Regulation of Salmonella-induced neutrophil transmigration by epithelial ADP-ribosylation factor 6

Salmonella typhimurium elicits an acute inflammatory response in the host intestinal epithelium, characterized by the movement of polymorphonuclear leukocytes (PMN) across the epithelial monolayer to the intestinal lumen. It was recently shown that SipA, a protein secreted by S. typhimurium, is necessary and sufficient to drive PMN transmigration across model intestinal epithelia (Lee, C. A., Silva, M., Siber, A. M., Kelly, A. J., Galyov, E., and McCormick, B. A. (2000) Proc. Natl. Acad Sci. USA 97, 12283-12288). However, the epithelial factors responsible for this process have not been identified. Here, for the first time, we demonstrate that S. typhimurium-induced PMN transmigration across Madin-Darby canine kidney-polarized monolayers is regulated by the GTPase ARF6. Apically added S. typhimurium promoted the translocation of ARF6 and its exchange factor ARNO to the apical surface. Overexpression of a dominant-negative mutant of ARF6 inhibited Salmonella-induced PMN transmigration, which was due to a reduction in apical release of the PMN chemoattractant PEEC (pathogen-elicited epithelial chemoattractant), without affecting bacterial internalization. Furthermore, ARF6 and its effector phospholipase D (PLD) were both required for bacteria-induced translocation of protein kinase C (PKC) to membranes. These results describe a novel signal transduction pathway, in which Salmonella initiates an ARF6- and PLD-dependent lipid signaling cascade that, in turn, directs activation of PKC, release of PEEC, and subsequent transepithelial PMN movement.

Activation of the luteinizing hormone/choriogonadotropin hormone receptor promotes ADP ribosylation factor 6 activation in porcine ovarian follicular membranes

Previously we demonstrated in a cell-free ovarian follicular plasma membrane model that agonist-dependent desensitization of the luteinizing hormone/choriogonadotropin receptor (LH/CG R) is GTP-dependent, mimicked by the addition of ADP-ribosylation factor (ARF) nucleotide binding site opener, which acts as a guanine nucleotide exchange factor for ARFs 1 and 6, and selectively inhibited by synthetic N-terminal ARF6 peptides. We therefore sought direct evidence that activation of the LH/CG R promotes activation of ARF1 and/or ARF6. Using a classic ARF activation assay, the cholera toxin-catalyzed ADP-ribosylation of G alpha(s), results show that LH/CG R activation stimulates an ARF protein by a brefeldin A-independent mechanism. Synthetic N-terminal inhibitory ARF6 but not ARF1 peptide blocks LH/CG R-stimulated ARF activity. LH/CG R activation also promotes the binding of a photoaffinity GTP analog to a protein that migrates on one- and two-dimensional polyacrylamide gel electrophoresis with ARF6. These results suggest that ARF6 is the predominant ARF activated by the LH/CG R. To activate ARF6, the LH/CG R does not appear to signal through the C-terminal regions of G alpha(i) or G alpha(q) or through the second or third intracellular loops or the N terminus of the cytoplasmic tail of the LH/CG R. Although exogenous recombinant ARNO promotes only a small increase in ARF6 activation in the presence of activated LH/CG R, hCG-stimulated ARF6 activation is reduced to basal levels by catalytically inactive ARF nucleotide binding-site opener. These results provide direct evidence that LH/CG R activation leads to the activation of membrane-delimited ARF6.

Activation of ARF6 by ARNO stimulates epithelial cell migration through downstream activation of both Rac1 and phospholipase D

Migration of epithelial cells is essential for tissue morphogenesis, wound healing, and metastasis of epithelial tumors. Here we show that ARNO, a guanine nucleotide exchange factor for ADP-ribosylation factor (ARF) GTPases, induces Madin-Darby canine kidney epithelial cells to develop broad lamellipodia, to separate from neighboring cells, and to exhibit a dramatic increase in migratory behavior. This transition requires ARNO catalytic activity, which we show leads to enhanced activation of endogenous ARF6, but not ARF1, using a novel pulldown assay. We further demonstrate that expression of ARNO leads to increased activation of endogenous Rac1, and that Rac activation is required for ARNO-induced cell motility. Finally, ARNO-induced activation of ARF6 also results in increased activation of phospholipase D (PLD), and inhibition of PLD activity also inhibits motility. However, inhibition of PLD does not prevent activation of Rac. Together, these data suggest that ARF6 activation stimulates two distinct signaling pathways, one leading to Rac activation, the other to changes in membrane phospholipid composition, and that both pathways are required for cell motility.

Expression of constitutively active Rab5 uncouples maturation of the Salmonella-containing vacuole from intracellular replication

The enteric bacterial pathogen Salmonella typhimurium enters and proliferates within both phagocytic and non-phagocytic host cells. Upon entry, the bacteria reside in membrane-bound vacuoles (SCVs) that mature with time, as evidenced by the sequential loss of early endosomal markers, followed by the selective recruitment of a number of lysosomal membrane glycoproteins (LAMPs). This remodelling process renders the SCVs non-fusogenic with lysosomes and is also thought to create a vacuolar environment permissive for replication. We demonstrate that disruption of the endocytic pathway by the expression of a constitutively active form of the small GTPase rab5 (rab5Q79L) significantly altered the biogenesis of the SCVs without inhibiting bacterial replication in HeLa cells. Expression of rab5Q79L caused the retention of early endosomal markers on SCVs and early acquisition of LAMP2, and led to an increase in the kinetics of intracellular replication. We also demonstrate that a significant fraction of LAMP2 in SCVs is derived from the cell surface via endocytosis rather than via the biosynthetic route. Further, in fibroblasts lacking a functional AP3 adaptor complex, in which all newly synthesized LAMP is delivered to the cell surface, recruitment of LAMP to the SCVs remained unaffected. These findings raise the possibility that all the SCV-associated LAMP could be derived by endocytosis from the cell surface.

Intra-endosomal pH-sensitive recruitment of the Arf-nucleotide exchange factor ARNO and Arf6 from cytoplasm to proximal tubule endosomes

Kidney proximal tubule epithelial cells have an extensive apical endocytotic apparatus that is critical for the reabsorption and degradation of proteins that traverse the glomerular filtration barrier and that is also involved in the extensive recycling of functionally important apical plasma membrane transporters. We show here that an Arf-nucleotide exchange factor, ARNO (ADP-ribosylation factor nucleotide site opener) as well as Arf6 and Arf1 small GTPases are located in the kidney proximal tubule receptor-mediated endocytosis pathway, and that ARNO and Arf6 recruitment from cytosol to endosomes is pH-dependent. In proximal tubules in situ, ARNO and Arf6 partially co-localized with the V-ATPase in apical endosomes in proximal tubules. Arf1 was localized both at the apical pole of proximal tubule epithelial cells, but also in the Golgi. By Western blot analysis ARNO, Arf6, and Arf1 were detected both in purified endosomes and in proximal tubule cytosol. A translocation assay showed that ATP-driven endosomal acidification triggered the recruitment of ARNO and Arf6 from proximal tubule cytosol to endosomal membranes. The translocation of both ARNO and Arf6 was reversed by V-type ATPase inhibitors and by uncouplers of endosomal intralumenal pH, and was correlated with the magnitude of intra-endosomal acidification. Our data suggest that V-type ATPase-dependent acidification stimulates the selective recruitment of ARNO and Arf6 to proximal tubule early endosomes. This mechanism may play an important role in the pH-dependent regulation of receptor-mediated endocytosis in proximal tubules in situ.

The GTPase Rac1 selectively regulates Salmonella invasion at the apical plasma membrane of polarized epithelial cells

The bacterial pathogen Salmonella typhimurium colonizes its animal hosts by inducing its internalization into intestinal epithelial cells. This process requires reorganization of the actin cytoskeleton of the apical plasma membrane into elaborate membrane ruffles that engulf the bacteria. Members of the Ρ family of small GTPases are critical regulators of actin structure, and in nonpolarized cells, the GTPase Cdc42 has been shown to modulate Salmonella entry. Because the actin architecture of epithelial cells is organized differently from that of nonpolarized cells, we examined the role of two ‘Rgr; family GTPases, Cdc42 and Rac1, in invasion of polarized monolayers of MDCK cells by S. typhimurium. Surprisingly, we found that endogenous Rac1, but not Cdc42, was activated during bacterial entry at the apical pole, and that this activation required the bacterial effector protein SopE. Furthermore, expression of dominant inhibitory Rac1 but not Cdc42 significantly inhibited apical internalization of Salmonella, indicating that Rac1 activation is integral to the bacterial entry process. In contrast, during basolateral internalization, both Cdc42 and Rac1 were activated; however, neither GTPase was required for entry. These findings, which differ significantly from previous observations in nonpolarized cells, indicate that the host cell signaling pathways activated by bacterial pathogens may vary with cell type, and in epithelial tissues may further differ between plasma membrane domains.

Desensitization of the luteinizing hormone/choriogonadotropin receptor in ovarian follicular membranes is inhibited by catalytically inactive ARNO(+)

We have investigated the participation of endogenous ADP-ribosylation factor (ARF) nucleotide-binding site opener (ARNO) in desensitization of the luteinizing hormone/choriogonadotropin (LH/CG) receptor, independent of receptor internalization, using a cell-free plasma membrane model. We recently showed that the addition of recombinant ARNO promotes binding of beta-arrestin1 to the third intracellular (3i) loop of the active LH/CG receptor, thereby reducing the ability of the receptor to activate the stimulatory G protein and signal to adenylyl cyclase. In the present report we determined whether ARNO is detectable in follicular membranes and whether the catalytically inactive E156K ARNO mutant, containing a mutation in the Sec7 domain, can act in a dominant negative manner to block LH/CG receptor desensitization. Results show that ARNO is readily detected in follicular membranes and that levels of membrane-associated ARNO increase with follicular maturation. The addition of catalytically inactive E156K ARNO blocks both the release of beta-arrestin1 from its membrane docking site, based on Western blot analysis, and development of LH/CG receptor desensitization. We also investigated whether a point mutation in the pleckstrin homology (PH) domain of ARNO (R280D), which blocks binding of phosphoinositides like phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 4,5-bisphosphate (PIP(2)) but not catalytic activity, disrupts LH/CG receptor desensitization. R280D ARNO neither promotes nor inhibits LH/CG receptor desensitization, consistent with a requirement of the PH domain of ARNO for its association with the plasma membrane. LH/CG receptor activation of ARNO is not mediated by activation of phosphatidylinositol 3-kinase (PI 3-kinase) or by G protein beta gamma subunits. Taken together, these results suggest that LH/CG receptor promotes beta-arrestin1 release from its membrane docking site to bind to the 3i loop of the LH/CG receptor via activation of membrane delimited endogenous ARNO. As ARNO activation is independent of PI 3-kinase and G beta gamma, our results are consistent with a role for PIP(2) in receptor-stimulated ARNO activation.

Regulation of vesicle trafficking in madin-darby canine kidney cells by Rab11a and Rab25

Polarized epithelial cells maintain the polarized distribution of basolateral and apical membrane proteins through a process of receptor-mediated endocytosis, sorting, and then recycling to the appropriate membrane domain. We have previously shown that the small GTP-binding proteins, Rab11a and Rab25, are associated with the apical recycling system of Madin-Darby canine kidney cells. Here we have utilized inducible expression of wild-type, dominant negative, and constitutively active mutants to directly compare the functions of Rab25 and Rab11a in postendocytic vesicular transport. We found that a Rab11a mutant deficient in GTP binding, Rab11aS25N, potently inhibited both transcytosis and apical recycling yet failed to inhibit transferrin recycling. Similarly, expression of either wild type Rab25 or the active mutant Rab25S21V inhibited both apical recycling and transcytosis of IgA by greater than 50% but had no effect on basolateral recycling of transferrin. Interestingly, the GTPase-deficient mutant Rab11aS20V inhibited basolateral to apical transcytosis of IgA, but had no effect on either apical or basolateral recycling. These results indicate that neither Rab11a nor Rab25 function in the basolateral recycling of transferrin in polarized Madin-Darby canine kidney cells cells, consistent with recent morphological observations by others. Thus, transferrin receptors must be recycled to the plasma membrane prior to sorting of apically directed cargoes into Rab11a/Rab25-positive apical recycling endosomes.

Identification of a plasma membrane-associated guanine nucleotide exchange factor for ARF6 in chromaffin cells. Possible role in the regulated exocytotic pathway

ADP-ribosylation factors (ARFs) constitute a family of structurally related proteins that forms a subset of the Ras superfamily of regulatory GTP-binding proteins. Like other GTPases, activation of ARFs is facilitated by specific guanine nucleotide exchange factors (GEFs). In chromaffin cells, ARF6 is associated with the membrane of secretory granules. Stimulation of intact cells or direct elevation of cytosolic calcium in permeabilized cells triggers the rapid translocation of ARF6 to the plasma membrane and the concomitant activation of phospholipase D (PLD) in the plasma membrane. Both calcium-evoked PLD activation and catecholamine secretion in permeabilized cells are strongly inhibited by a synthetic peptide corresponding to the N-terminal domain of ARF6, suggesting that the ARF6-dependent PLD activation near the exocytotic sites represents a key event in the exocytotic reaction in chromaffin cells. In the present study, we demonstrate the occurrence of a brefeldin A-insensitive ARF6-GEF activity in the plasma membrane and in the cytosol of chromaffin cells. Furthermore, reverse transcriptase-polymerase chain reaction and immunoreplica analysis indicate that ARNO, a member of the brefeldin A-insensitive ARF-GEF family, is expressed and predominantly localized in the cytosol and in the plasma membrane of chromaffin cells. Using permeabilized chromaffin cells, we found that the introduction of anti-ARNO antibodies into the cytosol inhibits, in a dose-dependent manner, both PLD activation and catecholamine secretion in calcium-stimulated cells. Furthermore, co-expression in PC12 cells of a catalytically inactive ARNO mutant with human growth hormone as a marker of secretory granules in transfected cells resulted in a 50% inhibition of growth hormone secretion evoked by depolarization with high K(+). The possibility that the plasma membrane-associated ARNO participates in the exocytotic pathway by activating ARF6 and downstream PLD is discussed.

The ADP ribosylation factor nucleotide exchange factor ARNO promotes beta-arrestin release necessary for luteinizing hormone/choriogonadotropin receptor desensitization

Desensitization of guanine nucleotide binding protein-coupled receptors is a ubiquitous phenomenon characterized by declining effector activity upon persistent agonist stimulation. The luteinizing hormone/choriogonadotropin receptor (LH/CGR) in ovarian follicles exhibits desensitization of effector adenylyl cyclase activity in response to the mid-cycle surge of LH. We have previously shown that uncoupling of the agonist-activated LH/CGR from the stimulatory G protein (G(s)) is dependent on GTP and attributable to binding of beta-arrestin present in adenylyl cyclase-rich follicular membrane fraction to the third intracellular (3i) loop of the receptor. Here, we report that LH/CGR-dependent desensitization is mimicked by ADP ribosylation factor nucleotide-binding site opener, a guanine nucleotide exchange factor of the small G proteins ADP ribosylation factors (Arfs) 1 and 6, and blocked by synthetic N-terminal Arf6 peptide, suggesting that the GTP-dependent step of LH/CGR desensitization is receptor-dependent Arf6 activation. Arf activation by GTP and ADP ribosylation factor nucelotide-binding site opener promotes the release of docked beta-arrestin from the membrane, making beta-arrestin available for LH/CGR; Arf6 but not Arf1 peptides block beta-arrestin release from the membrane. Thus, LH/CGR appears to activate two membrane delimited signaling cascades via two types of G proteins: heterotrimeric G(s) and small G protein Arf6. Arf6 activation releases docked beta-arrestin necessary for receptor desensitization, providing a feedback mechanism for receptor self-regulation.

Recycling of AQP2 occurs through a temperature- and bafilomycin-sensitive trans-Golgi-associated compartment

The exo- and endocytotic pathway in which aquaporin-2 (AQP2) travels between the plasma membrane and intracellular vesicles is only partially characterized. It is known that the antidiuretic hormone vasopressin induces a translocation of AQP2 from an intracellular to a plasma membrane location, both in kidney collecting duct principal cells and in transfected epithelial cells. Here we provide evidence suggesting that while AQP2 shifts from an intracellular location to the cell surface in response to vasopressin, AQP2 also constitutively recycles through a similar pathway in transfected LLC-PK(1) cells even in the absence of hormonal stimulation. Incubating cells at 20 degrees C blocks AQP2 recycling in a perinuclear compartment, regardless of whether vasopressin is present. The H(+)-ATPase inhibitor bafilomycin A1 also blocks the recycling pathway of AQP2 in a perinuclear compartment adjacent to the Golgi in the presence and absence of vasopressin stimulation, indicating a role of vesicle acidification in both the constitutive and regulated recycling of AQP2. Colocalization of AQP2 with clathrin, but not with giantin, after both bafilomycin treatment and a 20 degrees C block suggests that the compartment in which recycling AQP2 is blocked may be the trans-Golgi, and not cis- and medial-Golgi cisternae.

Turning on ARF: the Sec7 family of guanine-nucleotide-exchange factors

ARF proteins are important regulators of membrane dynamics and protein transport within the eukaryotic cell. The Sec7 domain is approximately 200 amino acids in size and stimulates guanine-nucleotide exchange on members of the ARF class of small GTPases. The members of one subclass of Sec7-domain proteins are direct targets of the secretion-inhibiting drug brefeldin A, which blocks the exchange reaction by trapping a reaction intermediate in an inactive, abortive complex. A separate subclass of Sec7-domain proteins is involved in signal transduction and possess a domain that mediates membrane binding in response to extracellular signals.

Regulation of ARNO nucleotide exchange by a PH domain electrostatic switch

ARNO is a member of a family of guanine nucleotide exchange factors that activate small GTPases called ADP-ribosylation factors (ARFs) [1] [2] [3], which regulate vesicular trafficking and, in one case (ARF6), also regulate cortical actin structure [4]. ARNO is located at the plasma membrane, and in the presence of activated protein kinase C (PKC) can induce cortical actin rearrangements reminiscent of those produced by active ARF6 [5] [6] [7] [8]. High-affinity binding of ARNO to membranes, which is required for exchange activity, is mediated cooperatively by a pleckstrin homology (PH) domain and an adjacent carboxy-terminal polybasic domain [3] [9]. ARNO is phosphorylated in vivo by PKC on a single serine residue, S392, located within the carboxy-terminal polybasic domain. Mutation of S392 to alanine does not prevent ARNO-mediated actin rearrangements, suggesting that phosphorylation does not lead to ARNO activation [6]. Here, we report that phosphorylation negatively regulates ARNO exchange activity through a 'PH domain electrostatic switch'. Introduction of a negatively charged phosphate into the polybasic domain reduced interaction of ARNO with membranes both in vitro and in vivo, and inhibited exchange in vitro. This regulated membrane association is similar to the myristoyl electrostatic switch that controls membrane binding of the myristoylated alanine-rich C kinase substrate (MARCKS) [10], but to our knowledge is the first demonstration of an electrostatic switch regulating the membrane interaction of a protein containing a PH domain. This mechanism allows regulation of ARNO lipid binding and exchange activity at two levels, phosphoinositide-dependent recruitment and PKC-dependent displacement from the membrane.

Association of Rab25 and Rab11a with the apical recycling system of polarized Madin-Darby canine kidney cells

Recent evidence suggests that apical and basolateral endocytic pathways in epithelia converge in an apically located, pericentriolar endosomal compartment termed the apical recycling endosome. In this compartment, apically and basolaterally internalized membrane constituents are thought to be sorted for recycling back to their site of origin or for transcytosis to the opposite plasma membrane domain. We report here that in the epithelial cell line Madin-Darby Canine Kidney (MDCK), antibodies to Rab11a label an apical pericentriolar endosomal compartment that is dependent on intact microtubules for its integrity. Furthermore, this compartment is accessible to a membrane-bound marker (dimeric immunoglobulin A [IgA]) internalized from either the apical or basolateral pole, functionally defining it as the apical recycling endosome. We have also examined the role of a closely related epithelial-specific Rab, Rab25, in the regulation of membrane recycling and transcytosis in MDCK cells. When cDNA encoding Rab25 was transfected into MDCK cells, the protein colocalized with Rab11a in subapical vesicles. Rab25 transfection also altered the distribution of Rab11a, causing the coalescence of immunoreactivity into multiple denser vesicular structures not associated with the centrosome. Nevertheless, nocodazole still dispersed these vesicles, and dimeric IgA internalized from either the apical or basolateral membrane was detected in endosomes labeled with antibodies to both Rab11a and Rab25. Overexpression of Rab25 decreased the rate of IgA transcytosis and of apical, but not basolateral, recycling of internalized ligand. Conversely, expression of the dominant-negative Rab25T26N did not alter either apical recycling or transcytosis. These results indicate that both Rab11a and Rab25 associate with the apical recycling system of epithelial cells and suggest that Rab25 may selectively regulate the apical recycling and/or transcytotic pathways.

Remodeling of the actin cytoskeleton is coordinately regulated by protein kinase C and the ADP-ribosylation factor nucleotide exchange factor ARNO

ARNO is a member of a family of guanine-nucleotide exchange factors with specificity for the ADP-ribosylation factor (ARF) GTPases. ARNO possesses a central catalytic domain with homology to yeast Sec7p and an adjacent C-terminal pleckstrin homology (PH) domain. We have previously shown that ARNO localizes to the plasma membrane in vivo and efficiently catalyzes ARF6 nucleotide exchange in vitro. In addition to a role in endocytosis, ARF6 has also been shown to regulate assembly of the actin cytoskeleton. To determine whether ARNO is an upstream regulator of ARF6 in vivo, we examined the distribution of actin in HeLa cells overexpressing ARNO. We found that, while expression of ARNO leads to disassembly of actin stress fibers, it does not result in obvious changes in cell morphology. However, treatment of ARNO transfectants with the PKC agonist phorbol 12-myristate 13-acetate results in the dramatic redistribution of ARNO, ARF6, and actin into membrane protrusions resembling lamellipodia. This process requires ARF activation, as actin rearrangement does not occur in cells expressing a catalytically inactive ARNO mutant. PKC phosphorylates ARNO at a site immediately C-terminal to its PH domain. However, mutation of this site had no effect on the ability of ARNO to regulate actin rearrangement, suggesting that phosphorylation of ARNO by PKC does not positively regulate its activity. Finally, we demonstrate that an ARNO mutant lacking the C-terminal PH domain no longer mediates cytoskeletal reorganization, indicating a role for this domain in appropriate membrane localization. Taken together, these data suggest that ARNO represents an important link between cell surface receptors, ARF6, and the actin cytoskeleton.

US family physicians' experiences with practice guidelines

BACKGROUND

Practice guidelines were developed to improve medical outcomes and cost-effectiveness. The experiences of family physicians, who may need to use multiple guidelines in their practices, are crucial for effective development and implementation of practice guidelines. We surveyed a national sample of US family physicians about factors that affect their adoption and use of practice guidelines.

METHODS

We mailed a structured survey to a national random sample of 400 family physician members of the American Academy of Family Physicians.

RESULTS

The response rate was 51%. Most respondents (69%) reported a positive attitude about practice guidelines, but only 44% reported using any guidelines. More younger physicians thought that guidelines could be useful tools. Most preferred guidelines that could be modified (87%) and that were no longer than two pages. Only 27% of respondents knew where to locate a guideline on a particular topic. Forty-three percent of respondents reported that it would be useful if guidelines were a component of an electronic medical record.

CONCLUSIONS

If guidelines are to be used by practicing family physicians, a generalist perspective needs to be considered in future guideline development and implementation. Younger physicians had more positive attitudes toward guidelines.

ARNO is a guanine nucleotide exchange factor for ADP-ribosylation factor 6

ADP-ribosylation factors (ARFs) constitute a family of small monomeric GTPases. ARFs 1 and 3 function in the recruitment of coat proteins to membranes of the Golgi apparatus, whereas ARF6 is localized to the plasma membrane, where it appears to modulate both the assembly of the actin cytoskeleton and endocytosis. Like other GTPases, ARF activation is facilitated by specific guanine nucleotide exchange factors (GEFs). ARNO (ARF nucleotide-binding site opener) is a member of a growing family of ARF-GEFs that share a common, tripartite structure consisting of an N-terminal coiled-coil domain, a central domain with homology to the yeast protein Sec7p, and a C-terminal pleckstrin homology domain. Recently, ARNO and its close homologue cytohesin-1 were found to catalyze in vitro nucleotide exchange on ARF1 and ARF3, respectively, raising the possibility that these GEFs function in the Golgi. However, the actual function of these proteins may be determined in part by their ability to interact with specific ARFs and in part by their subcellular localization. We report here that in vitro ARNO can stimulate nucleotide exchange on both ARF1 and ARF6. Furthermore, based on subcellular fractionation and immunolocalization experiments, we find that ARNO is localized to the plasma membrane in mammalian cells rather than the Golgi. It is therefore likely that ARNO functions in plasma membrane events by modulating the activity of ARF6 in vivo. These findings are consistent with the previous observation that cytohesin-1 regulates the adhesiveness of alphaLbeta2 integrins at the plasma membrane of lymphocytes.

Cloning and characterization of human phosphomannomutase, a mammalian homologue of yeast SEC53

Phosphomannomutase (PMM) catalyzes the conversion of mannose-6-phosphate to mannose-1-phosphate, which is a substrate for the synthesis of GDP-mannose. This nucleotide sugar is then used in the synthesis of dolichol-phosphate-mannose, which is essential for N-linked glycosylation and thus the secretion of several glycoproteins as well as for the synthesis of glycosyl-phosphatidyl-inositol (GPI) anchored proteins. In the yeast Saccharomyces cerevisiae, SEC53, a gene required for viability, encodes PMM. Given the importance of PMM in glycoprotein synthesis, it is surprising that very little is known about the enzyme in higher eukaryotes. Recently, an autosomal recessive human disease, Carbohydrate-deficient glycoprotein syndrome type I (CDGS-I) has been correlated with severely reduced PMM activity. Here we report the isolation of a cDNA encoding human PMM, a protein of 29 kDa that is 55% identical and 66% similar to yeast Sec53p. Northern blot analysis shows a single 1.4 kb transcript that is ubiquitously expressed, although levels vary markedly among tissues. Expression of the human cDNA in a temperature-sensitive mutant sec53 yeast strain confers growth at the restrictive temperature, strongly suggesting that this gene encodes a functional PMM. Finally, when expressed in BHK cells, PMM is localized exclusively to the cytosol corresponding to its localization in yeast.

Altered hepatic transport of immunoglobulin A in mice lacking the J chain

We have created J chain knockout mice to define the physiologic role of the J chain in immunoglobulin synthesis and transport. The J chain is covalently associated with pentameric immunoglobulin (Ig) M and dimeric IgA and is also expressed in most IgG-secreting cells. J chain-deficient mice have normal serum IgM and IgG levels but markedly elevated serum IgA. Although polymeric IgA was present in the mutant mice, a larger proportion of their serum IgA was monomeric than was found in wild-type mouse serum. Bile and fecal IgA levels were decreased in J chain-deficient mice compared with wild-type mice, suggesting inefficient transport of J chain-deficient IgA by hepatic polymeric immunoglobulin receptors (pIgR). The pIgR-mediated transport of serum-derived IgA from wild-type and mutant mice was assessed in Madin-Darby canine kidney (MDCK) cells transfected with the pIgR. These studies revealed selective transport by pIgR-expressing MDCK cells of wild-type IgA but not J chain-deficient IgA. We conclude that although the J chain is not required for IgA dimerization, it does affect the efficiency of polymerization or have a role in maintaining IgA dimer stability. Furthermore, the J chain is essential for efficient hepatic pIgR transport of IgA.

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits transcytosis in polarized epithelial cells

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits both basolateral to apical and apical to basolateral transcytosis of ricin in Fisher rat thyroid (FRT) cells by 50% at 100 nM in a continuous transcytosis assay. In MDCK cells, a similar effect of wortmannin on basolateral to apical transcytosis of ricin was found, whereas apical to basolateral transcytosis was inhibited to a lesser degree. Transcytosis of dimeric IgA in MDCK cells expressing the polymeric immunoglobulin receptor was also reduced to 50% of controls, suggesting that wortmannin inhibits membrane translocation rather than sorting of specific proteins in the transcytotic pathway. This effect of wortmannin is selective, however, in that endocytosis at the basolateral domain and recycling at both the basolateral and apical membrane domains are unaffected, and apical endocytosis and apical secretion are only moderately reduced. We have shown previously that cAMP stimulates a late stage in basolateral to apical transcytosis in MDCK cells through activation of protein kinase A (Hansen, S. H., and Casanova, J.E. (1994) J. Cell Biol. 126, 677-687). Elevation of cellular cAMP still induced a 100% increase in transcytosis in wortmannin-treated cells, but transcytosis was no longer increased when compared to cells which received no drugs. In contrast, in experiments using a 17 degrees C block to accumulate ricin internalized from the basolateral surface in the apical compartment of MDCK cells, wortmannin had little effect on the stimulation of transcytosis by activators of protein kinase A observed under these conditions. The data thus suggest the existence of a wortmannin-sensitive step in the transcytotic pathway, positioned after endocytosis but prior to translocation into the protein kinase A-sensitive apical compartment, implying a role for phosphoinositide 3-kinase in an intermediate step in transcytosis in polarized epithelial cells.

Evidence for a regulated interaction between heterotrimeric G proteins and caveolin

Caveolae are flask-shaped plasma membrane specializations. A 22-kDa protein, caveolin, is a principal component of caveolar membranes in vivo. As recent evidence suggests that caveolae may participate in G protein-coupled signaling events, we have investigated the potential interaction of caveolin with heterotrimeric G proteins. Using cell fractionation techniques, we found that mutational or pharmacologic activation of Gs alpha prevents its cofractionation with caveolin. In a second independent approach, we directly examined the interaction of G proteins with caveolin. For this purpose, we recombinantly expressed caveolin as a glutathione S-transferase fusion protein. Using an in vitro binding assay, we found that caveolin interacts with G protein alpha subunits (Gs, Go, and Gi). Mutational or pharmacologic activation (with guanosine 5'-O-(thiotriphosphate)) of G alpha subunits prevents this interaction, indicating that the inactive GDP-bound form of G alpha subunits preferentially interacts with caveolin. This G protein binding activity is located within a 41-amino acid region of caveolin's cytoplasmic N-terminal domain (residues 61-101). Further functional analysis shows that a polypeptide derived from this region of caveolin (residues 82-101) effectively suppresses the basal activity of purified G proteins, apparently by inhibiting GDP/GTP exchange. This caveolin sequence is homologous to a region of the Rab GDP dissociation inhibitor, a known inhibitor of GDP/GTP exchange for Rab proteins. These data suggest that caveolin could function to negatively regulate the activation state of heterotrimeric G proteins.

Mutational analysis of polymeric immunoglobulin receptor/ligand interactions. Evidence for the involvement of multiple complementarity determining region (CDR)-like loops in receptor domain I

The polymeric Ig receptor (pIgR) mediates the transport of IgA and IgM across a variety of mucosal epithelia. The ectodomain of this receptor consists of five immunoglobulin-like domains (I-V), the first four being structurally similar to immunoglobulin variable regions, and the fifth to Ig constant regions. This study examines the structural features of the pIgR that participate in binding of the ligand, dimeric IgA (dIgA). Recent evidence suggests that a highly conserved region of the first Ig-like domain (domain I) may be important in this process (Bakos, M.A., Kurosky, A., and Goldblum, R. M. (1991) J. Immunol. 147, 3419-3426). In support of this hypothesis, molecular modeling of domain I places this conserved region in an exposed loop analogous to the CDR1 loop of Ig, suggesting that interactions between dIgA and the pIgR may be similar to those between antibodies and their cognate antigens. To test this hypothesis directly, we performed a mutagenic analysis of all three CDR-like loops in domain I of the pIgR. We found that point mutations in multiple residues of CDR1 produced effects on IgA binding ranging from minimal (90% of control) to profound (7%). In addition, we replaced regions corresponding to the CDR2 and CDR3 loops of domain I with their counterparts from domain II (which does not bind IgA), which in both cases resulted in complete abrogation of IgA binding. Taken together, these data suggest that each of the three CDR-like loops of domain I of the rabbit pIgR participates in the binding of dimeric IgA.

In vitro phosphorylation of caveolin-rich membrane domains: identification of an associated serine kinase activity as a casein kinase II-like enzyme

Caveolae are flask-shaped micro-invaginations associated with the plasma membrane of a wide variety of cell types. Caveolin, an integral membrane component of caveolae, was first identified as the major phosphoprotein whose phosphorylation was elevated in v-Src transformed cells. As both v-Src transformation and elevated caveolin phosphorylation were dependent on membrane attachment of v-Src, it has been suggested that caveolin is a critical target in v-Src transformation. Although an increase in tyrosine phosphorylation of caveolin was evident, the increase in caveolin phosphorylation was predominantly on serine residues. In accordance with these in vivo observations, isolated caveolin-rich membrane domains undergo phosphorylation in vitro predominantly on serine and contain an unidentified serine kinase activity. Here, we have identified this serine kinase activity as a casein kinase II-like enzyme, since the phosphorylation of caveolin-rich membrane domains is stimulated and inhibited by known effectors of casein kinase II (poly-L-lysine, endogenous polyamines, and a casein kinase II inhibitor peptide), but is unaffected by modulators of other known kinases. In support of these observations, caveolin contains a consensus sequence for casein kinase II phosphorylation in its cytoplasmic N-terminal domain (Ser-88). A peptide containing this sequence inhibits the in vitro phosphorylation of caveolin-rich membrane domains, while many other peptides derived from the N-terminal domain of caveolin do not affect phosphorylation. Caveolin-rich membrane domains were also a substrate for exogenously added purified casein kinase II, but not casein kinase I. Finally, immunoblotting of these domains with an antibody directed against the alpha and alpha' subunits of casein kinase II reveals two bands with apparent molecular weights consistent with the known molecular weights of the alpha and alpha' subunits of casein kinase II. As casein kinase II appears to play a role in mitogenic signalling events and casein kinase II activators (endogenous polyamines) are required for v-Src transformation, our results may have implications for understanding the mechanism of v-Src oncogenesis.

Gs alpha stimulates transcytosis and apical secretion in MDCK cells through cAMP and protein kinase A

Recent evidence suggests a role for heterotrimeric G proteins in vesicular transport. Cholera toxin, which activates Gs alpha by ADP-ribosylation, has been reported to stimulate both apical secretion (Pimplikar, S.W., and K. Simons. 1993. Nature (Lond.). 352:456-458) and apically directed transcytosis (Bomsel, M., and K.E. Mostov. 1993. J. Biol. Chem. 268:25824-25835) in MDCK cells, via a cAMP-independent mechanism. Here, we demonstrate that apical secretion and apically directed transcytosis are significantly stimulated by agents that elevate cellular cAMP. Forskolin, which activates adenylyl cyclase directly, and 8BrcAMP augment both transport processes in MDCK cells. The increase is not limited to receptor-mediated transport (polymeric Ig receptor), since transcytosis of ricin, a galactose-binding lectin, is similarly stimulated. The effects of elevated cellular cAMP on apical secretion and transcytosis are apparently mediated via protein kinase A (PKA), as they are inhibited by H-89, a selective PKA inhibitor. Experiments employing a 17 degrees C temperature block indicate that cAMP/PKA acts at a late, possibly rate-limiting stage in the transcytotic pathway, after translocation of internalized markers into the apical cytoplasm. However, no significant stimulus of apical recycling was observed in the presence of FSK, suggesting that cAMP/PKA either affects transcytosis at a level proximal to apical early endosomes and/or specifically increases the efficiency by which transcytosing molecules are delivered to the apical plasma membrane. Finally, we overexpressed wild-type Gs alpha and a mutant, Q227L, which constitutively activates adenylyl cyclase, in MDCK cells. Although Q227L increased transcytosis more than wild-type Gs alpha, neither construct was as effective as FSK in stimulating transcytosis, arguing against a significant role of Gs alpha in transcytosis independent of cAMP and PKA.

Hand dexterity in hospital personnel with multiple needlestick injuries

OBJECTIVE

To test the hypothesis that multiple needlestick injuries in hospital employees may be due to intrinsic deficits in hand dexterity.

DESIGN

A case-control study comparing employees with multiple reported needlestick injuries to those with none. Hand dexterity was tested using the Purdue Pegboard Test, a standardized validated test of hand dexterity.

SETTING

A 300-bed, acute care teaching hospital.

PARTICIPANTS

Fifteen hospital employees who sustained four or more injuries were compared to 19 controls.

RESULTS

No differences were detected in hand dexterity between the case and control groups.

CONCLUSIONS

While employees with multiple needlestick injuries accounted for 85% of reported injuries, underlying factors responsible for injuries in this high-risk subgroup do not include measurable deficits in hand dexterity.

Utility of pulmonary function testing in the management of chronic obstructive pulmonary disease

This study sought to measure the effect of pulmonary function testing (PFT) data on the decisions made by generalist physicians in the management of chronic obstructive pulmonary disease (COPD). 148 physicians were randomly assigned to two groups, both of which were asked to manage two identical fictitious but representative cases of COPD, which included history, physical, x-ray, and laboratory results. The experimental group received PFT results in addition. No significant difference was noted between the two groups in management based on availability of PFT data. The optimum utility of PFT data in the management of COPD may be exaggerated and has yet to be determined.

Lessons from the Soviet health care system

The United States and the former Soviet Union have historically organized health care delivery systems according to totally different paradigms. These two divergent approaches have constituted a kind of natural experiment. At the present time, our systems may be becoming more alike, with the former Soviet system decentralizing and even experimenting with forms of medical insurance. Our system, on the other hand, has become much more regulated and, if some have their way, would become increasingly monopsonistic. At this critical point, it may be useful to learn from each other's experiences as we plan for the future.

Direct apical sorting of rat liver dipeptidylpeptidase IV expressed in Madin-Darby canine kidney cells

In Madin-Darby canine kidney (MDCK) cells, apical and basolateral membrane proteins are segregated from each other in the trans-Golgi network (TGN) and are transported to the appropriate membrane domain via separate vesicle populations. In hepatocytes, however, all plasma membrane proteins are delivered basolaterally. Apical proteins are then selectively retrieved and reach the apical surface by transcytosis. The sorting of apical proteins in different cell types may be the result of differences in the cellular sorting machinery, or alternatively, due to expression of cell-specific sorting signals on the proteins themselves. To test this directly, we have stably expressed cDNA encoding an apical protein from rat liver, dipeptidylpeptidase IV (DPPIV), in MDCK cells. We found that approximately 90% of the exogenous DPPIV is expressed on the apical cell surface at steady state. Furthermore, we demonstrate that this distribution is primarily due to vectorial transport from the TGN to the apical plasma membrane. The small pool of mis-sorted DPPIV that appears basolaterally is slowly endocytosed (t1/2 approximately 60 min) and is subsequently transcytosed. These data are consistent with the notion that both hepatocytes and MDCK cells are capable of correctly sorting rat liver DPPIV, but that this sorting occurs at different sites in the two cell types.

Hand function in patients with diabetes mellitus

Using validated clinical tests and standardized controls, we measured hand function in diabetic subjects. Fifteen diabetic patients (median age 48 years), all having used insulin for a minimum of 5 years, were randomly selected from inpatient and outpatient services of an academic medical center. Testing was done on an outpatient basis. The three hand function tests used were the Purdue Pegboard, O'Connor Tweezer Dexterity, and Smith Hand Function. A symptom questionnaire was also included in the study. Function was significantly decreased in the diabetic group, and the decrease was out of proportion to their own subjective pretest assessments. These findings may have implications for occupational performance as well as activities of daily living.

An autonomous signal for basolateral sorting in the cytoplasmic domain of the polymeric immunoglobulin receptor

The polymeric immunoglobulin receptor is normally delivered from the Golgi to the basolateral surface of epithelial cells and then transports polymeric IgA and IgM to the apical surface. We now report that a 14 residue segment of the 103 amino acid cytoplasmic domain, proximal to the plasma membrane, directs the receptor to the basolateral surface. A mutant receptor lacking these 14 amino acids is sorted directly to the apical surface from the Golgi. Furthermore, this sequence is sufficient to redirect an apical membrane protein, placental alkaline phosphatase, to the basolateral plasma membrane. We conclude that this sequence contains an autonomous signal, which specifies sorting from the Golgi to the basolateral surface, a process previously postulated to occur by default.

Deletions in the cytoplasmic domain of the polymeric immunoglobulin receptor differentially affect endocytotic rate and postendocytotic traffic

We have examined the function of the cytoplasmic domain of the polymeric immunoglobulin receptor (pIg-R) by producing two separate deletions in the cytoplasmic domain of the pIg-R, expressing the mutant receptors in polarized MDCK cells, and analyzing each for their effects on receptor and ligand traffic. Deletion of the C-terminal 30 amino acids (726-755) reduces the rate of internalization of receptor-bound ligand from the basolateral surface. However, this mutation has no effect on delivery of receptor from the Golgi to the basolateral surface or the post-endocytotic traffic of receptor and ligand. Mutation of a tyrosine at position 734 to serine produces a receptor with a similar phenotype. If residues 670-707 are deleted from the middle of the cytoplasmic domain, both basolateral delivery and internalization are unaffected. However, unlike wild type, after endocytosis from the basolateral surface, both receptor and ligand are largely degraded. We reported previously that deletion of the entire cytoplasmic domain prevents the basolateral delivery of newly synthesized receptor (Mostov, K.E., de Bruyn Kops, A., and Deitcher, D.L. (1986) Cell 47, 359-364). In contrast, the mutants reported here are delivered to the basolateral surface, suggesting that only residues 653-669 and/or 708-725 are necessary for basolateral delivery. Thus, different deletions in the cytoplasmic domain of the pIg-R can produce mutant receptors which alter different aspects of receptor traffic.

Phosphorylation of the polymeric immunoglobulin receptor required for its efficient transcytosis

The endosomal compartment of polarized epithelial cells is a major crossroads for membrane traffic. Proteins entering this compartment from the cell surface are sorted for transport to one of several destinations: recycling to the original cell surface, targeting to lysosomes for degradation, or transcytosis to the opposite surface. The polymeric immunoglobulin receptor (pIgR), which is normally transcytosed from the basolateral to the apical surface, was used as a model to dissect the signals that mediate this sorting event. When exogenous receptor was expressed in Madin-Darby Canine Kidney (MDCK) cells, it was shown that phosphorylation of pIgR at the serine residue at position 664 is required for efficient transcytosis. Replacement of this serine with alanine generated a receptor that is transcytosed only slowly, and appears to be recycled. Conversely, substitution with aspartic acid (which mimics the negative charge of the phosphate group) results in rapid transcytosis. It was concluded that phosphorylation is the signal that directs the pIgR from the endosome into the transcytotic pathway.

Plasminogen activator expression in F9 teratocarcinoma embryoid bodies and their endoderm derivatives

Plasminogen activators are believed to play an important role in tissue remodeling and cell migration. During mouse embryogenesis, visceral endoderm secretes urokinase-type plasminogen activator (uPA) whereas parietal endoderm secretes tissue-type plasminogen activator (tPA). Visceral endoderm from F9 embryoid bodies can transdifferentiate into parietal endoderm under the appropriate culture conditions. We have examined at the protein and mRNA levels the type of plasminogen activator expressed in whole embryoid bodies, visceral endoderm and its parietal endoderm derivatives. Our experiments show that the visceral endoderm on F9 embryoid bodies synthesizes and secretes substantial amounts of both tPA and uPA. In contrast, the parietal endoderm derived directly from the visceral endoderm secretes dramatically increased levels of tPA and decreases production of uPA to low or below detectable levels. These data support the finding that visceral endoderm can transdifferentiate to parietal endoderm. In addition, this transition provides an excellent model for studying the molecular basis of the coincident down- and upregulation of the two plasminogen activators as well as their potential function during embryogenesis.

Bronchial hyperreactivity and cough due to angiotensin-converting enzyme inhibitors

Seventeen patients using angiotensin-converting enzyme (ACE) inhibitors for hypertension were evaluated with baseline spirometry followed by determination of bronchial reactivity by challenge with methacholine. There were nine coughers and eight noncoughers in the study. Among the nine coughers, eight demonstrated bronchial hyperreactivity. Conversely, none of the noncoughers disclosed bronchial hyperreactivity. Eight of the nine coughers were rechallenged two to six months following cessation of ACE inhibitor therapy. Six of these eight showed persistent bronchial hyperreactivity. We conclude that cough is associated with the use of ACE inhibitors in patients with underlying bronchial hyperreactivity. The findings indicate caution in administration of ACE inhibitors in asthmatic patients and those with known bronchial hyperreactivity.

The role of cell interactions in the differentiation of teratocarcinoma-derived parietal and visceral endoderm

Cell interactions have been implicated in the differentiation of visceral and parietal endoderm in the developing mouse embryo. Embryoid bodies formed from F9 embryonal carcinoma cells have been useful in characterizing the events which lead to endoderm formation. As part of our effort to specify the interactions which may be involved in this process we have isolated visceral endoderm-like cells (VE) from F9 embryoid bodies and cultured them under various conditions. Using a combination of immunoprecipitation and enzyme-linked immunosorbent assay, we demonstrate that monolayer culture of these cells on a number of different substrates leads to a dramatic decrease in the level of alphafetoprotein (AFP), a VE-specific marker. Northern blot analysis of AFP mRNA indicates very low levels of this message are present after 48 hr in monolayer culture. Coincident with the drop in AFP levels is an increase in the levels of the cytokeratin Endo C and tissue plasminogen activator, both markers for parietal endoderm (PE). Morphological evidence at the ultrastructural level supports a transition from VE to PE. In contrast, the VE phenotype can be maintained in vitro by interaction with aggregates, but not monolayers, of stem cells. In addition, culturing the cells on the curved surface of gelatin-coated dextran beads, but not on a flat gelatin surface facilitates AFP expression and the cells are morphologically intermediate between VE and PE cells. The potential role of junctional complexes and cell shape are discussed.

The outgrowth of parietal endoderm from mouse teratocarcinoma stem-cell embryoid bodies

Teratocarcinoma stem cells can be used to study certain events occurring during early mouse embryogenesis. We report that the outgrowth of parietal endoderm from teratocarcinoma stem-cell embryoid bodies in vitro is analogous to the same process in vivo in terms of the spatial distribution of endoderm types: only parietal endoderm migrates away from the aggregate, whereas visceral endoderm remains associated with the embryoid body. The outgrowths generated on a substrate of type-I collagen from PSA-1 and retinoic-acid-treated F 9 embryoid bodies were found to be comparable, even though these aggregates express different endoderm types. We demonstrated that retinoic-acid-treated F 9 embryoid bodies that contain essentially only visceral endoderm in suspension culture can nonetheless generate parietal-endoderm outgrowth when plated on type-I collagen, suggesting that substrate interaction plays an important role in inducing parietal-endoderm differentiation. These data indicate the usefulness and relevance of studying endoderm differentiation and outgrowth in vitro employing the teratocarcinoma model system.