked to cancer progression. NM23-H1 silencing disrupted cell-cell adhesion mediated by E-cadherin, resulting in β-catenin nuclear translocation and T-cell factor/lymphoid-enhancing factor-1 transactivation. Further, NM23-H1 silencing promoted cellular scattering, motility, and extracellular matrix invasion by promoting invadopodia formation and upregulating several matrix metalloproteinases (MMP), including membrane type 1 MMP. In contrast, silencing the related NM23-H2 gene was ineffective at promoting invasion. NM23-H1 silencing activated proinvasive signaling pathways involving Rac1, mitogen-activated protein kinases, phosphatidylinositol 3-kinase (PI3K)/Akt, and src kinase. Conversely, NM23-H1 was dispensable for cancer cell proliferation in vitro and liver regeneration in NM23-M1 null mice, instead inducing cellular resistance to chemotherapeutic drugs in vitro. Analysis of NM23-H1 expression in clinical specimens revealed high expression in premalignant lesions (liver cirrhosis and colon adenoma) and the central body of primary liver or colon tumors, but downregulation at the invasive front of tumors. Our findings reveal that NM23-H1 is critical for control of cell-cell adhesion and cell migration at early stages of the invasive program in epithelial cancers, orchestrating a barrier against conversion of in situ carcinoma into invasive malignancy.

Cigarette smoke induces epidermal growth factor receptor-dependent redistribution of apical MUC1 and junctional beta-catenin in polarized human airway epithelial cells

Cigarette smoke (CS) accounts for nearly 90% of lung cancer deaths worldwide; however, an incomplete understanding of how CS initiates preneoplastic changes in the normal airway hinders early diagnosis. Short-term exposure to CS causes aberrant activation of epidermal growth factor receptor (EGFR) and canonical Wnt/beta-catenin signaling pathways in human bronchial epithelial (HBE) cells. We hypothesize that this response is elicited through the disruption of spatially segregated cell membrane proteins in the polarized airway epithelium. Using an in vitro model of highly differentiated HBE cells, we observed membrane characteristics consistent with the native airway, including the presence of a membrane mucin, MUC1, at the apical cell pole, beta-catenin at the apical-lateral membrane, and EGFR at the basolateral membrane. Following exposure to smoke, intercellular spaces enlarge and cilia disappear. This histopathology is accompanied by molecular events that include perinuclear trafficking of basolateral EGFR, EGFR phosphorylation, pEGFR-mediated phosphorylation of MUC1's cytoplasmic tail (CT), loss of E-cadherin/beta-catenin complexes at the adherens junctions (AJs), intracellular formation and nuclear shuffling of beta-catenin/MUC1-CT complexes, and, ultimately, up-regulation and nuclear localization of Wnt nuclear effector, Lef-1. In the presence of EGFR inhibitor, AG1478, CS-induced histopathology and molecular events were inhibited. These data point to EGFR as a portal through which CS mediates its damaging effects on AJ-mediated cell polarity and activation of canonical Wnt/beta-catenin signaling.

p120ctn and P-cadherin but not E-cadherin regulate cell motility and invasion of DU145 prostate cancer cells

Background

Adherens junctions consist of transmembrane cadherins, which interact intracellularly with p120ctn, ß-catenin and α-catenin. p120ctn is known to regulate cell-cell adhesion by increasing cadherin stability, but the effects of other adherens junction components on cell-cell adhesion have not been compared with that of p120ctn.

Methodology/Principal Findings

We show that depletion of p120ctn by small interfering RNA (siRNA) in DU145 prostate cancer and MCF10A breast epithelial cells reduces the expression levels of the adherens junction proteins, E-cadherin, P-cadherin, ß-catenin and α-catenin, and induces loss of cell-cell adhesion. p120ctn-depleted cells also have increased migration speed and invasion, which correlates with increased Rap1 but not Rac1 or RhoA activity. Downregulation of P-cadherin, β-catenin and α-catenin but not E-cadherin induces a loss of cell-cell adhesion, increased migration and enhanced invasion similar to p120ctn depletion. However, only p120ctn depletion leads to a decrease in the levels of other adherens junction proteins.

Conclusions/Significance

Our data indicate that P-cadherin but not E-cadherin is important for maintaining adherens junctions in DU145 and MCF10A cells, and that depletion of any of the cadherin-associated proteins, p120ctn, ß-catenin or α-catenin, is sufficient to disrupt adherens junctions in DU145 cells and increase migration and cancer cell invasion.

A modifier screen for Bazooka/PAR-3 interacting genes in the Drosophila embryo epithelium

Background

The development and homeostasis of multicellular organisms depends on sheets of epithelial cells. Bazooka (Baz; PAR-3) localizes to the apical circumference of epithelial cells and is a key hub in the protein interaction network regulating epithelial structure. We sought to identify additional proteins that function with Baz to regulate epithelial structure in the Drosophila embryo.

Methodology/Principal Findings

The baz zygotic mutant cuticle phenotype could be dominantly enhanced by loss of known interaction partners. To identify additional enhancers, we screened molecularly defined chromosome 2 and 3 deficiencies. 37 deficiencies acted as strong dominant enhancers. Using deficiency mapping, bioinformatics, and available single gene mutations, we identified 17 interacting genes encoding known and predicted polarity, cytoskeletal, transmembrane, trafficking and signaling proteins. For each gene, their loss of function enhanced adherens junction defects in zygotic baz mutants during early embryogenesis. To further evaluate involvement in epithelial polarity, we generated GFP fusion proteins for 15 of the genes which had not been found to localize to the apical domain previously. We found that GFP fusion proteins for Drosophila ASAP, Arf79F, CG11210, Septin 5 and Sds22 could be recruited to the apical circumference of epithelial cells. Nine of the other proteins showed various intracellular distributions, and one was not detected.

Conclusions/Significance

Our enhancer screen identified 17 genes that function with Baz to regulate epithelial structure in the Drosophila embryo. Our secondary localization screen indicated that some of the proteins may affect epithelial cell polarity by acting at the apical cell cortex while others may act through intracellular processes. For 13 of the 17 genes, this is the first report of a link to baz or the regulation of epithelial structure.

Mutations in N-cadherin and a Stardust homolog, Nagie oko, affect cell-cycle exit in zebrafish retina

It has been reported that the loss of apicobasal cell polarity and the disruption of adherens junctions induce hyperplasia in the mouse developing brain. However, it is not fully understood whether hyperplasia is caused by an enhanced cell proliferation, an inhibited neurogenesis, or both. In this study, we found that the ratio of the number of proliferating progenitor cells to the total number of retinal cells increases in the neurogenic stages in zebrafish n-cadherin (ncad) and nagie oko (nok) mutants, in which the apicobasal cell polarity and adherens junctions in the retinal epithelium are disrupted. The cell-cycle progression was not altered in the ncad and nok mutants. Rather, the ratio of the number of cells undergoing neurogenic cell division to the total number of cells undergoing mitosis decreased in the ncad and nok mutant retinas, suggesting that the switching from proliferative cell division to neurogenic cell division was compromised in these mutant retinas. These findings suggest that the inhibition of neurogenesis is a primary defect that causes hyperplasia in the ncad and nok mutant retinas. The Hedgehog-protein kinase A signaling pathway and the Notch signaling pathway regulate retinal neurogenesis in zebrafish. We found that both signaling pathways are involved in the generation of neurogenic defects in the ncad and nok mutant retinas. Taken together, these findings suggest that apicobasal cell polarity and epithelial integrity are essential for retinal neurogenesis in zebrafish.

The intercalated disk protein, mXinalpha, is capable of interacting with beta-catenin and bundling actin filaments [corrected]

Targeted deletion of mXinalpha results in cardiac hypertrophy and cardiomyopathy with conduction defects (Gustafson-Wagner, E., Sinn, H. W., Chen, Y.-L., Wang, D.-Z., Reiter, R. S., Lin, J. L.-C., Yang, B., Williamson, R. A., Chen, J. N., Lin, C.-I., and Lin, J. J.-C. (2007) Am. J. Physiol. 293, H2680-H2692). To understand the underlying mechanisms leading to such cardiac defects, the functional domains of mXinalpha and its interacting proteins were investigated. Interaction studies using co-immunoprecipitation, pull-down, and yeast two-hybrid assays revealed that mXinalpha directly interacts with beta-catenin. The beta-catenin-binding site on mXinalpha was mapped to amino acids 535-636, which overlaps with the known actin-binding domains composed of the Xin repeats. The overlapping nature of these domains provides insight into the molecular mechanism for mXinalpha localization and function. Purified recombinant glutathione S-transferase- or His-tagged mXinalpha proteins are capable of binding and bundling actin filaments, as determined by co-sedimentation and electron microscopic studies. The binding to actin was saturated at an approximate stoichiometry of nine actin monomers to one mXinalpha. A stronger interaction was observed between mXinalpha C-terminal deletion and actin as compared with the interaction between full-length mXinalpha and actin. Furthermore, force expression of green fluorescent protein fused to an mXinalpha C-terminal deletion in cultured cells showed greater stress fiber localization compared with force-expressed GFP-mXinalpha. These results suggest a model whereby the C terminus of mXinalpha may prevent the full-length molecule from binding to actin, until the beta-catenin-binding domain is occupied by beta-catenin. The binding of mXinalpha to beta-catenin at the adherens junction would then facilitate actin binding. In support of this model, we found that the actin binding and bundling activity of mXinalpha was enhanced in the presence of beta-catenin.

Cellular behavior in the developing Drosophila pupal retina

Correct patterning of cells within an epithelium is key to establishing their normal function. However, the precise mechanisms by which individual cells arrive at their final developmental niche remains poorly understood. We developed an optimized system for imaging the developing Drosophila retina, an ideal tissue for the study of cell positioning. Using this technique, we characterized the cellular dynamics of developing wild-type pupal retinas. We also analyzed two mutants affecting eye patterning and demonstrate that cells mutant for Notch or Roughest signaling were aberrantly dynamic in their cell movements. Finally, we establish a role for the adherens junction regulator P120-Catenin in retinal patterning through its regulation of normal adherens junction integrity. Our results indicate a requirement for P120-Catenin in the developing retina, the first reported developmental function of this protein in the epithelia of lower metazoa. Based upon our live visualization of the P120-Catenin mutant as well as genetic data, we conclude that P120-Catenin is acting to stabilize E-cadherin and adherens junction integrity during eye development.

Mutations in the Drosophila alphaPS2 integrin subunit uncover new features of adhesion site assembly

The Drosophila alphaPS2betaPS integrin is required for diverse development events, including muscle attachment. We characterized six unusual mutations in the alphaPS2 gene that cause a subset of the null phenotype. One mutation changes a residue in alphaPS2 that is equivalent to the residue in alphaV that contacts the arginine of RGD. This change severely reduced alphaPS2betaPS affinity for soluble ligand, abolished the ability of the integrin to recruit laminin to muscle attachment sites in the embryo and caused detachment of integrins and talin from the ECM. Three mutations that alter different parts of the alphaPS2 beta-propeller, plus a fourth that eliminated a late phase of alphaPS2 expression, all led to a strong decrease in alphaPS2betaPS at muscle ends, but, surprisingly, normal levels of talin were recruited. Thus, although talin recruitment requires alphaPS2betaPS, talin levels are not simply specified by the amount of integrin at the adhesive junction. These mutations caused detachment of talin and actin from integrins, suggesting that the integrin-talin link is weaker than the ECM-integrin link.

Gene Expression Profiling and Its Relevance to the Blood-Epididymal Barrier in the Human Epididymis1

The luminal environment along the epididymal duct is important for spermatozoal maturation. This environment is unique and created by the blood-epididymal barrier, which is formed by tight and adhering junctions. For the human epididymis, little information exists on the proteins that comprise these junctions. Our objectives were to assess the gene expression profiles in the different segments of the human epididymis and to identify the proteins that make up the blood-epididymal barrier. Using microarrays, we identified 2980 genes that were differentially expressed by at least 2-fold between the various segments. Of the many genes involved in diverse functions, were those that encoded adhesion proteins (cadherins and catenins) and tight junctional proteins (claudins [CLDN] and others). PCR analyses confirmed the microarray data. Immunolocalization of CLDNs 1, 3, 4, 8, and 10 revealed that the localization of CLDNs differed along the epididymis. In all three segments, CLDNs 1, 3, and 4 were localized to tight junctions, along the lateral margins of adjacent principal cells, and at the interface between basal and principal cells. CLDN8 was localized to tight junctions in all three segments, in addition to being localized in the caput along the lateral margins of principal cells, and in the corpus, at the interface between principal and basal cells. CLDN10, tight junction protein 1, and occludin were localized exclusively to tight junctions in all three epididymal segments. These data indicate that the epididymis displays a complex pattern of gene expression, which includes genes that are implicated in the formation of the blood-epididymal barrier, which suggests complex regulation of this barrier.

Conditional knock-out reveals that zygotic vezatin-null mouse embryos die at implantation

Vezatin, a protein associated to adherens junctions in epithelial cells, is already expressed in mouse oocytes and during pre-implantation development. Using a floxed strategy to generate a vezatin-null allele, we show that the lack of zygotic vezatin is embryonic lethal, indicating that vezatin is an essential gene. Homozygous null embryos are able to elicit a decidual response but as early as day 6.0 post-coitum mutant implantation sites are devoid of embryonic structures. Mutant blastocysts are morphologically normal, but only half of them are able to hatch upon in vitro culture and the blastocyst outgrowths formed after 3.5 days in culture exhibit severe abnormalities, in particular disrupted intercellular adhesion and clear signs of cellular degeneration. Notably, the junctional proteins E-cadherin and beta-catenin are delocalized and not observed at the plasma membrane anymore. These in vitro observations reinforce the idea that homozygous vezatin-null mutants die at the time of implantation because of a defect in intercellular adhesion. Together these results indicate that the absence of zygotic vezatin is deleterious for the implantation process, most likely because cadherin-dependent intercellular adhesion is impaired in late blastocysts when the maternal vezatin is lost.