Phosphatidylinositol 4-phosphate formation at ER exit sites regulates ER export.

The mechanisms that regulate endoplasmic reticulum (ER) exit-site (ERES) assembly and COPII-mediated ER export are currently unknown. We analyzed the role of phosphatidylinositols (PtdIns) in regulating ER export. Utilizing pleckstrin homology domains and a PtdIns phosphatase to specifically sequester or reduce phosphorylated PtdIns levels, we found that PtdIns 4-phosphate (PtsIns4P) is required to promote COPII-mediated ER export. Biochemical and morphological in vitro analysis revealed dynamic and localized PtsIns4P formation at ERES. PtdIns4P was utilized to support Sar1-induced proliferation and constriction of ERES membranes. PtdIns4P also assisted in Sar1-induced COPII nucleation at ERES. Therefore, localized dynamic remodeling of PtdIns marks ERES membranes to regulate COPII-mediated ER export.

Specific N-glycans direct apical delivery of transmembrane, but not soluble or glycosylphosphatidylinositol-anchored forms of endolyn in Madin-Darby canine kidney cells.

The sialomucin endolyn is a transmembrane protein with a unique trafficking pattern in polarized Madin-Darby canine kidney cells. Despite the presence of a cytoplasmic tyrosine motif that, in isolation, is sufficient to mediate basolateral sorting of a reporter protein, endolyn predominantly traverses the apical surface en route to lysosomes. Apical delivery of endolyn is disrupted in tunicamycin-treated cells, implicating a role for N-glycosylation in apical sorting. Site-directed mutagenesis of endolyn's eight N-glycosylation sites was used to identify two N-glycans that seem to be the major determinants for efficient apical sorting of the protein. In addition, apical delivery of endolyn was disrupted when terminal processing of N-glycans was blocked using glycosidase inhibitors. Missorting of endolyn occurred independently of the presence or absence of the basolateral sorting signal, because apical delivery was also inhibited by tunicamycin when the cytoplasmic tyrosine motif was mutated. However, we found that apical secretion of a soluble mutant of endolyn was N-glycan independent, as was delivery of glycosylphosphatidylinositol-anchored endolyn. Thus, specific N-glycans are only essential for the apical sorting of transmembrane endolyn, suggesting fundamental differences in the mechanisms by which soluble, glycosylphosphatidylinositol-anchored, and transmembrane proteins are sorted.

Resveratrol inhibits the epithelial sodium channel via phopshoinositides and AMP-activated protein kinase in kidney collecting duct cells.

Resveratrol, a naturally occurring phytoalexin, has reported cardioprotective, anti-inflammatory, chemopreventative and antidiabetic properties. Several studies indicate the multiple effects of resveratrol on cellular function are due to its inhibition of class 1A phosphoinositide 3-kinase (PI3K) mediated signaling pathways, but it also activates AMP-activated protein kinase (AMPK). As sodium transport in the kidney via the Epithelial Sodium Channel (ENaC) is highly sensitive to changes in phosphoinositide signaling in the membrane and AMPK, we employed resveratrol to probe the relative effects of phosphatidylinositol species in the plasma membrane and AMPK activity and their impact on ENaC activity in mouse cortical collecting duct (mpkCCDc14) cells. Here we demonstrate that resveratrol acutely reduces amiloride-sensitive current in mpkCCDc14 cells. The time course and dose dependency of this inhibition paralleled depletion of the PI(3,4,5)P3 reporter (AKT-PH) in live-cell microscopy, indicating the early inhibition is likely mediated by resveratrol's known effects on PI3K activity. Additionally, resveratrol induces a late inhibitory effect (4-24 hours) that appears to be mediated via AMPK activation. Resveratrol treatment induces significant AMPK activation compared with vehicle controls after 4 h, which persists through 16 h. Knockdown of AMPK or treatment with the AMPK inhibitor Compound C reduced the late phase of current reduction but had no effect on the early inhibitory activity of resveratrol. Collectively, these data demonstrate that resveratrol inhibits ENaC activity by a dual effect: an early reduction in activity seen within 5 minutes related to depletion of membrane PIP3, and a sustained late (4-24 h) effect secondary to activation of AMPK.

Membrane traffic and turnover in TRP-ML1-deficient cells: a revised model for mucolipidosis type IV pathogenesis.

The lysosomal storage disorder mucolipidosis type IV (MLIV) is caused by mutations in the transient receptor potential-mucolipin-1 (TRP-ML1) ion channel. The "biogenesis" model for MLIV pathogenesis suggests that TRP-ML1 modulates postendocytic delivery to lysosomes by regulating interactions between late endosomes and lysosomes. This model is based on observed lipid trafficking delays in MLIV patient fibroblasts. Because membrane traffic aberrations may be secondary to lipid buildup in chronically TRP-ML1-deficient cells, we depleted TRP-ML1 in HeLa cells using small interfering RNA and examined the effects on cell morphology and postendocytic traffic. TRP-ML1 knockdown induced gradual accumulation of membranous inclusions and, thus, represents a good model in which to examine the direct effects of acute TRP-ML1 deficiency on membrane traffic. Ratiometric imaging revealed decreased lysosomal pH in TRP-ML1-deficient cells, suggesting a disruption in lysosomal function. Nevertheless, we found no effect of TRP-ML1 knockdown on the kinetics of protein or lipid delivery to lysosomes. In contrast, by comparing degradation kinetics of low density lipoprotein constituents, we confirmed a selective defect in cholesterol but not apolipoprotein B hydrolysis in MLIV fibroblasts. We hypothesize that the effects of TRP-ML1 loss on hydrolytic activity have a cumulative effect on lysosome function, resulting in a lag between TRP-ML1 loss and full manifestation of MLIV.

Association of gag multimers with filamentous actin during equine infectious anemia virus assembly.

A role for the actin cytoskeleton in retrovirus assembly has long been speculated. However, specific mechanisms by which actin facilitates the assembly process remain elusive. We previously demonstrated differential effects of experimentally modified actin dynamics on virion production of equine infectious anemia virus (EIAV), a lentivirus related to HIV-1, suggesting an involvement of actin dynamics in retrovirus production. In the current study, we used bimolecular fluorescence complementation (BiFC) to reveal intimate (<15 nm) and specific associations between EIAV Gag and actin, but not tubulin. Specific interaction between Gag and filamentous actin was also demonstrated by co-immunoprecipitation experiments combined with the actin severing protein gelsolin to solubilize F-actin. Deletion of capsid (CA) or nucleocapsid (NC) genes reduced Gag association with F-actin by 40% and 95%, respectively. Interestingly, GCN4, a leucine zipper motif, could substitute for the NC domain in mediating F-actin association. Furthermore, deficiency of the DeltaNC Gag in F-actin interaction was restored upon co-expression of Gag constructs containing both CA and NC or the GCN4, suggesting a requirement for Gag polyprotein multimerization prior to F-actin association. The observed Gag-F-actin association appeared to correlate with viral budding, as enhanced budding of the DeltaNC mutant was evident upon restoration of F-actin association. Intracellular association of Gag complexes with F-actin was also detected by immunoscanning electron microscopy of Triton-extracted EIAV-infected cells. Together, these data suggest that Gag multimers induced by CA and NC domains interact with F-actin and that this association is important for efficient virion production.

Phosphatidylinositol 4-phosphate 5-kinase reduces cell surface expression of the epithelial sodium channel (ENaC) in cultured collecting duct cells.

Ubiquitination of ENaC subunits has been shown to negatively regulate the cell surface expression of ENaC channels. We have previously demonstrated that epsin links ubiquitinated ENaC to clathrin adaptors for clathrin-mediated endocytosis. Epsin is thought to directly modify the curvature of membranes upon binding to phosphatidylinositol 4,5-bisphosphate (PIP2) where it recruits clathrin and stimulates lattice assembly. Murine phosphatidylinositol 4-phosphate 5-kinase alpha (PI5KIalpha) has been shown to enhance endocytosis in a PIP2-dependent manner. We tested the hypothesis that PI5KIalpha-mediated PIP2 production would negatively regulate ENaC current by enhancing epsin-mediated endocytosis of the channel. Expression of PI5KIalpha decreased ENaC currents in Xenopus oocytes by 80%, entirely because of a decrease in cell surface ENaC levels. Catalytically inactive mutants of PI5Kalpha had no effect on ENaC activity. Expression of the PIP2 binding region of epsin increased ENaC current in oocytes, an effect completely reversed by co-expression of PI5KIalpha. Overexpression of epsin reduced amiloride-sensitive current in CCD cells. Overexpression of PI5KIalpha enhanced membrane PIP2 levels and reduced apical surface expression of ENaC in CCD cells, down-regulating amiloride-sensitive current. Knockdown of PI5KIalpha with isoform-specific siRNA resulted in a 4-fold enhancement of ENaC activity. PI5KIalpha localized exclusively to the apical plasma membrane domain when overexpressed in mouse CCD cells, consistent for a role in regulating PIP2 production at the apical plasma membrane. We conclude that membrane turnover events regulating ENaC surface expression and activity in oocytes and CCD cells can be regulated by PI5KIalpha.

N-glycans mediate apical recycling of the sialomucin endolyn in polarized MDCK cells.

Apical and basolateral proteins are maintained within distinct membrane subdomains in polarized epithelial cells by biosynthetic and postendocytic sorting processes. Sorting of basolateral proteins in these processes has been well studied; however, the sorting signals and mechanisms that direct proteins to the apical surface are less well understood. We previously demonstrated that an N-glycan-dependent sorting signal directs the sialomucin endolyn to the apical surface in polarized Madin-Darby canine kidney cells. Terminal processing of a subset of endolyn's N-glycans is key for polarized biosynthetic delivery to the apical membrane. Endolyn is subsequently internalized, and via a cytoplasmic tyrosine-based sorting motif is targeted to lysosomes from where it constitutively cycles to the cell surface. Here, we examine the polarized sorting of endolyn along the postendocytic pathway in polarized cells. Our results suggest that similar N-glycan sorting determinants are required for apical delivery of endolyn along both the biosynthetic and the postendocytic pathways.

Phosphatidylinositol 5-kinase stimulates apical biosynthetic delivery via an Arp2/3-dependent mechanism.

The mechanisms by which polarized epithelial cells target distinct carriers enriched in newly synthesized proteins to the apical or basolateral membrane remain largely unknown. Here we investigated the effect of phosphatidylinositol metabolism and modulation of the actin cytoskeleton, two regulatory mechanisms that have individually been suggested to function in biosynthetic traffic, on polarized traffic in Madin-Darby canine kidney cells. Overexpression of phosphatidylinositol 5-kinase (PI5K) increased actin comet frequency in Madin-Darby canine kidney cells and concomitantly stimulated trans-Golgi network (TGN) to apical membrane delivery of the raft-associated protein influenza hemagglutinin (HA), but did not affect delivery of a non-raft-associated apical protein or a basolateral marker. Modulation of actin comet formation by pharmacologic means, by overexpression of the TGN-localized inositol polyphosphate 5-phosphatase Ocrl, or by blockade of Arp2/3 function had parallel effects on the rate of apical delivery of HA. Moreover, HA released from a TGN block was colocalized in transport carriers in association with PI5K and actin comets. Inhibition of Arp2/3 function in combination with microtubule depolymerization led to a virtual block in HA delivery, suggesting synergistic coordination of these cytoskeletal assemblies in membrane transport. Our results suggest a previously unidentified role for actin comet-mediated propulsion in the biosynthetic delivery of a subset of apical proteins.

Posttranslational cleavage and adaptor protein complex-dependent trafficking of mucolipin-1.

Mucolipin-1 (ML1) is a member of the transient receptor potential ion channel superfamily that is thought to function in the biogenesis of lysosomes. Mutations in ML1 result in mucolipidosis type IV, a lysosomal storage disease characterized by the intracellular accumulation of enlarged vacuolar structures containing phospholipids, sphingolipids, and mucopolysaccharides. Little is known about how ML1 trafficking or activity is regulated. Here we have examined the processing and trafficking of ML1 in a variety of cell types. We find that a significant fraction of ML1 undergoes cell type-independent cleavage within the first extracellular loop of the protein during a late step in its biosynthetic delivery. To determine the trafficking route of ML1, we systematically examined the effect of ablating adaptor protein complexes on the localization of this protein. Whereas ML1 trafficking was not apparently affected in fibroblasts from mocha mice that lack functional adaptor protein complex (AP)-3, small interfering RNA-mediated knockdown revealed a requirement for AP-1 in Golgi export of ML1. Knockdown of functional AP-2 had no effect on ML1 localization. Interestingly, cleavage of ML1 was not compromised in AP-1-deficient cells, suggesting that proteolysis occurs in a prelysosomal compartment, possibly the trans-Golgi network. Our results suggest that posttranslational processing of ML1 is more complex than previously described and that this protein is delivered to lysosomes primarily via an AP-1-dependent route that does not involve passage via the cell surface.

Clathrin-mediated endocytosis of the epithelial sodium channel. Role of epsin.

Here we present evidence that the epithelial sodium channel (ENaC), a heteromeric membrane protein whose surface expression is regulated by ubiquitination, is present in clathrin-coated vesicles in epithelial cells that natively express ENaC. The channel subunits are ubiquitinated and co-immunoprecipitate with both epsin and clathrin adaptor proteins, and epsin, as expected, co-immunoprecipitates with clathrin adaptor proteins. The functional significance of these interactions was evaluated in a Xenopus oocyte expression system where co-expression of epsin and ENaC resulted in a down-regulation of ENaC activity; conversely, co-expression of epsin sub-domains acted as dominant-negative effectors and stimulated ENaC activity. These results identify epsin as an accessory protein linking ENaC to the clathrin-based endocytic machinery thereby regulating the activity of this ion channel at the cell surface.

Recycling of MUC1 is dependent on its palmitoylation.

MUC1 is a mucin-like transmembrane protein expressed on the apical surface of epithelia, where it protects the cell surface. The cytoplasmic domain has numerous sites for phosphorylation and docking of proteins involved in signal transduction. In a previous study, we showed that the cytoplasmic YXXphi motif Y20HPM and the tyrosine-phosphorylated Y60TNP motif are required for MUC1 clathrin-mediated endocytosis through binding AP-2 and Grb2, respectively (Kinlough, C. L., Poland, P. A., Bruns, J. B., Harkleroad, K. L., and Hughey, R. P. (2004) J. Biol. Chem. 279, 53071-53077). Palmitoylation of transmembrane proteins can affect their membrane trafficking, and the MUC1 sequence CQC3RRK at the boundary of the transmembrane and cytoplasmic domains mimics reported site(s) of S-palmitoylation. [3H]Palmitate labeling of Chinese hamster ovary cells expressing MUC1 with mutations in CQC3RRK revealed that MUC1 is dually palmitoylated at the CQC motif independent of RRK. Lack of palmitoylation did not affect the cold detergent solubility profile of a chimera (Tac ectodomain and MUC1 transmembrane and cytoplasmic domains), the rate of chimera delivery to the cell surface, or its half-life. Calculation of rate constants for membrane trafficking of wild-type and mutant Tac-MUC1 indicated that the lack of palmitoylation blocked recycling, but not endocytosis, and caused the chimera to accumulate in a EGFP-Rab11-positive endosomal compartment. Mutations CQC/AQA and Y20N inhibited Tac-MUC1 co-immunoprecipitation with AP-1, although mutant Y20N had reduced rates of both endocytosis and recycling, but a normal subcellular distribution. The double mutant chimera AQA+Y20N had reduced endocytosis and recycling rates and accumulated in EGFP-Rab11-positive endosomes, indicating that palmitoylation is the dominant feature modulating MUC1 recycling from endosomes back to the plasma membrane.

Distinct Golgi populations of phosphatidylinositol 4-phosphate regulated by phosphatidylinositol 4-kinases.

Phosphatidylinositol 4-phosphate (PI4P) regulates biosynthetic membrane traffic at multiple steps and differentially affects the surface delivery of apically and basolaterally destined proteins in polarized cells. Two phosphatidylinositol 4-kinases (PI4Ks) have been localized to the Golgi complex in mammalian cells, type III PI4Kbeta (PI4KIIIbeta) and type II PI4Kalpha (PI4KIIalpha). Here we report that PI4KIIIbeta and PI4KIIalpha localize to discrete subcompartments of the Golgi complex in Madin-Darby canine kidney (MDCK) cells. PI4KIIIbeta was enriched in early Golgi compartments, whereas PI4KIIalpha colocalized with markers of the trans-Golgi network (TGN). To understand the temporal and spatial control of PI4P generation across the Golgi complex, we quantitated the steady state distribution of a fluorescent PI4P-binding domain relative to cis/medial Golgi and TGN markers in transiently transfected MDCK cells. The density of the signal from this PI4P reporter was roughly 2-fold greater in the early Golgi compartments compared with that of the TGN. Furthermore, this ratio could be modulated in vivo by overexpression of catalytically inactive PI4KIIIbeta and PI4KIIalpha or in vitro by the PI4KIIIbeta inhibitor wortmannin. Our data suggest that both PI4KIIIbeta and PI4KIIalpha contribute to the compartmental regulation of PI4P synthesis within the Golgi complex. We discuss our results with respect to the kinetic effects of modulating PI4K activity on polarized biosynthetic traffic in MDCK cells.

Differential current decay profiles of epithelial sodium channel subunit combinations in polarized renal epithelial cells.

In many epithelial tissues in the body, the rate of Na(+) reabsorption is governed by the activity of the epithelial sodium channel (ENaC). The assembly, trafficking, and turnover of the three ENaC subunits (alpha, beta, and gamma) is complex and not well understood. Recent experiments suggest that ENaC must be proteolytically cleaved for maximal activity and may explain the discrepancies reported in prior biochemical approaches focused on quantitating the trafficking and half-life of full-length subunits. As an alternative approach to examining the dynamics of ENaC subunits, we have generated doxycycline-repressible replication-defective recombinant adenoviruses encoding individual epitope-tagged mouse ENaC subunits and expressed these in polarized MDCK I cells. Co-infection with these viruses encoding all three subunits generates robust amiloride-sensitive currents in polarized MDCK cells. Significant current was also observed in cells expressing alpha- and gamma-mENaC in the absence of beta-mENaC. These currents did not appear to result from association with endogenous canine beta-ENaC. Treatment of alpha beta gamma-expressing cells with cycloheximide (CHX) resulted in the rapid inhibition (within 3 h) of approximately 50-80% of the initial current; however, a sizable fraction of the initial current remained even after 6 h of CHX. By contrast, CHX addition to cells expressing only alpha- and gamma-mENaC resulted in rapid decay in current with no residual fraction. Our data suggest that ENaC channels of differing stoichiometries are differentially trafficked and degraded and provide support for the possibility that noncoordinate trafficking of ENaC subunits may function in vivo as a mechanism to modulate ENaC activity.