The many ways to cross the plasma membrane.
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Biological Transport↗
Biomedical subjects
D J Stephens
Publications and source records attributed to D J Stephens.
Illuminating the secretory pathway: when do we need vesicles?
Recent studies using GFP-tagged markers and time-lapse microscopy have allowed direct visualisation of membrane traffic in the secretory pathway in living mammalian cells. This work shows that larger membrane structures, 300-500 nm in size, are the vehicles responsible for long distance, microtubule-dependent ER-to-Golgi and trans-Golgi to plasma membrane transport of secretory markers. At least two retrograde transport pathways from the Golgi to the ER exist, both of which are proposed to involve a further class of long, tubular membrane carrier that forms from the Golgi and fuses with the ER. Together, this has challenged established transport models, raising the question of whether larger pleiomorphic structures, rather than small 60-80 nm transport vesicles, mediate long-range transport between the ER and Golgi and between the Golgi and plasma membrane.
Animals↗
In vivo dynamics of the F-actin-binding protein neurabin-II.
Neurabin-II (spinophilin) is a ubiquitously expressed F-actin-binding protein containing an N-terminal actin-binding domain, a PDZ (PSD95/discs large/ZO-1) domain and a C-terminal domain predicted to form a coiled-coil structure. We have stably expressed a green fluorescent protein (GFP)-tagged version of neurabin-II in PC12 cells, and characterized the in vivo dynamics of this actin-binding protein using confocal fluorescence microscopy. We show that GFP-neurabin-II localizes to actin filaments, especially at cortical sites and areas underlying sites of active membrane remodelling. GFP-neurabin-II labels only a subset of F-actin within these cells, as indicated by rhodamine-phalloidin staining. Both actin filaments and small, highly motile structures within the cell body are seen. Photobleaching experiments show that GFP-neurabin-II also exhibits highly dynamic behaviour when bound to actin filaments. Latrunculin B treatment results in rapid relocalization of GFP-neurabin-II to the cytosol, whereas cytochalasin D treatment causes the collapse of GFP-neurabin-II fluorescence to intensely fluorescent foci of F-actin within the cell body. This collapse is reversed on cytochalasin D removal, recovery from which is greatly accelerated by stimulation of cells with epidermal growth factor (EGF). Furthermore, we show that this EGF-induced relocalization of GFP-neurabin-II is dependent on the activity of the small GTPase Rac1 but not the activity of ADP-ribosylation factor 6.
ADP-Ribosylation Factor 6↗
The use of yeast two-hybrid screens in studies of protein:protein interactions involved in trafficking.
The yeast two-hybrid system has provided a convenient means to both screen for proteins that interact with a protein of interest and to characterise the known interaction between two proteins. Several groups with an interest in the molecular mechanisms that underlie discrete steps along trafficking pathways have exploited the yeast two-hybrid system. Here, we provide a brief background to the technology, attempt to point out some of the pitfalls and benefits of the different systems that can be employed, and mention some of the areas (within the trafficking field) where yeast two-hybrid interaction assays have been particularly informative.
Protein Binding↗
COPI-coated ER-to-Golgi transport complexes segregate from COPII in close proximity to ER exit sites.
Transport of proteins between the endoplasmic reticulum and Golgi apparatus is mediated by two distinct membrane coat complexes, COPI and COPII. Genetic, biochemical and morphological data have accumulated into a model which suggests a sequential mode of action with COPII mediating the selection of cargo and formation of transport vesicles at the ER membrane for ER-to-Golgi transport and COPI mediating recycling of the transport machinery from post-ER membranes. To test this transport model directly in vivo, and to study the precise temporal sequence of COPI and COPII action in ER-to-Golgi transport, we have used time lapse microscopy of living cells to visualise simultaneously the dynamics of COPII and COPI, as well as COPII and GFP tagged secretory markers in living cells. The majority of COPII labelling appears tightly associated with ER membranes that move only within a limited area (less than 2 microm). Secretory cargo segregates from these sites and is then transported to the Golgi apparatus without any apparent association with COPII. COPI-coated transport complexes are seen to form adjacent to the COPII sites on the ER before segregating and moving directionally towards the Golgi apparatus. COPII is not present on these transport complexes and remains associated with the ER. These data demonstrate for the first time directly in vivo that ER-to-Golgi transport is organised in two steps characterised by a sequential mode of action of COPII and COPI.
Animals↗
Lumenal targeted GFP, used as a marker of soluble cargo, visualises rapid ERGIC to Golgi traffic by a tubulo-vesicular network.
The mechanism by which soluble proteins without sorting motifs are transported to the cell surface is not clear. Here we show that soluble green fluorescent protein (GFP) targeted to the lumen of the endoplasmic reticulum but lacking any known retrieval, retention or targeting motifs, was accumulated in the lumen of the ERGIC if cells were kept at reduced temperature. Upon activation of anterograde transport by rewarming of cells, lumenal GFP stained a microtubule-dependent, pre-Golgi tubulo-vesicular network that served as transport structure between peripheral ERGIC-elements and the perinuclear Golgi complex. Individual examples of these tubular elements up to 20 microm in length were observed. Time lapse imaging indicated rapid anterograde flow of soluble lumenal GFP through this network. Transport tubules, stained by lumenal GFP, segregated rapidly from COPI-positive membranes after transport activation. A transmembrane cargo marker, the temperature sensitive glycoprotein of the vesicular stomatitis virus, ts-045 G, is also not present in tubules which contained the soluble cargo marker lum-GFP. These results suggest a role for pre-Golgi vesicular tubular membranes in long distance anterograde transport of soluble cargo. http://www. biologists.com/JCS/movies/jcs1334.html
Animals↗
Direct interaction of the trans-Golgi network membrane protein, TGN38, with the F-actin binding protein, neurabin.
TGN38 is a type I integral membrane protein that constitutively cycles between the trans-Golgi network (TGN) and plasma membrane. The cytosolic domain of TGN38 interacts with AP2 clathrin adaptor complexes via the tyrosine-containing motif (-SDYQRL-) to direct internalization from the plasma membrane. This motif has previously been shown to direct both internalization and subsequent TGN targeting of TGN38. We have used the cytosolic domain of TGN38 in a two-hybrid screen, and we have identified the brain-specific F-actin binding protein neurabin-I as a TGN38-binding protein. We demonstrate a direct interaction between TGN38 and the ubiquitous homologue of neurabin-I, neurabin-II (also called spinophilin). We have used a combination of yeast two-hybrid and in vitro protein interaction assays to show that this interaction is dependent on the serine (but not tyrosine) residue of the known TGN38 trafficking motif. We show that TGN38 interacts with the coiled coil region of neurabin in vitro and binds preferentially with the dimeric form of neurabin. TGN38 and neurabin also interact in vivo as demonstrated by coimmunoprecipitation from stably transfected PC12 cells. These data suggest that neurabin provides a direct physical link between TGN38-containing membranes and the actin cytoskeleton.
Actins↗
The role of cholesterol in the biosynthesis of beta-amyloid.
Addition of the beta-hydroxy-beta-methylglutaryl-CoA (HmG-CoA) reductase inhibitor lovastatin to human HEK cells transfected with the amyloid precursor protein (APP) reduces intracellular cholesterol/protein ratios by 50%, and markedly inhibits beta-secretase cleavage of newly-synthesized APP. Exogenous water-solubilized cholesterol at 200 microg/ml concentration increases newly synthesized beta-amyloidogenic products four-fold. These intracellular changes are detectable by immunoprecipitation and immunofluorescent labelling. Analyses of the fragments captured from culture medium by an N-terminal anti-beta-amyloid antibody on ProteinChip arrays and detected using surface-enhanced laser desorption/ionization (SELDI) mass spectrometry revealed that culture with cholesterol (200 microg/ml) increased secretion of beta-amyloid 1-40 by 1.8-fold, and increased secretion of beta-amyloid 1-42. Changes in APP processing by cholesterol may mediate the way in which the ApoE4 allele increases risk of developing Alzheimer's disease (AD) in western populations.
Amyloid beta-Peptides↗
Specificity of interaction between adaptor-complex medium chains and the tyrosine-based sorting motifs of TGN38 and lgp120.
Multiple sorting steps within eukaryotic cells are mediated by tyrosine-based sorting motifs. Motifs conforming to the consensus -YXXO- (where O indicates a bulky hydrophobic residue) have been shown to specify high-efficiency internalization from the plasma membrane, targeting from the plasma membrane to the trans-Golgi network and targeting to lysosomal compartments as well as being involved in basolateral sorting in polarized cells. These motifs are recognized by the medium-chain subunits of heterotetrameric adaptor complexes. Whereas these motifs have been shown to be sufficient to mediate interaction with the mu-chains, we and others have shown that their context is important in determining the affinity of interaction. In this study we have investigated the interaction between the tyrosine motifs of the type-1 integral membrane proteins TGN38 and lgp120 with medium-chain subunits using the yeast two-hybrid system. Whereas the wild-type version of the cytosolic domain of TGN38 interacts with highest affinity with mu2, we show that the cytosolic domain of lgp120 interacts almost exclusively with mu3A. The specificity of binding of tyrosine-based sorting motifs to mu-chains is shown to be highly sensitive to the context in which the motif lies. For example, the -YQTI- motif of lgp120 is effectively non-functional with regard to mu-chain binding when placed in the context of the TGN38 cytosolic domain. Deletion of four amino acids (NLKL) at the extreme C-terminus of TGN38, leaving the YXXO motif as the C-terminus, greatly enhances the affinity of interaction with mu2. Furthermore, addition of these same residues to the extreme C-terminus of lgp120 effectively abolishes the interaction of the cytosolic domain of lgp120 with mu-chains. We also show that the newly identified mu-adaptin-related protein 2 (mu4) only interacts weakly with tyrosine-based sorting motifs.
Amino Acid Sequence↗
Inhibition of the interaction between tyrosine-based motifs and the medium chain subunit of the AP-2 adaptor complex by specific tyrphostins.
Several intracellular membrane trafficking events are mediated by tyrosine-containing motifs found within the cytosolic domains of certain integral membrane proteins. Many of these tyrosine motifs conform to the consensus YXXPhi (where Phi represents a bulky hydrophobic residue). This YXXPhi motif has been shown to interact with the medium chain subunits of adaptor complexes that generally link relevant integral membrane protein cytosolic domains to the clathrin coat involved in vesicle formation. The motif YXXPhi is also very similar to motifs that are targets for phosphorylation by tyrosine kinases. Tyrosine kinase inhibitors known as tyrphostins are structural analogues of tyrosine, and so it is possible that tyrphostins could also inhibit interactions between medium chains and YXXPhi motifs. TGN38 is a type I integral membrane protein containing a tyrosine motif, YQRL, within the cytosolic domain. We have previously shown that this motif interacts directly with the medium chain subunit of the plasma membrane localized AP-2 adaptor complex (mu2). We have investigated a range of tyrphostins and demonstrated a specific inhibition of the interaction between mu2 and the TGN38 cytosolic domain by tyrphostin A23 through in vitro analysis and the yeast two-hybrid system. These data raise the exciting possibility that different membrane traffic events could be inhibited by specific tyrphostins.
Adaptor Protein Complex 1↗
Characterization of beta-secretase.
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Alzheimer Disease↗
Insulin dependent tyrosine phosphorylation of the tyrosine internalisation motif of TGN38 creates a specific SH2 domain binding site.
Tyrosine-based motifs are involved in both protein targeting and, via SH2 domain binding, intracellular signalling. To date there has only been one example of such a motif acting as both an intracellular sorting signal and SH2 binding determinant, namely that of the T cell costimulation receptor, CTLA-4. We show that insulin stimulation of cultured rat hepatoma cells results in increased cell surface expression of TGN38. Furthermore, the cytosolic domain of TGN38 can be phosphorylated by the insulin receptor in vitro and tyrosine phosphorylated TGN38 can specifically bind to the SH2 domains of the spleen tyrosine kinase Syk. These data imply that tyrosine-based motifs may play a broader role than has previously been accepted and could help to integrate trafficking and signalling events.
Amino Acid Sequence↗
Serine 331 and tyrosine 333 are both involved in the interaction between the cytosolic domain of TGN38 and the mu2 subunit of the AP2 clathrin adaptor complex.
TGN38 is a type I integral membrane protein that cycles between the trans-Golgi network and the plasma membrane. Internalization at the cell surface and targeting back to the trans-Golgi network is dependent on a hexapeptide motif, SDYQRL, in the cytosolic tail of the protein. It was recently demonstrated that this motif specifically interacts with the mu2 subunit of the AP2 adaptor complex. We have studied the interaction between the entire cytosolic domain of TGN38 and mu2 using the yeast two hybrid system, in vitro binding of recombinant fusion proteins and IAsys optical biosensor technology. A specific interaction has been demonstrated in each of the systems we have employed. We have shown an absolute requirement for Tyr-333 of TGN38 in binding to mu2. In addition we found that mutation of Ser-331 to alanine reduces the affinity of the interaction. By measuring tryptophan fluorescence at equilibrium, we have also determined the dissociation constant for the interaction between the entire cytosolic tail of TGN38 and mu2 as 58 nM. In contrast to previously published work, our data suggest that not only Tyr-333 but also its context is important in determining the specificity of binding of TGN38 to mu2.
Adaptor Protein Complex 1↗
Metabolites of the beta-amyloid precursor protein generated by beta-secretase localise to the trans-Golgi network and late endosome in 293 cells.
Deposition of beta-amyloid occurs in the brains of all sufferers of Alzheimer's disease. beta-amyloid is proteolytically derived from the beta-amyloid precursor protein by as yet unidentified enzymes termed secretases. We have generated and characterised antisera to the carboxy-terminal domain and beta-secretase cleavage site of the Alzheimer's amyloid precursor protein. The beta-secretase cleavage event occurs at the extreme N-terminus of the beta-amyloid peptide. Our antiserum to the N-terminus of the beta-amyloid peptide (NT beta 4) specifically recognises beta-secretase cleaved species as opposed to intact beta APP. NT beta 4 specifically immunoprecipitates a 13 kDa fragment of beta APP (p13) which is potentially amyloidogenic. We have used these antisera in confocal laser scanning immunofluorescence microscopy to localise the intracellular location of potentially amyloidogenic beta APP processing fragments such as p13. Using a number of marker antisera of known intracellular location, we have defined the major location of beta APP fragments possessing the Asp-1 N-terminus of beta-amyloid as the trans-Golgi network or late endosome on the basis of colocalisation with a monoclonal antibody to the cation-independent mannose-6-phosphate receptor. The colocalisation was further investigated using brefeldin A which demonstrated that the p13 fragment and mannose-6-phosphate receptor are trafficked by alternative pathways from the trans-Golgi network.
Amino Acid Sequence↗
The in vitro and in vivo ocular pharmacology of olopatadine (AL-4943A), an effective anti-allergic/antihistaminic agent.
Olopatadine (AL-4943A; KW-4679) [(z)-11-[3-(dimethylamino)propylidene]-6, 11-dihydrodibenz[b,e]oxepine-2 acetic acid hydrochloride] is an anti-allergic agent which inhibits mast cell mediator release and possesses histamine H1 receptor antagonist activity. Studies were conducted to characterize the in vitro and in vivo pharmacological profile of this drug relevant to its topical ocular use. AL-4943A inhibits histamine release in a concentration-dependent fashion (IC50 = 559 microM) from human conjunctival mast cell preparations in vitro. Histamine release was not stimulated by AL-4943A at concentrations as high as 10 mM. In contrast, ketotifen stimulated histamine release at concentrations slightly higher than effective inhibitory concentrations. AL-4943A did not display any in vitro cyclooxygenase or 5-lipoxygenase inhibition. Topical ocular application of AL-4943A effectively inhibits antigen- and histamine-stimulated conjunctivitis in guinea pigs. Passive anaphylaxis in guinea pig conjunctiva was attenuated by AL-4943A applied 30 min prior to intravenous or topical ocular antigen challenge (ED50 values 0.0067% and 0.0170%, w/v, respectively). Antihistaminic activity in vivo was demonstrated using a model of histamine-induced vascular permeability in guinea pig conjunctiva. AL-4943A applied topically from 5 min to 24 hrs prior to histamine challenge effectively and concentration-dependently inhibited the vascular permeability response, indicating the compound has an acceptable onset and a long duration of action. Drug concentrations 5-fold greater than those effective against histamine-stimulated conjunctival responses failed to inhibit vascular permeability responses induced with either serotonin or Platelet-Activating-Factor. These data indicate that the anti-histaminic effect observed with AL-4943A is specific. These anti-allergic/antihistaminic activities of AL-4943A observed in preclinical model systems have been confirmed in clinical trials in allergic patients.
Administration, Topical↗
Cleavage of a beta-amyloid precursor sequence by cathepsin D.
The identify of the proteases that release beta-amyloid, found in the senile plaques of Alzheimer's disease, from its precursor APP, have not been rigorously identified. As senile plaques contain lysosomal enzymes, and production of some of the amyloidogenic intermediates are inhibited by lysosomotrophic agents, it has been suggested that cathepsins are involved in amyloidogenesis. A synthetic 31-residue peptide overlapping the beta-secretase cleavage site is found to be digested at two mutually exclusive sites, one and three residues on the N-terminal side of the N-terminal Asp residue of beta-amyloid. Coupled with the action of aminopeptidases, lysosomal or endosomal cathepsin D could be responsible for generating the N-terminus of beta-amyloid in vivo.
Alzheimer Disease↗
Preclinical efficacy of emedastine, a potent, selective histamine H1 antagonist for topical ocular use.
Emedastine [1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)- benzimidazole difumarate] was evaluated for topical ocular anti-histaminic activity in histamine and antigen stimulated conjunctivitis models. Concentration-dependent inhibition of histamine induced vascular permeability changes occurring in the conjunctiva was observed when the time interval between topical ocular administration and histamine challenge ranged from 1 min to 8 hr. The calculated ED50 values obtained using intervals of 1 min, 30 min, 2, 4 and 8 hr were 0.0002%, 0.000035%, 0.0029%, 0.019% and 0.19%, w/v, respectively. Comparisons of relative potency 30 min post dosing between emedastine and other anti-histamines demonstrated that emedastine is equipotent to ketotifen, and 7, 7, 10, 10, 100, 357, 3333, and 5813 times more potent than brompheniramine, chlorpheniramine, clemastine, pyrilamine, levocabastine, pheniramine, diphenhydramine, and antazoline, respectively. Emedastine (0.1%) failed to significantly attenuate either serotonin or platelet-activating-factor induced vascular permeability changes indicating high selectivity for the histamine H1 receptor. In a passive conjunctival anaphylaxis model in guinea pigs, significant inhibition of the allergic response was observed following topical ocular administration of emedastine 5 min or 30 min prior to antigen challenge (ED50s 0.0046% and 0.00022%, respectively). These data clearly indicate that emedastine has potential as a topical ocular anti-histamine for treating allergic conjunctivitis.
Administration, Topical↗
Supreme Court strikes down National Labor Relations Board's restrictive definition of health care supervisor. N.L.R.B. v. Health Care & Retirement Corporation of America.
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Labor Unions↗