Disruption of microtubules reveals two independent apical targeting mechanisms for G-protein-coupled receptors in polarized renal epithelial cells

Friend or Foe: The Role of the Cytoskeleton in Influenza A Virus Assembly

Influenza A Virus (IAV) is a respiratory virus that causes seasonal outbreaks annually and pandemics occasionally. The main targets of the virus are epithelial cells in the respiratory tract. Like many other viruses, IAV employs the host cell’s machinery to enter cells, synthesize new genomes and viral proteins, and assemble new virus particles. The cytoskeletal system is a major cellular machinery, which IAV exploits for its entry to and exit from the cell. However, in some cases, the cytoskeleton has a negative impact on efficient IAV growth. In this review, we highlight the role of cytoskeletal elements in cellular processes that are utilized by IAV in the host cell. We further provide an in-depth summary of the current literature on the roles the cytoskeleton plays in regulating specific steps during the assembly of progeny IAV particles.

Regulation of E-cadherin localization by microtubule targeting agents: rapid promotion of cortical E-cadherin through p130Cas/Src inhibition by eribulin

Microtubule targeting agents (MTAs) are some of the most effective anticancer drugs used to treat a wide variety of adult and pediatric cancers. Building evidence suggests that these drugs inhibit interphase signaling events and that this contributes to their anticancer actions. The effects of diverse MTAs were evaluated following a 2 hour incubation with clinically relevant concentrations to test the hypothesis that these drugs rapidly and differentially disrupt epithelial-to-mesenchymal transition (EMT)-related signaling. The MTAs rapidly promoted the cortical localization of internal pools of E-cadherin in HCC1937 breast cancer cells, with the most robust effects observed with the microtubule destabilizers eribulin and vinorelbine. Cortical E-cadherin localization was also promoted by the Src kinase inhibitor dasatinib or by siRNA-mediated depletion of the p130Cas scaffold. Mechanistic studies demonstrate that eribulin disrupts the interaction between p130Cas and Src, leading to decreased cortical Src phosphorylation that precedes the accumulation of cortical E-cadherin. These results suggest that microtubules can be actively co-opted by cancer cells to inhibit cortical E-cadherin localization, a hallmark of EMT, and provide a direct link between the initial disruption of the microtubule network and reversal of EMT phenotypes demonstrated by eribulin in long-term studies.

The angiotensin II type 1 receptor C-terminal Lys residues interact with tubulin and modulate receptor export trafficking

The physiological and pathological functions of angiotensin II are largely mediated through activating the cell surface angiotensin II type 1 receptor (AT1R). However, the molecular mechanisms underlying the transport of newly synthesized AT1R from the endoplasmic reticulum (ER) to the cell surface remain poorly defined. Here we demonstrated that the C-terminus (CT) of AT1R directly and strongly bound to tubulin and the binding domains were mapped to two consecutive Lys residues at positions 310 and 311 in the CT membrane-proximal region of AT1R and the acidic CT of tubulin, suggestive of essentially ionic interactions between AT1R and tubulin. Furthermore, mutation to disrupt tubulin binding dramatically inhibited the cell surface expression of AT1R, arrested AT1R in the ER, and attenuated AT1R-mediated signaling measured as ERK1/2 activation. These data demonstrate for the first time that specific Lys residues in the CT juxtamembrane region regulate the processing of AT1R through interacting with tubulin. These data also suggest an important role of the microtubule network in the cell surface transport of AT1R.

Regulation of post-Golgi traffic of G protein-coupled receptors

Anterograde trafficking of newly synthesized G protein-coupled -receptors (GPCRs) from the endoplasmic reticulum to the cell surface represents a crucial checkpoint in controlling the amount of the functional receptors at the cell surface and the strength of signaling initiated by the receptors. In contrast to the extensively studied, well-understood endocytic and recycling pathways, the molecular mechanisms underlying the cell-surface targeting of the receptors remain poorly defined. In this chapter, I will discuss current advances in understanding post-Golgi transport of GPCRs by focusing on specific motifs or sequences that may function as sorting signals regulating export from the Golgi and subsequent transport to the plasma membrane of GPCRs.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

Small GTPase regulation of GPCR anterograde trafficking

The physiological functions of heterotrimeric G protein-coupled receptors (GPCRs) are dictated by their intracellular trafficking and precise targeting to the functional destinations. Over the past decades, most studies on the trafficking of GPCRs have focused on the events involved in endocytosis and recycling. By contrast, the molecular mechanisms underlying anterograde transport of newly synthesized GPCRs from the endoplasmic reticulum (ER) to the cell surface have only now begun to be revealed. In this review we discuss current advances in understanding the role of Ras-like GTPases, specifically the Rab and Sar1/ARF subfamilies, in regulating cell-surface transport of GPCRs en route from the ER and the Golgi.

Alpha2B-adrenergic receptor interaction with tubulin controls its transport from the endoplasmic reticulum to the cell surface

It is well recognized that the C terminus (CT) plays a crucial role in modulating G protein-coupled receptor (GPCR) transport from the endoplasmic reticulum (ER) to the cell surface. However the molecular mechanisms that govern CT-dependent ER export remain elusive. To address this issue, we used α(2B)-adrenergic receptor (α(2B)-AR) as a model GPCR to search for proteins interacting with the CT. By using peptide-conjugated affinity matrix combined with proteomics and glutathione S-transferase fusion protein pull-down assays, we identified tubulin directly interacting with the α(2B)-AR CT. The interaction domains were mapped to the acidic CT of tubulin and the basic Arg residues in the α(2B)-AR CT, particularly Arg-437, Arg-441, and Arg-446. More importantly, mutation of these Arg residues to disrupt tubulin interaction markedly inhibited α(2B)-AR transport to the cell surface and strongly arrested the receptor in the ER. These data provide the first evidence indicating that the α(2B)-AR C-terminal Arg cluster mediates its association with tubulin to coordinate its ER-to-cell surface traffic and suggest a novel mechanism of GPCR export through physical contact with microtubules.

Soluble levels of cytosolic tubulin regulate ciliary length control

We show that manipulation of either the microtubule or the actin cytoskeleton has unexpected influences on cilia length control.

The primary cilium is an evolutionarily conserved dynamic organelle important for regulating numerous signaling pathways, and, as such, mutations disrupting ciliogenesis result in a variety of developmental abnormalities and postnatal disorders. The length of the cilium is regulated by the cell through largely unknown mechanisms. Normal cilia length is important, as either shortened or elongated cilia have been associated with disease and developmental defects. Here we explore the importance of cytoskeletal dynamics in regulating cilia length. Using pharmacological approaches in different cell types, we demonstrate that actin depolymerization or stabilization and protein kinase A activation result in a rapid elongation of the primary cilium. The effects of pharmacological agents on cilia length are associated with a subsequent increase in soluble tubulin levels and can be impaired by depletion of soluble tubulin with taxol. In addition, subtle nocodazole treatment was able to induce ciliogenesis under conditions in which cilia are not normally formed and also increases cilia length on cells that have already established cilia. Together these data indicate that cilia length can be regulated through changes in either the actin or microtubule network and implicate a possible role for soluble tubulin levels in cilia length control.

KIF17 stabilizes microtubules and contributes to epithelial morphogenesis by acting at MT plus ends with EB1 and APC

Epithelial polarization is associated with selective stabilization and reorganization of microtubule (MT) arrays. However, upstream events and downstream consequences of MT stabilization during epithelial morphogenesis are still unclear. We show that the anterograde kinesin KIF17 localizes to MT plus ends, stabilizes MTs, and affects epithelial architecture. Targeting of KIF17 to plus ends of growing MTs requires kinesin motor activity and interaction with EB1. In turn, KIF17 participates in localizing adenomatous polyposis coli (APC) to the plus ends of a subset of MTs. We found that KIF17 affects MT dynamics, polymerization rates, and MT plus end stabilization to generate posttranslationally acetylated MTs. Depletion of KIF17 from cells growing in three-dimensional matrices results in aberrant epithelial cysts that fail to generate a single central lumen and to polarize apical markers. These findings implicate KIF17 in MT stabilization events that contribute to epithelial polarization and morphogenesis.

Actin cytoskeleton-dependent Rab GTPase-regulated angiotensin type I receptor lysosomal degradation studied by fluorescence lifetime imaging microscopy

The dynamic regulation of the cellular trafficking of human angiotensin (Ang) type 1 receptor (AT1R) is not well understood. Therefore, we investigated the cellular trafficking of AT1R-enhanced green fluorescent protein (EGFP) (AT1R-EGFP) heterologously expressed in HEK293 cells by determining the change in donor lifetime (AT1R-EGFP) in the presence or absence of acceptor(s) using fluorescence lifetime imaging-fluorescence resonance energy transfer (FRET) microscopy. The average lifetime of AT1R-EGFP in our donor-alone samples was ∼2.33 ns. The basal state lifetime was shortened slightly in the presence of Rab5 (2.01±0.10 ns) or Rab7 (2.11±0.11 ns) labeled with Alexa 555, as the acceptor fluorophore. A 5-min Ang II treatment markedly shortened the lifetime of AT1R-EGFP in the presence of Rab5-Alexa 555 (1.78±0.31 ns) but was affected minimally in the presence of Rab7-Alexa 555 (2.09±0.37 ns). A 30-min Ang II treatment further decreased the AT1R-EGFP lifetime in the presence of both Rab5- and Rab7-Alexa 555. Latrunculin A but not nocodazole pretreatment blocked the ability of Ang II to shorten the AT1R-EGFP lifetime. The occurrence of FRET between AT1R-EGFP (donor) and LAMP1-Alexa 555 (acceptor) with Ang II stimulation was impaired by photobleaching the acceptor. These studies demonstrate that Ang II-induced AT1R lysosomal degradation through its association with LAMP1 is regulated by Rab5/7 via mechanisms that are dependent on intact actin cytoskeletons.

Different N-terminal motifs determine plasma membrane targeting of the human concentrative nucleoside transporter 3 in polarized and nonpolarized cells

Human concentrative nucleoside transporter 3 (hCNT3) is a broad-selectivity, high-affinity protein implicated in the uptake of most nucleoside-derived anticancer and antiviral drugs. Regulated trafficking of hCNT3 has been recently postulated as a suitable way to improve nucleoside-based therapies. Moreover, the recent identification of a putative novel hCNT3-type transporter lacking the first 69 amino acids and retained at the endoplasmic reticulum anticipated that the N terminus of hCNT3 contains critical motifs implicated in trafficking. In the current study, we have addressed this issue by using deletions and site-directed mutagenesis and plasma membrane expression and nucleoside uptake kinetic analysis. Data reveal that 1) a segment between amino acids 50 and 62 contains plasma membrane-sorting determinants in nonpolarized cells; 2) in particular, the Val(57)-Thr(58)-Val(59) tripeptide seems to be the core of the export signal, whereas acidic motifs upstream and downstream of it seem to be important for the kinetics of the process; and 3) in polarized epithelia, the β-turn-forming motif (17)VGFQ(20) is necessary for proper apical expression of the protein.

Adenosine and renal tubular function

PURPOSE OF REVIEW

Intrarenal adenosine is present in the cytoplasm of renal epithelial cells and in the extracellular space. Adenosine is generated at high levels in response to imbalance between energy demand and supply (e.g. increased tubular sodium chloride transport or hypoxia) and activates cell membrane adenosine receptors to affect renal vascular and tubular functions. Adenosine regulates renal sodium and water excretion via a myriad of effects on renal hemodynamic, glomerular filtration rate, renin secretion and direct effects on the renal tubule epithelium. This review examines the direct effects of adenosine on renal tubular epithelial transport in light of the most recent evidence and discusses some physiologic and pathophysiologic implications.

RECENT FINDINGS

Intrarenal adenosine affects proximal fluid and solute transport in a biphasic fashion. Under physiological conditions adenosine stimulates proximal tubular re-absorption, thus reducing the load delivered to the distal nephron. A supra-physiologic increase in adenosine such as in ischemia reduces reabsorption in the proximal tubule, thus reducing renal oxygen consumption.

SUMMARY

Intrarenal adenosine and its receptors have important regulatory functions in the renal epithelium. A complete understanding of this autocrine/paracrine system holds great potential for novel therapeutic strategies, such as the use of nucleoside analogues for reno-protection in renal ischemia.

Mucus secretion and cytoskeletal modifications in cultured nasal epithelial cells exposed to wall shear stresses

The nasal epithelium is continuously subjected to wall shear stresses (WSS) induced by respiratory airflows. An in vitro experimental model was developed to expose nasal epithelial cells cultured under air-liquid interface conditions to steady airflow-induced WSS. Mucus secretion from epithelial goblet cells was quantified using an enzyme-linked lectinosorbent assay, and modifications of the cytoskeletal structure were qualitatively evaluated from fluorescent stains of actin and beta-tubulin fibers. The results show increased mucus secretion from cells subjected to WSS of 0.1 and 1.0 dyne/cm(2) for more than 15 min in comparison with unstressed cells. The integrity levels of beta-tubulin fibers were significantly lower in cells subjected to WSS than in unstressed cells. The increased mucus secretion in response to WSS was approximately the same in Taxol-free and Taxol-treated cultures, which indicates that there is no direct connection between beta-tubulin fragmentation and mucus secretion. The stressed cells regained their normal cytoskeletal appearance 24 h after the exposure to WSS. The results of this study suggest that WSS have an important role in the mechanical regulation of the nasal surface epithelium function.

The role of anti-alpha-enolase autoantibodies in pathogenicity of autoimmune-mediated retinopathy

Autoantibodies against alpha-enolase, a glycolytic enzyme, have been frequently associated with visual loss and retinal degeneration in patients with autoimmune and cancer-associated retinopathy; however their role in the pathogenicity of retinopathy has not been fully explained. Thus, we examined the causative role of anti-enolase antibodies on retinal cells. In the in vitro studies reported here, we found that Enol-1 monoclonal antibody against alpha-enolase significantly inhibited the catalytic function of enolase, which resulted in the depletion of glycolytic ATP. Enol-1 significantly increased intracellular Ca(2+), which led to Bax translocation to the mitochondria, and the release of cytochrome c into the cytoplasm--events that correlated with the initiation of apoptosis. Normal IgG did not induce intracellular calcium or reduce cytosolic ATP. L-type voltage-gated calcium channel blockers (nifedipine, D-cis-diltiazem, and verapamil) were effective in blocking the Ab-induced intracellular Ca(2+) rise and induction of Bax. Based on these findings we propose that chronic access of autoantibodies to the retina results in the inhibition of enolase catalytic function, depletion of ATP, and elevation in intracellular Ca(2+), leading to deregulation of glycolysis in retinal neurons and their destruction.

Differential regulation of AQP2 trafficking in endosomes by microtubules and actin filaments

Vasopressin-induced trafficking of aquaporin-2 (AQP2) water channels in kidney collecting duct cells is critical to regulate the urine concentration. To better understand the mechanism of subcellular trafficking of AQP2, we examined MDCK cells expressing AQP2 as a model. We first performed double-immunolabeling of AQP2 with endosomal marker proteins, and showed that AQP2 is stored at a Rab11-positive subapical compartment. After the translocation to the plasma membrane, AQP2 was endocytosed to EEA1-positive early endosomes, and then transferred back to the original Rab11-positive compartment. When Rab11 was depleted by RNA interference, retention of AQP2 at the subapical storage compartment was impaired. We next examined the role of cytoskeleton in the AQP2 trafficking and localization. By the treatment with microtubule-disrupting agent such as nocodazole or colcemid, the distribution of AQP2 storage compartment was altered. The disruption of actin filaments with cytochalasin D or latrunculin B induced the accumulation of AQP2 in EEA1-positive early endosomes. Altogether, our data suggest that Rab11 and microtubules maintain the proper distribution of the subapical AQP2 storage compartment, and actin filaments regulate the trafficking of AQP2 from early endosomes to the storage compartment.

N-glycosylation and microtubule integrity are involved in apical targeting of prostate-specific membrane antigen: implications for immunotherapy

Prostate-specific membrane antigen (PSMA) is an important biomarker expressed in prostate cancer cells with levels proportional to tumor grade. The membrane association and correlation with disease stage portend a promising role for PSMA as an antigenic target for antibody-based therapies. Successful application of such modalities necessitates a detailed knowledge of the subcellular localization and trafficking of target antigen. In this study, we show that PSMA is expressed predominantly in the apical plasma membrane in epithelial cells of the prostate gland and in well-differentiated Madin-Darby canine kidney cells. We show that PSMA is targeted directly to the apical surface and that sorting into appropriate post-Golgi vesicles is dependent upon N-glycosylation of the protein. Integrity of the microtubule cytoskeleton is also essential for delivery and retention of PSMA at the apical plasma membrane domain, as destabilization of microtubules with nocodazole or commonly used chemotherapeutic Vinca alkaloids resulted in the basolateral expression of PSMA and increased the uptake of anti-PSMA antibody from the basolateral domain. These results may have important relevance to PSMA-based immunotherapy and imaging strategies, as prostate cancer cells can maintain a well-differentiated morphology even after metastasis to distal sites. In contrast to antigens on the basolateral surface, apical antigens are separated from the circulation by tight junctions that restrict transport of molecules across the epithelium. Thus, antigens expressed on the apical plasma membrane are not exposed to intravenously administered agents. The ability to reverse the polarity of PSMA from apical to basolateral could have significant implications for the use of PSMA as a therapeutic target.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

A capillary electrophoretic method for monitoring the presence of α-tubulin in nuclear preparations

A sensitive capillary electrophoretic method was developed to detect the presence of alpha-tubulin, a microtubular cytoskeletal component, in isolated nuclear preparations. These preparations are treated with anti-alpha-tubulin primary mouse antibodies and then stained with a fluorescently labeled anti-mouse IgG antibody. The stained preparation is then analyzed by capillary electrophoresis with laser-induced fluorescence detection, a technique that allows for sensitive detection of fluorescently labeled species. Using this method, it is feasible to count individual subcellular aggregates containing alpha-tubulin (SATs), estimate the number of alpha-tubulin molecules per SAT, determine the cumulative intensity of all SATs as an estimate of the relative level of alpha-tubulin in a preparation, and obtain their apparent electrophoretic mobility distribution. The method was validated by comparing SATs from untreated cells with those from colchicine-treated cells. Since colchicine is a microtubule-disrupting agent, treatment reduced the number of SATs per cell as well as the cumulative intensity of all SATs in a preparation. In contrast, the apparent electrophoretic mobility distribution was not influenced by colchicine treatment, suggesting that this parameter is not strongly dependent on the alpha-tubulin content. Given the zeptomolar sensitivity of laser-induced fluorescence detection and the widespread availability of antibodies, the approach used here represents an improvement in the detection of cytoskeletal impurities in subcellular fractions.

Association of the 16-kDa subunit c of vacuolar proton pump with the ileal Na+-dependent bile acid transporter: protein-protein interaction and intracellular trafficking

The rat ileal apical sodium-dependent bile acid transporter (Asbt) transports conjugated bile acids in a Na+-dependent fashion and localizes specifically to the apical surface of ileal enterocytes. The mechanisms that target organic anion transporters to different domains of the ileal enterocyte plasma membrane have not been well defined. Previous studies (Sung, A.-Q., Arresa, M. A., Zeng, L., Swaby, I'K., Zhou, M. M., and Suchy, F. J. (2001) J. Biol. Chem. 276, 6825-6833) from our laboratory demonstrated that rat Asbt follows an apical sorting pathway that is brefeldin A-sensitive and insensitive to protein glycosylation, monensin treatment, and low temperature shift. Furthermore, a 14-mer signal sequence that adopts a beta-turn conformation is required for apical localization of rat Asbt. In this study, a vacuolar proton pump subunit (VPP-c, the 16-kDa subunit c of vacuolar H+-ATPase) has been identified as an interacting partner of Asbt by a bacterial two-hybrid screen. A direct protein-protein interaction between Asbt and VPP-c was confirmed in an in vitro pull-down assay and in an in vivo mammalian two-hybrid analysis. Indirect immunofluorescence confocal microscopy demonstrated that the Asbt and VPP-c colocalized in transfected COS-7 and MDCK cells. Moreover, bafilomycin A1 (a specific inhibitor of VPP) interrupted the colocalization of Asbt and VPP-c. A taurocholate influx assay and membrane biotinylation analysis showed that treatment with bafilomycin A1 resulted in a significant decrease in bile acid transport activity and the apical membrane localization of Asbt in transfected cells. Thus, these results suggest that the apical membrane localization of Asbt is mediated in part by the vacuolar proton pump associated apical sorting machinery.

The Type 1 Angiotensin II Receptor Tail Affects Receptor Targeting, Internalization, and Membrane Fusion Properties

Endocytosis modulates cell responses by removing and recycling receptors from the cell surface. Type I angiotensin II receptors (AT1R) are somewhat unique in that they are expressed at apical (AP) and basolateral (BL) membranes in proximal tubule cells and both receptor sites undergo endocytosis. We analyzed AT1R cytoplasmic (-COOH) tail deletion mutants to determine whether classic AT1R endocytosis motifs functioned similarly in polarized cells and simultaneously altered receptor properties. Serially truncating the AT1R tail had little effect on AP/BL AT1R distribution as determined by 125I-angiotensin II binding in LLCPK(Cl4) cells transfected with an AT1R transcript. AP AT1R expression required the proximal 12 amino acids in the AT1R-COOH tail. Deleting all but the proximal 12 aa of the AT1R-COOH tail (T316L mutant) decreased AP AT1R internalization at 20 min (17 +/- 6%; p < 0.05 versus full-length; n = 5) and inhibited AP AT1R-stimulated arachidonic acid release (counts released per milligram of protein at 20 min: full-length, 18,762 +/- 4018; T316L, 2430 +/- 1711; n = 4; p < 0.02). Endosomal fusion assays were performed using peptide sequences of regions in the AT1R tail involved in endocytosis (YFLQLLKYIPP [LL] and LSTKMSTLSY [STL]). Peptide STL significantly inhibited endosomal fusion (22 +/- 10% of control; n = 5; p < 0.05 versus positive control). Peptide LL had no significant inhibitory effect. AT(1)R in polarized cells contain dominant endocytosis signals but these motifs do not correlate with AP or BL AT1R expression. Moreover, peptide sequences within the AT1R-COOH tail necessary for endocytosis also modulate endosomal fusion properties.

Capillary Electrophoretic Separation of Nuclei Released from Single Cells

We report here the first capillary electrophoresis analysis of intact nuclei released on-column from single cells. Expression of the nuclear-targeted protein nuDsRed2 and the plasma membrane-bound farnesylated enhanced green fluorescent protein in cultured human DeltaH2-1 cells allowed fluorescent monitoring of the fate of these subcellular compartments upon injection of a single cell into the separation capillary. On-column treatment with digitonin allowed for the separation of the plasma membrane from the nucleus as indicated by their selective laser-induced fluorescence detection in two separate spectral regions. The data suggest that less than 0.1% of the plasma membrane remains bound to individually detected nuclei. In digitonin-treated cells, the electropherograms consisted of a prominent fluorescent peak attributed to nuDsRed2 localized to the nucleus and a collection of weakly fluorescent events (barely distinguishable from scattering) that seem to indicate additional localization of this protein to other subcellular regions. Taken together, this report points to the feasibility of studying intact organelles released from a single mammalian cell by capillary electrophoresis, which is a prerequisite to understanding the relevance of subcellular heterogeneity in biological systems.

Heterotrimeric G-proteins associate with microtubules during differentiation in PC12 pheochromocytoma cells

Tubulin modifies G-protein signaling and heterotrimeric G-proteins regulate microtubule assembly. Here we report an interplay among G-protein-coupled receptor and receptor tyrosine kinase (such as nerve growth factor-NGF) signaling systems in PC12 pheochromocytoma cells that resulted in a translocation of Galpha(s), Galpha(i1), and Galpha(o) from cell bodies to cellular processes where they appear to localize with tubulin-containing structures. This relocation appeared to depend on the integrity of microtubules, as it was blocked and reversed by nocodazole. Latrunculin, which promotes actin filament depolymerization, had no effect. Both deconvolution microscopy and immunoprecipitation showed a significant increase of Galpha association with microtubules that was coincident with the extension of "neurites." There were distinctions among the Galpha subtypes, with Galpha(s) showing the most profound NGF-induced colocalization with tubulin. Translocation of Galpha was blocked by agents that inhibit the MAP kinases required for neuronal differentiation, suggesting that G-protein relocation is triggered by the intracellular signals for differentiation. Consistent with this, Galpha in Neuro-2A cells, which spontaneously differentiate, showed a similar translocation coincident with differentiation. Thus, diverse signals that promote neuronal differentiation and changes in cell morphology may use specific G-proteins to evoke cytoskeletal rearrangement.

Three-dimensional analysis of post-Golgi carrier exocytosis in epithelial cells

Targeted delivery of proteins to distinct plasma membrane domains is critical to the development and maintenance of polarity in epithelial cells. We used confocal and time-lapse total internal reflection fluorescence microscopy (TIR-FM) to study changes in localization and exocytic sites of post-Golgi transport intermediates (PGTIs) carrying GFP-tagged apical or basolateral membrane proteins during epithelial polarization. In non-polarized Madin Darby Canine Kidney (MDCK) cells, apical and basolateral PGTIs were present throughout the cytoplasm and were observed to fuse with the basal domain of the plasma membrane. During polarization, apical and basolateral PGTIs were restricted to different regions of the cytoplasm and their fusion with the basal membrane was completely abrogated. Quantitative analysis suggested that basolateral, but not apical, PGTIs fused with the lateral membrane in polarized cells, correlating with the restricted localization of Syntaxins 4 and 3 to lateral and apical membrane domains, respectively. Microtubule disruption induced Syntaxin 3 depolarization and fusion of apical PGTIs with the basal membrane, but affected neither the lateral localization of Syntaxin 4 or Sec6, nor promoted fusion of basolateral PGTIs with the basal membrane.

Gap junctions assemble in the presence of cytoskeletal inhibitors, but enhanced assembly requires microtubules

The role of cytoskeletal elements in gap junction (GJ) assembly has been studied using Novikoff hepatoma cells treated with cytochalasin B (CB) to disrupt actin filaments or with colchicine or nocodazole to disrupt microtubules. After 60 min of cell reaggregation, freeze-fracture was used to evaluate quantitatively the "initiation," "maturation," and "growth" phases of GJ assembly. The development of junctional permeability to fluorescent dyes was also analyzed. The only effects of CB on the structure or permeability of the developing junctions involved an elongation of GJ aggregates and a small decrease in formation plaque areas. Colchicine (but not the inactive form, lumicolchicine) prevented the enhancement of GJ growth by cholesterol, but its effect on basal growth was equivocal. Nocodazole inhibited the growth of GJ, even under basal conditions, without an effect on initiation. Nocodazole also blocked the forskolin-enhanced increase in the growth of GJs and, in living MDCK cells, reduced the movement of transport intermediates containing green fluorescent protein-tagged connexin43. Thus, neither actin filaments nor microtubules appear to restrict GJ assembly by anchoring intramembrane GJ proteins, nor are they absolutely required for functional GJs to form. However, microtubules are necessary for enhanced GJ growth and likely for facilitating connexin trafficking under basal conditions.

Morphological and biochemical strategies for monitoring trafficking of epitope-tagged G protein-coupled receptors in agonist-naive and agonist-occupied states

Epitope tagged alpha 2-AR subtypes have been used to address a variety of cell biological questions, and the strategies used are readily applicable to all GPCR as well as other cell surface proteins. We have provided detailed protocols for successful utilization of the epitope-tagged receptor in the studies of protein localization and trafficking in epithelial cells, and the mechanisms by which this is achieved. We have also described reversible biotinytion strategies to examine agonist-dependent (and independent) receptor turnover at the cell surface.

Adenosine receptors: G protein-mediated signalling and the role of accessory proteins

Ever since the discovery of the effects of adenosine in the circulation, adenosine receptors continue to represent a promising drug target. Firstly, this is due to the fact that the receptors are expressed in a large variety of cells; in particular, the actions of adenosine (or, respectively, of the antagonistic methylxanthines) in the central nervous system, in the circulation, on immune cells and on other tissues can be beneficial in certain disorders. Secondly, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes have been identified by molecular cloning; they belong to the family of G protein-coupled receptors, which transfer signals by activating heterotrimeric G proteins. It has been appreciated recently that accessory proteins impinge on the receptor/G protein interaction and thus modulate the signalling reaction. These accessory components may be thought as adaptors that redirect the signalling pathway to elicit a cell-specific response. Here, we review the recent literature on adenosine receptors and place a focus on the role of accessory proteins in the organisation of adenosine receptor signalling. These components have been involved in receptor sorting, in the control of signal amplification and in the temporal regulation of receptor activity, while the existence of others is postulated on the basis of atypical cellular reactions elicited by receptor activation.

Cell-specific basolateral membrane sorting of the human liver Na(+)-dependent bile acid cotransporter

The human Na(+)-taurocholate cotransporting polypeptide (Ntcp) is located exclusively on the basolateral membrane of hepatocyte, but the mechanisms underlying its membrane sorting domain have not been fully elucidated. In the present study, a green fluorescent protein-fused human NTCP (NTCP-GFP) was constructed using the polymerase chain reaction and was stably transfected into Madin-Darby canine kidney (MDCK) and Caco-2 cells. Taurocholate uptake studies and confocal microscopy demonstrated that the polarity of basolateral surface expression of NTCP-GFP was maintained in MDCK cells but was lost in Caco-2 cells. Nocodazole (33 microM), an agent that causes microtubular depolymerization, partially disrupted the basolateral localization of NTCP-GFP by increasing apical surface expression to 33.5% compared with untreated cells (P < 0.05). Brefeldin A (BFA; 1-2 microM) disrupted the polarized basolateral localization of NTCP, but monensin (1.4 microM) had no affect on NTCP-GFP localization. In addition, low-temperature shift (20 degrees C) did not affect the polarized basolateral surface sorting of NTCP-GFP and repolarization of this protein after BFA interruption. In summary, these data suggest that the polarized basolateral localization of human NTCP is cell specific and is mediated by a novel sorting pathway that is BFA sensitive and monensin and low-temperature shift insensitive. The process may also involve microtubule motors.

Hypertonic activation of the renal betaine/GABA transporter is microtubule dependent

BACKGROUND

Epithelial cells in the renal inner medulla accumulate osmolytes such as betaine to maintain normal cell volume during prolonged extracellular hypertonic stress. Betaine accumulation is the result of activation of transcription of the BGT1 transporter gene followed by increased betaine transport.

METHODS

We studied the possible role of microtubules in this adaptive mechanism using renal cells in culture. RESULTS.: In cultured renal cell lines [Madin-Darby canine kidney (MDCK) and mouse inner medullary collecting duct (mIMCD-3)], up-regulation of BGT1 activity was maximal after 24 to 30 hours in growth medium made hypertonic (510 mOsm/kg) by the addition of sucrose or NaCl. Up-regulation was reversed within 24 to 36 hours after returning cells to isotonic medium. Both cycloheximide (20 micromol/L) and nocodazole (20 micromol/L) blocked the hypertonic up-regulation of BGT1. Nocodazole was partially effective even when added 16 to 20 hours after the switch to hypertonic medium. Recovery from nocodazole action was rapid, and there was full activation of BGT1 transport within three to six hours after nocodazole removal, suggesting rapid trafficking to the cell surface once microtubules repolymerized. Hypertonic activation of BGT1 transport was detected in an isolated membrane fraction and was blocked by cycloheximide but not by nocodazole. Confocal microscopy confirmed the increased abundance of BGT1 proteins in the plasma membrane of hypertonic cells and showed that BGT1 remained intracellular during nocodazole treatment.

CONCLUSIONS

Hypertonic activation of BGT1 in renal cells requires de novo protein synthesis and microtubule-dependent trafficking of additional transporters to the cell surface. The apparent resistance of membrane BGT1 to nocodazole blockade is likely due to the presence in the membrane fraction of an increased intracellular pool of active BGT1 transporters.

Localization of GFP-tagged concentrative nucleoside transporters in a renal polarized epithelial cell line

Many nucleosides undergo active reabsorption within the kidney, probably via nucleoside transporters. To date, two concentrative nucleoside transporters have been cloned, the sodium-dependent purine-selective nucleoside transporter (SPNT) and concentrative nucleoside transporter 1 (CNT1). We report the stable expression of green fluorescence protein (GFP)-tagged SPNT and CNT1 in Madin-Darby canine kidney (MDCK) cells, a polarized renal epithelial line. We demonstrate that the GFP tag does not alter the substrate selectivity and only modestly affects the kinetic activity of the transporters. By using confocal microscopy and functional studies, both SPNT and CNT1 are localized primarily to the apical membrane of MDCK and LLC-PK(1) cells. Apical localization of these transporters suggests a role in renal nucleoside reabsorption and regulation of tubular function via the adenosine pathway.

Metabotropic glutamate receptor type 1alpha and tubulin assemble into dynamic interacting complexes

Metabotropic glutamate receptors (mGlu receptors) are coupled to G-protein second messenger pathways and modulate glutamate neurotransmission in the brain, where they are targeted to specific synaptic locations. Very recently, we identified tubulin as an interacting partner of the mGlu(1alpha) receptor in rat brain. Using BHK-570 cells permanently expressing the receptor we have shown that this interaction occurs predominantly with soluble tubulin, following its translocation to the plasma membrane. In addition, treatment of the cells with the agonist quisqualic acid induce tubulin depolymerization and its translocation to the plasma membrane. Immunofluorescence detection of both the receptor and tubulin in agonist-treated cells reveals a disruption of the microtubule network and an increased clustering of the receptor. Collectively these data demonstrate that the mGlu(1alpha) receptor interacts with soluble tubulin and that this association can take place at the plasma membrane.

Effects of microtubule disruption on force, velocity, stiffness and [Ca(2+)](i) in porcine coronary arteries

Force generated by smooth muscle cells is believed to result from the interaction of actin and myosin filaments and is regulated through phosphorylation of the myosin regulatory light chain (LC(20)). The role of other cytoskeleton filaments, such as microtubules and intermediate filaments, in determining the mechanical output of smooth muscle is unclear. In cultured fibroblasts, microtubule disruption results in large increases in force similar to contractions associated with LC(20) phosphorylation (15). One hypothesis, the "tensegrity" or "push-pull" model, attributes this increase in force to the disruption of microtubules functioning as rigid struts to resist force generated by actin-myosin interaction (9). In porcine coronary arteries, the disruption of microtubules by nocodazole (11 microM) also elicited moderate but significant increases in isometric force (10-40% of a KCl contracture), which could be blocked or reversed by taxol (a microtubule stabilizer). We tested whether this nocodazole-induced force was accompanied by changes in coronary artery stiffness or unloaded shortening velocity, parameters likely to be highly sensitive to microtubule resistance elements. Few changes were seen, ruling out push-pull mechanisms for the increase in force by nocodazole. In contrast, the intracellular calcium concentration, measured by fura 2 in the intact artery, was increased by nocodazole in parallel with force, and this was inhibited and/or reversed by taxol. Our results indicate that microtubules do not significantly contribute to vascular smooth muscle mechanical characteristics but, importantly, may play a role in modulation of Ca(2+) signal transduction.

Polarity of A2b adenosine receptor expression determines characteristics of receptor desensitization

It is not known if, in polarized cells, desensitization events can be influenced by the domain on which the receptor resides. Desensitization was induced by 5'-(N-ethylcarboxamido)adenosine (NECA) and was quantitated by measurement of short-circuit current (I(sc)) in response to adenosine. NECA added to either the apical or basolateral compartments rapidly desensitized receptors on these respective domains. Although apical NECA had no effect on the basolateral receptor stimulation, basolateral NECA induced a complete desensitization of the apical receptor. We hypothesized that desensitization of apical receptor by basolateral desensitization could relate to a trafficking step in which A2b receptor is first targeted basolaterally upon synthesis and transported to the apical surface via vesicular transport/microtubules. Because desensitization is associated with downregulation of receptors, apical adenosine receptor can thus be affected by basolateral desensitization. Both low temperature and nocodazole inhibited I(sc) induced by apical and not basolateral adenosine.

IN CONCLUSION

1) a single receptor subtype, here modeled by the A2b receptor, differentially desensitizes based on the membrane domain on which it is expressed, 2) agonist exposure on one domain can result in desensitization of receptors on the opposite domain, 3) cross-domain desensitization can display strict polarity, and 4) receptor trafficking may play a role in the cross-desensitization process.

A low resolution model for the interaction of G proteins with G protein-coupled receptors

A model is presented for the interaction between G proteins and G protein-coupled receptors. The model is based on the fact that this interaction shows little specificity and thus conserved parts of the G proteins have to interact with conserved parts of the receptors. These parts are a conserved negative residue in the G protein, a fully conserved arginine in the receptor and a series of residues that are not conserved but always hydrophobic like the hydrophobic side of the C-terminal helix of the G protein and the hydrophobic side of a helix in the C-terminal domain of the receptor. Other, mainly cytosolic, factors determine the specificity and regulation of this interaction. The relation between binding and activation will be shown. A large body of experimental evidence supports this model. Despite the fact that the model does not provide atomic resolution, it can be used to explain some experimental data that would otherwise seem inexplicable, and it suggests experiments for its falsification or verification.

Expression of G protein alpha subunits in normal rat colon and in azoxymethane-induced colonic neoplasms

BACKGROUND & AIMS

Heterotrimeric G proteins are important in growth-regulating signal transduction. The aim of this study was to characterize the relative expression of G protein alpha subunits in rat colonocytes, colonocyte antipodal plasma membranes, and colonic neoplasms.

METHODS

Antipodal plasma membranes were prepared from isolated colonocytes. Azoxymethane was administered to rats to induce colonic neoplasms. K-ras mutations in the neoplasms were determined by oligonucleotide hybridization and confirmed by primer mediated-restriction fragment length polymorphism. Colonocyte and tumor homogenates or membranes were probed for Galpha subunits by Western blotting with isoform-specific antibodies.

RESULTS

The expressions of Galphai2, alphai3, and alphaq/11 were significantly enriched in the basolateral compared with brush border fraction of colonic antipodal plasma membranes. In neoplasms without K-ras mutations, the expression of Galphai2 increased 4-fold, Galphas(long) increased 2.5-fold, and Galphai3 increased 1.5-2-fold. Expression did not differ among tumor grades. K-ras mutations were associated with lowered expression of G proteins, especially Galphao.

CONCLUSIONS

In colonocytes, Galpha subunits are localized primarily in basolateral plasma membranes. The increased expressions of Galphai2 and, to a lesser degree, Galphai3 and Galphas(long) in tumors was independent of tumor grade but was modulated by the presence of K-ras mutations.

N-terminal domain of Gpa1 (G protein alpha) subunit) is sufficient for plasma membrane targeting in yeast Saccharomyces cerevisiae

G proteins play a central role in transmitting signals from cell surface receptors to effector proteins inside the cell. Signaling can only occur, however, if all these protein components are properly assembled and localized at the plasma membrane. Past studies have shown that certain segments within the N-terminal region of the G protein alpha subunit are necessary for membrane attachment. Here we identify a region within the yeast G alpha (Gpa1) that is sufficient for membrane attachment, as well as for specific targeting to the plasma membrane. Initially, we constructed chimeric proteins that replace the N terminus of mammalian Gsalpha with the corresponding sequence from Gpa1. Gsalpha is inefficiently targeted to the yeast plasma membrane and therefore cannot fully complement the loss of Gpa1. Gpa1-Gsaplha chimeras were assayed for proper membrane localization by functional complementation of a gpa1Delta;) mutant, and by sucrose density gradient fractionation of cell membranes. Most of the chimeras tested, including one with only the N-terminal 7 amino acids from Gpa1, exhibited normal membrane targeting and complementing activity. We also fused various lengths of N-terminal Gpa1 sequence to glutathione-S-transferase (GST), a heterologous protein normally expressed in the cytoplasm. The first 67- 36- or 9-amino acids of Gpa1 were all sufficient to direct GST specifically to the plasma membrane in yeast. This analysis defines the extreme N terminus of Gpa1 as the primary determinant of proper membrane targeting, and represents an essential step towards isolating and identifying G protein-targeting proteins within the plasma membrane.

Binding and phosphorylation of tubulin by G protein-coupled receptor kinases

Although the beta-adrenergic receptor kinase (betaARK) mediates agonist-dependent phosphorylation and desensitization of G protein-coupled receptors, recent studies suggest additional cellular functions. During our attempts to identify novel betaARK interacting proteins, we found that the cytoskeletal protein tubulin could specifically bind to a betaARK-coupled affinity column. In vitro analysis demonstrated that betaARK and G protein-coupled receptor kinase-5 (GRK5) were able to stoichiometrically phosphorylate purified tubulin dimers with a preference for beta-tubulin and, under certain conditions, the betaIII-isotype. Examination of the GRK/tubulin binding characteristics revealed that tubulin dimers and assembled microtubules bind GRKs, whereas the catalytic domain of betaARK contains the primary tubulin binding determinants. In vivo interaction of GRK and tubulin was suggested by the following: (i) co-purification of betaARK with tubulin from brain tissue; (ii) co-immunoprecipitation of betaARK and tubulin from COS-1 cells; and (iii) co-localization of betaARK and GRK5 with microtubule structures in COS-1 cells. In addition, GRK-phosphorylated tubulin was found preferentially associated with the microtubule fraction during in vitro assembly assays suggesting potential functional significance. These results suggest a novel link between the cytoskeleton and GRKs that may be important for regulating GRK and/or tubulin function.

Microtubule-interfering agents activate c-Jun N-terminal kinase/stress-activated protein kinase through both Ras and apoptosis signal-regulating kinase pathways

The essential cellular functions associated with microtubules have led to a wide use of microtubule-interfering agents in cancer chemotherapy with promising results. Although the most well studied action of microtubule-interfering agents is an arrest of cells at the G2/M phase of the cell cycle, other effects may also exist. We have observed that paclitaxel (Taxol), docetaxel (Taxotere), vinblastine, vincristine, nocodazole, and colchicine activate the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) signaling pathway in a variety of human cells. Activation of JNK/SAPK by microtubule-interfering agents is dose-dependent and time-dependent and requires interactions with microtubules. Functional activation of the JNKK/SEK1-JNK/SAPK-c-Jun cascade (where JNKK/SEK1 is JNK kinase/SAPK kinase) was demonstrated by activation of a 12-O-tetradecanoylphorbol-13-acetate response element (TRE) reporter construct in a c-Jun dependent fashion. Microtubule-interfering agents also activated both Ras and apoptosis signal-regulating kinase (ASK1) and coexpression of dominant negative Ras and dominant negative apoptosis signal-regulating kinase exerted individual and additive inhibition of JNK/SAPK activation by microtubule-interfering agents. These findings suggest that multiple signal transduction pathways are involved with cellular detection of microtubular disarray and subsequent activation of JNK/SAPK.

New aspects of G-protein-coupled receptor signalling and regulation

Research on the structure, regulation and signalling properties of the family of seven-transmembrane-helix, heterotrimeric guanine nucleotide-binding protein (G-protein)-coupled receptors (GPCRs) continues at a frantic pace. This reflects their central role in transmission of hormone- and neurotransmitter-encoded information across the plasma membrane of cells. The location of the ligand-binding sites on the extracellular face of the membrane has made them obvious targets for therapeutic intervention in a wide range of conditions resulting from endocrine imbalance. Furthermore, based on the identification of many novel GPCR sequences emerging from expressed sequence tags (ESTs) and other DNA sequencing programmes, it has become clear that the GPCR family is likely to be considerably larger than appreciated in even the recent past. Although neither the natural ligands nor synthetic pharmaceuticals have yet been identified for these so-called ;orphan' GPCRs, they offer the potential for a plethora of new therapeutic targets. Within a short review, it is impossible to cover all the current developments in this field and the topics selected represent a personal view of recent highlights of areas that provide both novel and general insights into the function and regulation of GPCRs.