Phosphorylation of Zona Occludens-2 by Protein Kinase Cε Regulates Its Nuclear Exportation

Here, we have analyzed the subcellular destiny of newly synthesized tight junction protein zona occludens (ZO)-2. After transfection in sparse cells, 74% of cells exhibit ZO-2 at the nucleus, and after 18 h the value decreases to 17%. The mutation S369A located within the nuclear exportation signal 1 of ZO-2 impairs the nuclear export of the protein. Because Ser369 represents a putative protein kinase C (PKC) phosphorylation site, we tested the effect of PKC inhibition and stimulation on the nuclear export of ZO-2. Our results strongly suggest that the departure of ZO-2 from the nucleus is regulated by phosphorylation at Ser369 by novel PKCε. To test the route taken by ZO-2 from synthesis to the plasma membrane, we devised a novel nuclear microinjection assay in which the nucleus served as a reservoir for anti-ZO-2 antibody. Through this assay, we demonstrate that a significant amount of newly synthesized ZO-2 goes into the nucleus and is later relocated to the plasma membrane. These results constitute novel information for understanding the mechanisms that regulate the intracellular fate of ZO-2.     

Interaction between O-GlcNAc Modification and Tyrosine Phosphorylation of Prohibitin: Implication for a Novel Binary Switch

Prohibitin (PHB or PHB1) is an evolutionarily conserved, multifunctional protein which is present in various cellular compartments including the plasma membrane. However, mechanisms involved in various functions of PHB are not fully explored yet. Here we report for the first time that PHB interacts with O-linked β-N-acetylglucosamine transferase (O-GlcNAc transferase, OGT) and is O-GlcNAc modified; and also undergoes tyrosine phosphorylation in response to insulin. Tyrosine 114 (Tyr114) and tyrosine 259 (Tyr259) in PHB are in the close proximity of potential O-GlcNAc sites serine 121 (Ser121) and threonine 258 (Thr258) respectively. Substitution of Tyr114 and Tyr259 residues in PHB with phenylalanine by site-directed mutagenesis results in reduced tyrosine phosphorylation as well as reduced O-GlcNAc modification of PHB. Surprisingly, this also resulted in enhanced tyrosine phosphorylation and activity of OGT. This is attributed to the presence of similar tyrosine motifs in PHB and OGT. Substitution of Ser121 and Thr258 with alanine and isoleucine respectively resulted in attenuation of O-GlcNAc modification and increased tyrosine phosphorylation of PHB suggesting an association between these two dynamic modifications. Sequence analysis of O-GlcNAc modified proteins having known O-GlcNAc modification site(s) or known tyrosine phosphorylation site(s) revealed a strong potential association between these two posttranslational modifications in various proteins. We speculate that O-GlcNAc modification and tyrosine phosphorylation of PHB play an important role in tyrosine kinase signaling pathways including insulin, growth factors and immune receptors signaling. In addition, we propose that O-GlcNAc modification and tyrosine phosphorylation is a novel previously unidentified binary switch which may provide new mechanistic insights into cell signaling pathways and is open for direct experimental examination.     

Vascular Endothelial Tight Junctions and Barrier Function Are Disrupted by 15(S)-Hydroxyeicosatetraenoic Acid Partly via Protein Kinase C?-mediated Zona Occludens-1 Phosphorylation at Threonine 770/772

Disruption of tight junctions (TJs) perturbs endothelial barrier function and promotes inflammation. Previously, we have shown that 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), the major 15-lipoxygenase 1 (15-LO1) metabolite of arachidonic acid, by stimulating zona occludens (ZO)-2 tyrosine phosphorylation and its dissociation from claudins 1/5, induces endothelial TJ disruption and its barrier dysfunction. Here, we have studied the role of serine/threonine phosphorylation of TJ proteins in 15(S)-HETE-induced endothelial TJ disruption and its barrier dysfunction. We found that 15(S)-HETE enhances ZO-1 phosphorylation at Thr-770/772 residues via PKCϵ-mediated MEK1-ERK1/2 activation, causing ZO-1 dissociation from occludin, disrupting endothelial TJs and its barrier function, and promoting monocyte transmigration; these effects were reversed by T770A/T772A mutations. In the arteries of WT mice ex vivo, 15(S)-HETE also induced ZO-1 phosphorylation and endothelial TJ disruption in a PKCϵ and MEK1-ERK1/2-dependent manner. In line with these observations, in WT mice high fat diet feeding induced 12/15-lipoxygenase (12/15-LO) expression in the endothelium and caused disruption of its TJs and barrier function. However, in 12/15-LO^−/− mice, high fat diet feeding did not cause disruption of endothelial TJs and barrier function. These observations suggest that the 12/15-LO-12/15(S)-HETE axis, in addition to tyrosine phosphorylation of ZO-2, also stimulates threonine phosphorylation of ZO-1 in the mediation of endothelial TJ disruption and its barrier dysfunction.     

Phosphorylation of Tyr-398 and Tyr-402 in Occludin Prevents Its Interaction with ZO-1 and Destabilizes Its Assembly at the Tight Junctions

Occludin is phosphorylated on tyrosine residues during the oxidative stress-induced disruption of tight junction, and in vitro phosphorylation of occludin by c-Src attenuates its binding to ZO-1. In the present study mass spectrometric analyses of C-terminal domain of occludin identified Tyr-379 and Tyr-383 in chicken occludin as the phosphorylation sites, which are located in a highly conserved sequence of occludin, YETDYTT; Tyr-398 and Tyr-402 are the corresponding residues in human occludin. Deletion of YETDYTT motif abolished the c-Src-mediated phosphorylation of occludin and the regulation of ZO-1 binding. Y398A and Y402A mutations in human occludin also abolished the c-Src-mediated phosphorylation and regulation of ZO-1 binding. Y398D/Y402D mutation resulted in a dramatic reduction in ZO-1 binding even in the absence of c-Src. Similar to wild type occludin, its Y398A/Y402A mutant was localized at the plasma membrane and cell-cell contact sites in Rat-1 cells. However, Y398D/Y402D mutants of occludin failed to localize at the cell-cell contacts. Calcium-induced reassembly of Y398D/Y402D mutant occludin in Madin-Darby canine kidney cells was significantly delayed compared with that of wild type occludin or its T398A/T402A mutant. Furthermore, expression of Y398D/Y402D mutant of occludin sensitized MDCK cells for hydrogen peroxide-induced barrier disruption. This study reveals a unique motif in the occludin sequence that is involved in the regulation of ZO-1 binding by reversible phosphorylation of specific Tyr residues.Epithelial tight junctions (TJs)² form a selective barrier to the diffusion of toxins, allergens, and pathogens from the external environment into the tissues in the gastrointestinal tract, lung, liver, and kidney (1). Disruption of TJs is associated with the gastrointestinal diseases such as inflammatory bowel disease, celiac disease, infectious enterocolitis, and colon cancer (2–4) as well as in diseases of lung and kidney (5, 6). Numerous inflammatory mediators such as tumor necrosis factor α, interferon γ, and oxidative stress (7–12) are known to disrupt the epithelial TJs and the barrier function. Several studies have indicated that hydrogen peroxide disrupts the TJs in intestinal epithelium by a tyrosine kinase-dependent mechanism (11, 12).Four types of integral proteins, occludin, claudins, junctional adhesion molecules, and tricellulin are associated with TJs. Occludin, claudins, and tricellulin are tetraspan proteins, and their extracellular domains interact with homotypic domains of the adjacent cells (1, 2, 13). The intracellular domains of these proteins interact with a variety of soluble proteins such as ZO-1, ZO-2, ZO-3, 7H6, cingulin, and symplekin (14–23); this protein complex interacts with the perijunctional actomyosin ring. The interactions among TJ proteins are essential for the assembly and the maintenance of TJs. Therefore, regulation of the interactions among TJ proteins may regulate the TJ integrity. A significant body of evidence indicates that numerous signaling molecules are associated with the TJs. Protein kinases and protein phosphatases such as protein kinase Cζ (PKCζ), PKCι/λ (24), c-Src (25), c-Yes (26, 27), mitogen-activated protein kinase (28), PP2A, and PP1 (29) interact with TJs, indicating that TJs are dynamically regulated by intracellular signal transduction involving protein phosphorylation. Additionally, other signaling molecules such as calcium (30), phosphatidylinositol 3-kinase (31), Rho (32), and Rac (33) are involved in the regulation of TJs.Occludin, a ∼65-kDa protein, has been well characterized to be assembled into the TJs. Although occludin knock-out mice showed the formation of intact TJs in different epithelia (34), numerous studies have emphasized that it plays an important role in the regulation of TJ integrity. Occludin spans the membrane four times to form two extracellular loops and one intracellular loop, and the N-terminal and C-terminal domains hang into the intracellular compartment (35–37). In epithelium with intact TJs, occludin is highly phosphorylated on Ser and Thr residues (38), whereas Tyr phosphorylation is undetectable. However, the disruption of TJs in Caco-2 cell monolayers by oxidative stress and acetaldehyde leads to Tyr phosphorylation of occludin; the tyrosine kinase inhibitors attenuate the disruption of TJs (39, 40). Furthermore, a previous in vitro study demonstrated that Tyr phosphorylation of the C-terminal domain of occludin leads to the loss of its interaction with ZO-1 and ZO-3 (25).In the present study we identified the Tyr residues in occludin that are phosphorylated by c-Src and determined their role in regulated interaction between occludin and ZO-1 and its assembly into the TJs. Results show that 1) Tyr-379 and Tyr-383 in chicken occludin and Tyr-398 and Tyr-402 in human occludin are the exclusive sites of phosphorylation by c-Src, and these Tyr residues are located in a highly conserved sequence of occludin, YET-DYTT, 2) deletion of YEDTYTT or point mutation of Tyr-398 and Tyr-402 in human occludin attenuates the phosphorylation-dependent regulation of ZO-1 binding, 3) Y398D/Y402D mutation of human occludin leads to loss of ZO-1 binding and prevents its translocation to the plasma membrane and cell-cell contact sites in Rat-1 cells, 4) Y398D/Y402D mutation of occludin delays its assembly into the intercellular junctions during the calcium-induced assembly of TJs, and 5) expression of Y398D/Y402D mutant occludin sensitizes cell monolayers for hydrogen peroxide-induced disruption of barrier function.     

Comparison of the phosphorylation events in membranes from proliferating vs. quiescent endothelial cells

In an attempt to elucidate the intracellular events regulating the proliferation of endothelial cells (EC), we have compared the phosphorylation events in membranes prepared from proliferating (sparse) and quiescent (confluent) EC. Triton-solubilized membranes from sparse and confluent EC were incubated at pH 6.5 in the presence of divalent cations and [32P]ATP. Membrane proteins were then separated by SDS-PAGE and the radiolabeled phosphoproteins visualized by autoradiography. The overall kinase activity per milligram protein was 1.7 +/- 0.2-fold greater in membranes prepared from proliferating than from quiescent cells. The extent of phosphorylation was dramatically elevated in sparse over confluent samples for four phosphoproteins having the following approximate molecular masses: 180, 100, 97, and 55 kDa. The 180 and 100 kDa phosphoproteins exhibited 3.6- and 7.4-fold higher labeling, respectively, in sparse than in confluent membranes and both were phosphorylated on serine residues exclusively. The 97 kDa phosphoprotein was 11.6-fold higher in sparse membranes and contained both phosphoserine (p-ser) and phosphotheronine (p-thr), the latter comprising 61% of the radioactivity. The 55 kDA phosphoprotein contained 62% p-ser, 16% p-thr, and 22% phosphotyrosine (p-tyr) and was 2.3-fold higher in sparse membranes. Of these four phosphoproteins, only the 55 kDa protein was phosphorylated in confluent samples to an appreciable degree. Whereas the p-ser and p-thr content of the 55 kDa band increased moderately in sparse vs. confluent sample (1.8-fold increase), the tyrosine residues of this protein in sparse membranes were radiolabeled to a much greater extent relative to confluent membranes (5.4-fold increase). Analysis of the cofactor requirements of the FC membrane kinase(s) revealed that Mn2+ is the optimum cofactor and that Mg2+ can replace Mn2+ only for the kinase acting on the 100 kDa band. This suggests the presence of multiple EC membrane kinases. In the presence of both cofactors, the phosphorylation pattern is similar to the pattern obtained with Mn2+ alone. The kinase activity acting on all four phosphoproteins was independent of Ca2+, cAMP, cGMP, and phorbol 12-myristate 13-acetate. The mechanism responsible for the difference in kinase activity of proliferating vs. quiescent cells was not due to an inhibitor or enhanced phosphatase activity in confluent cells; the phosphorylation patterns obtained with sparse solubilized membranes and a mixture of sparse and confluent solubilized membranes were similar.(ABSTRACT TRUNCATED AT 400 WORDS)     

Targeting O-Glycosyltransferase (OGT) to Promote Healing of Diabetic Skin Wounds

Non-healing wounds are a significant source of morbidity. This is particularly true for diabetic patients, who tend to develop chronic skin wounds. O-GlcNAc modification of serine and threonine residues is a common regulatory post-translational modification analogous to protein phosphorylation; increased intracellular protein O-GlcNAc modification has been observed in diabetic and hyperglycemic states. Two intracellular enzymes, UDP-N-acetylglucosamine-polypeptide β-N-acetylglucosaminyl transferase (OGT) and O-GlcNAc-selective N-acetyl-β-d-glucosaminidase (OGA), mediate addition and removal, respectively, of N-acetylglucosamine (GlcNAc) from intracellular protein substrates. Alterations in O-GlcNAc modification of intracellular proteins is linked to diabetes, and the increased levels of protein O-GlcNAc modification observed in diabetic tissues may in part explain some of the observed underlying pathophysiology that contributes to delayed wound healing. We have previously shown that increasing protein O-GlcNAc modification by overexpression of OGT in murine keratinocytes results in elevated protein O-GlcNAc modification and a hyperadhesive phenotype. This study was undertaken to explore the hypothesis that increased O-GlcNAc modification of cellular proteins in diabetic skin could contribute to the delayed wound healing observed in patients with diabetic skin ulcers. In the present study, we show that human keratinocytes cultured under hyperglycemic conditions display increased levels of O-GlcNAc modification as well as a delay in the rate of wound closure in vitro. We further show that specific knockdown of OGT by RNA interference (RNAi) reverses this effect, thereby opening up the opportunity for OGT-targeted therapies to promote wound healing in diabetic patients.     

Glucosamine Treatment-mediated O-GlcNAc Modification of Paxillin Depends on Adhesion State of Rat Insulinoma INS-1 Cells

Protein-protein interactions and/or signaling activities at focal adhesions, where integrin-mediated adhesion to extracellular matrix occurs, are critical for the regulation of adhesion-dependent cellular functions. Although the phosphorylation and activities of focal adhesion molecules have been intensively studied, the effects of the O-GlcNAc modification of their Ser/Thr residues on cellular functions have been largely unexplored. We investigated the effects of O-GlcNAc modification on actin reorganization and morphology of rat insulinoma INS-1 cells after glucosamine (GlcN) treatment. We found that paxillin, a key adaptor molecule in focal adhesions, could be modified by O-GlcNAc in INS-1 cells treated with GlcN and in pancreatic islets from mice treated with streptozotocin. Ser-84/85 in human paxillin appeared to be modified by O-GlcNAc, which was inversely correlated to Ser-85 phosphorylation (Ser-83 in rat paxillin). Integrin-mediated adhesion signaling inhibited the GlcN treatment-enhanced O-GlcNAc modification of paxillin. Adherent INS-1 cells treated with GlcN showed restricted protrusions, whereas untreated cells showed active protrusions for multiple-elongated morphologies. Upon GlcN treatment, expression of a triple mutation (S83A/S84A/S85A) resulted in no further restriction of protrusions. Together these observations suggest that murine pancreatic β cells may have restricted actin organization upon GlcN treatment by virtue of the O-GlcNAc modification of paxillin, which can be antagonized by a persistent cell adhesion process.     

Zona Occludens-2 Inhibits Cyclin D1 Expression and Cell Proliferation and Exhibits Changes in Localization along the Cell Cycle

Here, we have studied the effect of the tight junction protein zona occludens (ZO)-2 on cyclin D1 (CD1) protein expression. CD1 is essential for cell progression through the G1 phase of the cell cycle. We have found that in cultures of synchronized Madin-Darby canine kidney cells, ZO-2 inhibits cell proliferation at G0/G1 and decreases CD1 protein level. These effects occur in response to a diminished CD1 translation and an augmented CD1 degradation at the proteosome triggered by ZO-2. ZO-2 overexpression decreases the amount of Glycogen synthase kinase-3β phosphorylated at Ser9 and represses β-catenin target gene expression. We have also explored the expression of ZO-2 through the cell cycle and demonstrate that ZO-2 enters the nucleus at the late G1 phase and leaves the nucleus when the cell is in mitosis. These results thus explain why in confluent quiescent epithelia ZO-2 is absent from the nucleus and localizes at the cellular borders, whereas in sparse proliferating cultures ZO-2 is conspicuously present at the nucleus.     

Cross-talk between Two Essential Nutrient-sensitive Enzymes: O-GlcNAc TRANSFERASE (OGT) AND AMP-ACTIVATED PROTEIN KINASE (AMPK)*

Nutrient-sensitive pathways regulate both O-GlcNAc transferase (OGT) and AMP-activated protein kinase (AMPK), cooperatively connecting metabolic homeostasis to regulation of numerous intracellular processes essential for life. Similar to phosphorylation, catalyzed by kinases such as AMPK, O-GlcNAcylation is a highly dynamic Ser/Thr-specific post-translational modification of nuclear, cytoplasmic, and mitochondrial proteins catalyzed exclusively by OGT. OGT and AMPK target a multitude of intracellular proteins, with the net effect to protect cells from the damaging effects of metabolic stress. Despite hundreds of studies demonstrating significant overlap in upstream and downstream signaling processes, no study has investigated if OGT and AMPK can directly regulate each other. We show acute activation of AMPK alters the substrate selectivity of OGT in several cell lines and nuclear localization of OGT in C2C12 skeletal muscle myotubes. Nuclear localization of OGT affects O-GlcNAcylation of numerous nuclear proteins and acetylation of Lys-9 on histone 3 in myotubes. AMPK phosphorylates Thr-444 on OGT in vitro; phosphorylation of Thr-444 is tightly associated with AMPK activity and nuclear localization of OGT in myotubes, and phospho-mimetic T444E-OGT exhibits altered substrate selectivity. Conversely, the α- and γ-subunits of AMPK are O-GlcNAcylated, O-GlcNAcylation of the γ1-subunit increases with AMPK activity, and acute inhibition of O-GlcNAc cycling disrupts activation of AMPK. We have demonstrated significant cross-talk between the O-GlcNAc and AMPK systems, suggesting OGT and AMPK may cooperatively regulate nutrient-sensitive intracellular processes that mediate cellular metabolism, growth, proliferation, and/or tissue function.     

O-GlcNAc and the Epigenetic Regulation of Gene Expression

O-GlcNAcylation is an abundant nutrient-driven modification linked to cellular signaling and regulation of gene expression. Utilizing precursors derived from metabolic flux, O-GlcNAc functions as a homeostatic regulator. The enzymes of O-GlcNAc cycling, OGT and O-GlcNAcase, act in mitochondria, the cytoplasm, and the nucleus in association with epigenetic “writers” and “erasers” of the histone code. Both O-GlcNAc and O-phosphate modify repeats within the RNA polymerase II C-terminal domain (CTD). By communicating with the histone and CTD codes, O-GlcNAc cycling provides a link between cellular metabolic status and the epigenetic machinery. Thus, O-GlcNAcylation is poised to influence trans-generational epigenetic inheritance.     

Regulation of Insulin Receptor Substrate 1 (IRS-1)/AKT Kinase-mediated Insulin Signaling by O-Linked ��-N-Acetylglucosamine in 3T3-L1 Adipocytes

Increased O-linked β-N-acetylglucosamine (O-GlcNAc) is associated with insulin resistance in muscle and adipocytes. Upon insulin treatment of insulin-responsive adipocytes, O-GlcNAcylation of several proteins is increased. Key insulin signaling proteins, including IRS-1, IRS-2, and PDK1, are substrates for OGT, suggesting potential O-GlcNAc control points within the pathway. To elucidate the roles of O-GlcNAc in dampening insulin signaling (Vosseller, K., Wells, L., Lane, M. D., and Hart, G. W. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 5313–5318), we focused on the pathway upstream of AKT. Increasing O-GlcNAc in 3T3-L1 adipocytes decreases phosphoinositide 3-kinase (PI3K) interactions with both IRS-1 and IRS-2. Elevated O-GlcNAc also reduces phosphorylation of the PI3K p85 binding motifs (YXXM) of IRS-1 and results in a concomitant reduction in tyrosine phosphorylation of Y⁶⁰⁸XXM in IRS-1, one of the two main PI3K p85 binding motifs. Additionally, insulin signaling stimulates the interaction of OGT with PDK1. We conclude that one of the steps at which O-GlcNAc contributes to insulin resistance is by inhibiting phosphorylation at the Y⁶⁰⁸XXM PI3K p85 binding motif in IRS-1 and possibly at PDK1 as well.     

A Highlights from MBoC Selection: O-GlcNAc Cycling Enzymes Associate with the Translational Machinery and Modify Core Ribosomal Proteins

Protein synthesis is globally regulated through posttranslational modifications of initiation and elongation factors. Recent high-throughput studies have identified translation factors and ribosomal proteins (RPs) as substrates for the O-GlcNAc modification. Here we determine the extent and abundance of O-GlcNAcylated proteins in translational preparations. O-GlcNAc is present on many proteins that form active polysomes. We identify twenty O-GlcNAcylated core RPs, of which eight are newly reported. We map sites of O-GlcNAc modification on four RPs (L6, L29, L32, and L36). RPS6, a component of the mammalian target of rapamycin (mTOR) signaling pathway, follows different dynamics of O-GlcNAcylation than nutrient-induced phosphorylation. We also show that both O-GlcNAc cycling enzymes OGT and OGAse strongly associate with cytosolic ribosomes. Immunofluorescence experiments demonstrate that OGAse is present uniformly throughout the nucleus, whereas OGT is excluded from the nucleolus. Moreover, nucleolar stress only alters OGAse nuclear staining, but not OGT staining. Lastly, adenovirus-mediated overexpression of OGT, but not of OGAse or GFP control, causes an accumulation of 60S subunits and 80S monosomes. Our results not only establish that O-GlcNAcylation extensively modifies RPs, but also suggest that O-GlcNAc play important roles in regulating translation and ribosome biogenesis.     

Cell Density Impacts Epigenetic Regulation of Cytokine-Induced E-Selectin Gene Expression in Vascular Endothelium

Growing evidence suggests that the phenotype of endothelial cells during angiogenesis differs from that of quiescent endothelial cells, although little is known regarding the difference in the susceptibility to inflammation between both the conditions. Here, we assessed the inflammatory response in sparse and confluent endothelial cell monolayers. To obtain sparse and confluent monolayers, human umbilical vein endothelial cells were seeded at a density of 7.3×10³ cells/cm² and 29.2×10³ cells/cm², respectively, followed by culturing for 36 h and stimulation with tumor necrosis factor α. The levels of tumor necrosis factor α-induced E-selectin protein and mRNA expression were higher in the confluent monolayer than in the sparse monolayer. The phosphorylation of c-jun N-terminal kinase and p38 mitogen-activated protein kinase or nuclear factor-κB activation was not involved in this phenomenon. A chromatin immunoprecipitation assay of the E-selectin promoter using an anti-acetyl-histone H3 antibody showed that the E-selectin promoter was highly and specifically acetylated in the confluent monolayer after tumor necrosis factor α activation. Furthermore, chromatin accessibility real-time PCR showed that the chromatin accessibility at the E-selectin promoter was higher in the confluent monolayer than in the sparse monolayer. Our data suggest that the inflammatory response may change during blood vessel maturation via epigenetic mechanisms that affect the accessibility of chromatin.     

Regulation of AMPA Receptor Trafficking by O-Glycosylation

The present study investigated the role of O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) in AMPA receptor trafficking. Alloxan, an inhibitor of O-GlcNAc transferase, potentiated responses of AMPA receptors composed of the GluR1 subunit expressed in Xenopus oocytes. No potentiating effect of alloxan was obtained with mutant GluR1 (S831A) receptor lacking CaMKII phosphorylation site. Alloxan facilitated basal synaptic transmission to approximately 120% of basal levels and enhanced Schaffer collateral-CA1 long-term potentiation (LTP) in rat hippocampal slices, especially in the late phase of the LTP. Alloxan stimulated translocation of the GluR1 and GluR2 subunit from the cytosol towards the plasma membrane in rat hippocampal slices with the LTP, although it had no effect on subcellular distribution of the NR1 subunit. Taken together, the results of the present study show that alloxan regulates AMPA receptor trafficking by inhibiting O-GlcNAcylation, to modulate hippocampal synaptic transmission and synaptic plasticity.     

Assembly of the tight junction: the role of diacylglycerol

Extracellular Ca2+ triggers assembly and sealing of tight junctions (TJs) in MDCK cells. These events are modulated by G-proteins, phospholipase C, protein kinase C (PKC), and calmodulin. In the present work we observed that 1,2-dioctanoylglycerol (diC8) promotes the assembly of TJ in low extracellular Ca2+, as evidenced by translocation of the TJ-associated protein ZO-1 to the plasma membrane, formation of junctional fibrils observed in freeze-fracture replicas, decreased permeability of the intercellular space to [3H]mannitol, and reorganization of actin filaments to the cell periphery, visualized by fluorescence microscopy using rhodamine-phalloidin. In contrast, diC8 in low Ca2+ did not induce redistribution of the Ca-dependent adhesion protein E-cadherin (uvomorulin). Extracellular antibodies to E-cadherin block junction formation normally induced by adding Ca2+. diC8 counteracted this inhibition, suggesting that PKC may be in the signaling pathway activated by E-cadherin-mediated cell-cell adhesion. In addition, we found a novel phosphoprotein of 130 kD which coimmunoprecipitated with the ZO-1/ZO-2 complex. Although the assembly and sealing of TJs may involve the activation of PKC, the level of phosphorylation of ZO-1, ZO-2, and the 130-kD protein did not change after adding Ca2+ or a PKC agonist. The complex of these three proteins was present even in low extracellular Ca2+, suggesting that the addition of Ca2+ or diC8 triggers the translocation and assembly of preformed TJ subcomplexes.     

Pkh1/2-dependent phosphorylation of Vps27 regulates ESCRT-I recruitment to endosomes

Multivesicular endosomes (MVBs) are major sorting platforms for membrane proteins and participate in plasma membrane protein turnover, vacuolar/lysosomal hydrolase delivery, and surface receptor signal attenuation. MVBs undergo unconventional inward budding, which results in the formation of intraluminal vesicles (ILVs). MVB cargo sorting and ILV formation are achieved by the concerted function of endosomal sorting complex required for transport (ESCRT)-0 to ESCRT-III. The ESCRT-0 subunit Vps27 is a key player in this pathway since it recruits the other complexes to endosomes. Here we show that the Pkh1/Phk2 kinases, two yeast orthologues of the 3-phosphoinositide–dependent kinase, phosphorylate directly Vps27 in vivo and in vitro. We identify the phosphorylation site as the serine 613 and demonstrate that this phosphorylation is required for proper Vps27 function. Indeed, in pkh-ts temperature-sensitive mutant cells and in cells expressing vps27^S613A, MVB sorting of the carboxypeptidase Cps1 and of the α-factor receptor Ste2 is affected and the Vps28–green fluorescent protein ESCRT-I subunit is mainly cytoplasmic. We propose that Vps27 phosphorylation by Pkh1/2 kinases regulates the coordinated cascade of ESCRT complex recruitment at the endosomal membrane.     

O-Linked N-Acetylglucosamine Cycling Regulates Mitotic Spindle Organization

Any defects in the correct formation of the mitotic spindle will lead to chromosomal segregation errors, mitotic arrest, or aneuploidy. We demonstrate that O-linked N-acetylglucosamine (O-GlcNAc), a post-translational modification of serine and threonine residues in nuclear and cytoplasmic proteins, regulates spindle function. In O-GlcNAc transferase or O-GlcNAcase gain of function cells, the mitotic spindle is incorrectly assembled. Chromosome condensation and centrosome assembly is impaired in these cells. The disruption in spindle architecture is due to a reduction in histone H3 phosphorylation by Aurora kinase B. However, gain of function cells treated with the O-GlcNAcase inhibitor Thiamet-G restored the assembly of the spindle and partially rescued histone phosphorylation. Together, these data suggest that the coordinated addition and removal of O-GlcNAc, termed O-GlcNAc cycling, regulates mitotic spindle organization and provides a potential new perspective on how O-GlcNAc regulates cellular events.     

Analysis of the distribution and phosphorylation state of ZO-1 in MDCK and nonepithelial cells

We have examined the distribution and extent of phosphorylation of the tight junction-associated protein ZO-1 in the epithelial MDCK cell line, and in three cell types that do not form tight junctions: S180 (sarcoma) cells, S180 cells transfected with E-cadherin (S180L), and primary cultures of astrocytes. In shortterm calcium chelation experiments on MDCK cells, removal of extracellular calcium caused cells to pull apart. However, ZO-1 remained concentrated at the plasma membrane and no change in ZO-1 phosphorylation was observed. Maintenance of MDCK cells in low calcium medium, conditions where no tight junctions are found, resulted in altered ZO-1 distribution and lower total phosphorylation of the protein. In S180 cells, ZO-1 was diffusely distributed along the entire cell surface, with concentration of the antigen in motile regions of the cell. Cell-cell contact was not a prerequisite for ZO-1 localization at the plasma membrane in this cell type, and the phosphate content of ZO-1 was found to be lower in S180 cells relative to MDCK cells. Expression of Ecadherin in S180L cells did not alter either the distribution or phosphorylation of ZO-1. In contrast to S180 cells, ZO-1 in primary cultures of astrocytes was concentrated at sites of cell-cell contact, and the phosphorylation state was the same as that in control MDCK cells. Comparison of one-dimensional proteolytic digests of ³²P-labeled ZO-1 revealed the phosphorylation of two peptides in control MDCK cells that was absent in both MDCK cells grown in low calcium and in S180 cells.We would like to thank Cheryl Richards for her help with the cell culture and immunohistochemistry; David Begg, Gary Firestone, Vik Maraj, Manijeh Pasdar and Colin Rasmussen for helpful discussions; Jaclyn Peebles and Greg Morrison for help with graphics and photography; and Grace Martin and Bob Campenot for rat tail collagen. We are grateful to all the members of our laboratories for their friendship, advice and support. This work was supported by an Establishment Award to B.R.S. from the Alberta Heritage Foundation for Medical Research and grants to B.R.S. from the Kidney Foundation of Canada and the Medical Research Council of Canada. A.H. is funded by a Studentship from the AHFMR. K.L.S. was supported by a grant from the National Institutes of Health (DK-42799) to Gary L. Firestone. B.R.S. is a Medical Research Council of Canada and AHFMR Scholar.