Properties of GluR3 receptors tagged with GFP at the amino or carboxyl terminus

Functional impairment of cortical AMPA receptors in schizophrenia

Clinical and preclinical studies suggest that some of the behavioral alterations observed in schizophrenia (SZ) may be mechanistically linked to synaptic dysfunction of glutamatergic signaling. Recent genetic and proteomic studies suggest alterations of cortical glutamate receptors of the AMPA-type (AMPARs), which are the predominant ligand-gated ionic channels of fast transmission at excitatory synapses. The impact of gene and protein alterations on the electrophysiological activity of AMPARs is not known in SZ. In this proof of principle work, using human postmortem brain synaptic membranes isolated from the dorsolateral prefrontal cortex (DLPFC), we combined electrophysiological analysis from microtransplanted synaptic membranes (MSM) with transcriptomic (RNA-Seq) and label-free proteomics data in 10 control and 10 subjects diagnosed with SZ. We observed in SZ a reduction in the amplitude of AMPARs currents elicited by kainate, an agonist of AMPARs that blocks the desensitization of the receptor. This reduction was not associated with protein abundance but with a reduction in kainate's potency to activate AMPARs. Electrophysiologically-anchored dataset analysis (EDA) was used to identify synaptosomal proteins that linearly correlate with the amplitude of the AMPARs responses, gene ontology functional annotations were then used to determine protein-protein interactions. Protein modules associated with positive AMPARs current increases were downregulated in SZ, while protein modules that were upregulated in SZ were associated with decreased AMPARs currents. Our results indicate that transcriptomic and proteomic alterations, frequently observed in the DLPFC in SZ, converge at the synaptic level producing a functional electrophysiological impairment of AMPARs.

Microtransplantation of Synaptic Membranes to Reactivate Human Synaptic Receptors for Functional Studies

Excitatory and inhibitory ionotropic receptors are the major gates of ion fluxes that determine the activity of synapses during physiological neuronal communication. Therefore, alterations in their abundance, function, and relationships with other synaptic elements have been observed as a major correlate of alterations in brain function and cognitive impairment in neurodegenerative diseases and mental disorders. Understanding how the function of excitatory and inhibitory synaptic receptors is altered by disease is of critical importance for the development of effective therapies. To gain disease-relevant information, it is important to record the electrical activity of neurotransmitter receptors that remain functional in the diseased human brain. So far this is the closest approach to assess pathological alterations in receptors' function. In this work, a methodology is presented to perform microtransplantation of synaptic membranes, which consists of reactivating synaptic membranes from snap frozen human brain tissue containing human receptors, by its injection and posterior fusion into the membrane of Xenopus laevis oocytes. The protocol also provides the methodological strategy to obtain consistent and reliable responses of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and γ-aminobutyric acid (GABA) receptors, as well as novel detailed methods that are used for normalization and rigorous data analysis.

A novel voltage-clamp/dye uptake assay reveals saturable transport of molecules through CALHM1 and connexin channels

Large-pore channels permeable to small molecules such as ATP, in addition to atomic ions, are emerging as important regulators in health and disease. Nonetheless, their mechanisms of molecular permeation and selectivity remain mostly unexplored. Combining fluorescence microscopy and electrophysiology, we developed a novel technique that allows kinetic analysis of molecular permeation through connexin and CALHM1 channels in Xenopus oocytes rendered translucent. Using this methodology, we found that (1) molecular flux through these channels saturates at low micromolar concentrations, (2) kinetic parameters of molecular transport are sensitive to modulators of channel gating, (3) molecular transport and ionic currents can be differentially affected by mutation and gating, and (4) N-terminal regions of these channels control transport kinetics and permselectivity. Our methodology allows analysis of how human disease-causing mutations affect kinetic properties and permselectivity of molecular signaling and enables the study of molecular mechanisms, including selectivity and saturability, of molecular transport in other large-pore channels.

Electrophysiological evaluation of extracellular spermine and alkaline pH on synaptic human GABAA receptors

Polyamines have fundamental roles in brain homeostasis as key modulators of cellular excitability. Several studies have suggested alterations in polyamine metabolism in stress related disorders, suicide, depression, and neurodegeneration, making the pharmacological modulation of polyamines a highly appealing therapeutic strategy. Polyamines are small aliphatic molecules that can modulate cationic channels involved in neuronal excitability. Previous indirect evidence has suggested that polyamines can modulate anionic GABA_A receptors (GABA_ARs), which mediate inhibitory signaling and provide a direct route to reduce hyperexcitability. Here, we attempted to characterize the effect that spermine, the polyamine with the strongest reported effect on GABA_ARs, has on human postmortem native GABA_ARs. We microtransplanted human synaptic membranes from the dorsolateral prefrontal cortex of four cases with no history of mental or neurological disorders, and directly recorded spermine effects on ionic GABA_ARs responses on microtransplanted oocytes. We show that in human synapses, inhibition of GABA_ARs by spermine was better explained by alkalization of the extracellular solution. Additionally, spermine had no effect on the potentiation of GABA-currents by diazepam, indicating that even if diazepam binding is enhanced by spermine, it does not translate to changes in functional activity. Our results clearly demonstrate that while extracellular spermine does not have direct effects on human native synaptic GABA_ARs, spermine-mediated shifts of pH inhibit GABA_ARs. Potential spermine-mediated increase of pH in synapses in vivo may therefore participate in increased neuronal activity observed during physiological and pathological states, and during metabolic alterations that increase the release of spermine to the extracellular milieu.

Chemical labelling for visualizing native AMPA receptors in live neurons

The location and number of neurotransmitter receptors are dynamically regulated at postsynaptic sites. However, currently available methods for visualizing receptor trafficking require the introduction of genetically engineered receptors into neurons, which can disrupt the normal functioning and processing of the original receptor. Here we report a powerful method for visualizing native α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) which are essential for cognitive functions without any genetic manipulation. This is based on a covalent chemical labelling strategy driven by selective ligand-protein recognition to tether small fluorophores to AMPARs using chemical AMPAR modification (CAM) reagents. The high penetrability of CAM reagents enables visualization of native AMPARs deep in brain tissues without affecting receptor function. Moreover, CAM reagents are used to characterize the diffusion dynamics of endogenous AMPARs in both cultured neurons and hippocampal slices. This method will help clarify the involvement of AMPAR trafficking in various neuropsychiatric and neurodevelopmental disorders.

Subcellular localization of the five members of the human steroid 5α-reductase family

In humans the steroid 5α-reductase (SRD5A) family comprises five integral membrane enzymes that carry out reduction of a double bond in lipidic substrates: Δ⁴-3-keto steroids, polyprenol and trans-enoyl CoA. The best-characterized reaction is the conversion of testosterone into the more potent dihydrotestosterone carried out by SRD5A1-2. Some controversy exists on their possible nuclear or endoplasmic reticulum localization.

We report the cloning and transient expression in HeLa cells of the five members of the human steroid 5α-reductase family as both N- and C-terminus green fluorescent protein tagged protein constructs. Following the intrinsic fluorescence of the tag, we have determined that the subcellular localization of these enzymes is in the endoplasmic reticulum, upon expression in HeLa cells. The presence of the tag at either end of the polypeptide chain can affect protein expression and, in the case of trans enoyl-CoA reductase, it induces the formation of protein aggregates.

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All members of human testosterone 5α-reductase family were expressed in HeLa cells.

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Subcellular localization of SRD5A proteins in the endoplasmic reticulum is reported.

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The effect of GFP tagging at N- or C-term on SRD5A proteins expression was assessed.

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The TECRL gene is expressed for the first time and its product localizes in the ER.

QM/MM studies of contemporary and novel membrane raft fluorescent probes

We have studied a number of contemporary and novel membrane probes, selected for their structural similarity to membrane raft components, in order to properly anchor themselves within a sphingolipid/cholesterol rich region. A QM/MM approach was adopted in order to understand the structural and electrostatic influences of fluorescence emission shifts of the probes in different lipid and solvation environments. The proposed modifications to the membrane probes have shown encouraging data relating not only to emission shifts within the membrane, but also their ability to anchor within a membrane raft domain and the stability to internalization within a membrane system.

The endogenous GABA bioactivity of camel, bovine, goat and human milks

GABA orally administered has several beneficial effects on health, including the regulation of hyperglycaemic states in humans. Those effects are similar to the effects reported for camel milk (CMk); however, it is not known whether compounds with GABAergic activity are present in milk from camels or other species. We determined CMk free-GABA concentration by LS/MS and its bioactivity on human GABA receptors. We found that camel and goat milks have significantly more bioavailable GABA than cow and human milks and are able to activate GABAρ receptors. The relationship between GABA and taurine concentrations suggests that whole camel milk may be more efficient to activate GABAρ1 receptors than goat milk. Because GABAρ receptors are normally found in enteroendocrine cells in the lumen of the digestive tract, these results suggest that GABA in camel and goat milk may participate in GABA-modulated functions of enteroendocrine cells in the GI lumen.

Characterization of an outward rectifying chloride current of Xenopus tropicalis oocytes

Here, we describe an outward rectifying current in Xenopus tropicalis oocytes that we have called xtClC-or. The current has two components; the major component is voltage activated and independent of intracellular or extracellular Ca(2+), whereas the second is a smaller component that is Ca(2+) dependent. The properties of the Ca(2+)-independent current, such as voltage dependence and outward rectification, resemble those of ClC anion channels/transporters. This current is sensitive to NPPB and NFA, insensitive to 9AC and DIDS, and showed a whole-cell conductance sequence of SCN(-)>I(-)>Br(-)>CI(-). RT-PCR revealed the expression in oocytes of ClC-2 to ClC-7, and major reductions of current amplitudes were observed when a ClC-5 antisense oligonucleotide was injected into oocytes. The Ca(2+)-dependent component was abated after injection of 10mM BAPTA or EGTA, whereas 10mMMg(2+) inhibited the current to 26±3.1%. This component was blocked by 9-AC, NFA, and NPPB, whereas DIDS did not elicit any evident effect. The ion sequence selectivity was SCN=I(-)>Br(-)>Cl(-). To try to determine the molecular identity that gives rise to this component we assessed by RT-PCR the expression of the Ca(2+)-dependent Cl(-) channel TMEM16A, which was found to be present in the oocytes. However, injection of antisense TMEM16A oligonucleotides did not inhibit the transient outward current. This result fits well with the electrophysiological data. Together, these results suggest that ClC-5 is a major, but not the sole channel responsible for this outwardly rectifying Cl(-) current.

Impact of cell type and epitope tagging on heterologous expression of G protein-coupled receptor: a systematic study on angiotensin type II receptor

Despite heterologous expression of epitope-tagged GPCR is widely adopted for functional characterization, there is lacking of systematic analysis of the impact of expression host and epitope tag on GPCR expression. Angiotensin type II (AT2) receptor displays agonist-dependent and -independent activities, coupling to a spectrum of signaling molecules. However, consensus has not been reached on the subcellular distributions, signaling cascades and receptor-mediated actions. To examine the contributions of host cell and epitope tag on receptor expression and activity, epitope-tagged AT2 receptor variants were transiently or stably expressed in HEK293, CHO-K1 and PC12 cells. The epitope-tagged AT2 receptor variants were detected both on the cell membrane and in the perinuclear region. In transiently transfected HEK293 cells, Myc-AT2 existed predominantly as monomer. Additionally, a ladder of ubiquitinated AT2 receptor proteins was detected. By contrast, stably expressed epitope-tagged AT2 receptor variants existed as both monomer and high molecular weight complexes, and the latter was enriched in cell surface. Glycosylation promoted cell surface expression of Myc-AT2 but had no effect on AT2-GFP in HEK293 cells. In cells that stably expressed Myc-AT2, serum starvation induced apoptosis in CHO-K1 cells but not in HEK293 or PC12 cells. Instead, HEK293 and PC12 cells stably expressing Myc-AT2 exhibited partial cell cycle arrest with cells accumulating at G1 and S phases, respectively. Taken together, these results suggest that expression levels, subcellular distributions and ligand-independent constitutive activities of AT2 receptor were cell type-dependent while posttranslational processing of nascent AT2 receptor protein was modulated by epitope tag and mode of expression.

Virus-mediated swapping of zolpidem-insensitive with zolpidem-sensitive GABA(A) receptors in cortical pyramidal cells

Recently developed pharmacogenetic and optogenetic approaches, with their own advantages and disadvantages, have become indispensable tools in modern neuroscience. Here, we employed a previously described knock-in mouse line (GABA_ARγ2^77Ilox) in which the γ2 subunit of the GABA_A receptor (GABA_AR) was mutated to become zolpidem insensitive (γ2^77I) and used viral vectors to swap γ2^77I with wild-type, zolpidem-sensitive γ2 subunits (γ2^77F). The verification of unaltered density and subcellular distribution of the virally introduced γ2 subunits requires their selective labelling. For this we generated six N- and six C-terminal-tagged γ2 subunits, with which cortical cultures of GABA_ARγ2^−/− mice were transduced using lentiviruses. We found that the N-terminal AU1 tag resulted in excellent immunodetection and unimpaired synaptic localization. Unaltered kinetic properties of the AU1-tagged γ2 (^AU1γ2^77F) channels were demonstrated with whole-cell patch-clamp recordings of spontaneous IPSCs from cultured cells. Next, we carried out stereotaxic injections of lenti- and adeno-associated viruses containing Cre-recombinase and the ^AU1γ2^77F subunit (Cre-2A-^AU1γ2^77F) into the neocortex of GABA_ARγ2^77Ilox mice. Light microscopic immunofluorescence and electron microscopic freeze-fracture replica immunogold labelling demonstrated the efficient immunodetection of the AU1 tag and the normal enrichment of the ^AU1γ2^77F subunits in perisomatic GABAergic synapses. In line with this, miniature and action potential-evoked IPSCs whole-cell recorded from transduced cells had unaltered amplitudes, kinetics and restored zolpidem sensitivity. Our results obtained with a wide range of structural and functional verification methods reveal unaltered subcellular distributions and functional properties of γ2^77I and ^AU1γ2^77F GABA_ARs in cortical pyramidal cells. This transgenic–viral pharmacogenetic approach has the advantage that it does not require any extrinsic protein that might endow some unforeseen alterations of the genetically modified cells. In addition, this virus-based approach opens up the possibility of modifying multiple cell types in distinct brain regions and performing alternative recombination-based intersectional genetic manipulations.

The EGFP/hERG fusion protein alter the electrophysiological properties of hERG channels in HEK293 cells

EGFP (enhanced green fluorescent protein) tagged to either the N (amino)-terminus [EGFP/hERG (human ether-a-go-go-related gene)] or C (carboxyl)-terminus (hERG/EGFP) of hERG channel is used to study mutant channel protein trafficking for several years. However, it has been reported that the process can alter hERG channel properties. The aim of the study was to determine whether EGFP tagged to N-terminus of hERG channels would alter the cellular localizations and the electrophysiological properties of hERG channels compared with untagged hERG channels. The hERG channels tagged with or without EGFP were transiently expressed in HEK (human embryonic kidney) 293 cells using a lipofectamine method. HEK 293 cells expressing pCDNA3-hERG or pEGFP-hERG were double immunolabelled with anti-hERG and anti-calnexin (an ER marker protein) followed with FITC- and TRITC (tetramethylrhodamine β-isothiocyanate)-labelled secondary antibodies, respectively. Confocal laser scanning microscope was used to observe the cellular localization of EGFP-tagged hERG channels and untagged hERG channels. Patch-clamp technique was used to record whole cell currents. We found that the EGFP/hERG fusion protein and untagged hERG channels were both expressed not only on the cell surface membrane but also in the cytoplasm of HEK293 cells. The EGFP/hERG appeared to influence the hERG channel gating properties, including reduction of the peak tail current density, more rapid inactivation process, faster recovery from inactivation and faster deactivation kinetics compared with untagged hERG channels. Our results suggest that the EGFP/hERG channel alter the electrophysiological properties of hERG channel, but it does not seem to alter the cellular location of hERG channels. Thus, EGFP tagging to N-terminus might be used for research of subcellular location of hERG channels but not for the channel electrophysiological properties.

Revisiting rat spermatogenesis with MALDI imaging at 20-microm resolution

Matrix-assisted laser desorption/ionization (MALDI) molecular imaging technology attracts increasing attention in the field of biomarker discovery. The unambiguous correlation between histopathology and MALDI images is a key feature for success. MALDI imaging mass spectrometry (MS) at high definition thus calls for technological developments that were established by a number of small steps. These included tissue and matrix preparation steps, dedicated lasers for MALDI imaging, an increase of the robustness against cell debris and matrix sublimation, software for precision matching of molecular and microscopic images, and the analysis of MALDI imaging data using multivariate statistical methods. The goal of these developments is to approach single cell resolution with imaging MS. Currently, a performance level of 20-μm image resolution was achieved with an unmodified and commercially available instrument for proteins detected in the 2-16-kDa range. The rat testis was used as a relevant model for validating and optimizing our technological developments. Indeed, testicular anatomy is among the most complex found in mammalian bodies. In the present study, we were able to visualize, at 20-μm image resolution level, different stages of germ cell development in testicular seminiferous tubules; to provide a molecular correlate for its well established stage-specific classification; and to identify proteins of interest using a top-down approach and superimpose molecular and immunohistochemistry images.

Role of progerin-induced telomere dysfunction in HGPS premature cellular senescence

Hutchinson-Gilford Progeria Syndrome (HGPS) is a premature-aging syndrome caused by a dominant mutation in the gene encoding lamin A, which leads to an aberrantly spliced and processed protein termed progerin. Previous studies have shown that progerin induces early senescence associated with increased DNA-damage signaling and that telomerase extends HGPS cellular lifespan. We demonstrate that telomerase extends HGPS cellular lifespan by decreasing progerin-induced DNA-damage signaling and activation of p53 and Rb pathways that otherwise mediate the onset of premature senescence. We show further that progerin-induced DNA-damage signaling is localized to telomeres and is associated with telomere aggregates and chromosomal aberrations. Telomerase amelioration of DNA-damage signaling is relatively rapid, requires both its catalytic and DNA-binding functions, and correlates in time with the acquisition by HGPS cells of the ability to proliferate. All of these findings establish that HGPS premature cellular senescence results from progerin-induced telomere dysfunction.

Functional impact of serial deletions at the C-terminus of the human GABArho1 receptor

GABArho1 receptors are formed by homopentameric assemblies that gate a chloride ion-channel upon activation by the neurotransmitter. Very little is known about the structural and functional roles played by the different domains that form each subunit; but one of them, the fourth transmembrane segment (TM4), is known to form a hydrophobic bundle together with three other TM segments that are necessary to stabilize the structure of the receptor. In this study we progressively removed amino acid residues from the C-terminus of the human GABArho1 and studied the functional properties of the receptor mutants expressed in X. laevis oocytes. We found that deletions of up to the last four residues gave rise to receptors that were still functional, generating currents of 3.92 microA for the wt, 5.75 microA for S479X, 1.82 microA for F478X, 0.52 microA for I477X and 0.27 microA for S476X when exposed to 5 microM GABA; surprisingly, the mutant with one residue removed resulted more sensitive to the agonists. Further deletions, up to residue W475, resulted in receptors that did not gate an ion-channel. In addition, deleting the signal sequence, from R2-A15, in the N-terminus produced non-functional receptors. This study reveals that GABArho1 can tolerate removal of several residues that form the fourth transmembrane segment up to a critical point, signaled by W475, beyond which the mutant protein is translated but does not form functional receptors. A comparative study is presented of some electrophysiological and pharmacological properties of the deletion mutants that were able to generate GABA currents.

Using fluorometry and ion-sensitive microelectrodes to study the functional expression of heterologously-expressed ion channels and transporters in Xenopus oocytes

The Xenopus laevis oocyte is a model system for the electrophysiological study of exogenous ion transporters. Three main reasons make the oocyte suitable for this purpose: (a) it has a large cell size (approximately 1mm diameter), (b) it has an established capacity to produce-from microinjected mRNAs or cRNAs-exogenous ion transporters with close-to-physiological post-translational modifications and actions, and (c) its membranes contain endogenous ion-transport activities which are usually smaller in magnitude than the activities of exogenously-expressed ion transporters. The expression of ion transporters as green fluorescent protein fusions allows the fluorometric assay of transporter yield in living oocytes. Monitoring of transporter-mediated movement of ions such as Cl(-), H(+) (and hence base equivalents like OH(-) and HCO(3)(-)), K(+), and Na(+) is achieved by positioning the tips of ion-sensitive microelectrodes inside the oocyte and/or at the surface of the oocyte plasma membrane. The use of ion-sensitive electrodes is critical for studying net ion-movements mediated by electroneutral transporters. The combined use of fluorometry and electrophysiology expedites transporter study by allowing measurement of transporter yield prior to electrophysiological study and correlation of relative transporter yield with transport rates.

Kaitocephalin antagonism of glutamate receptors expressed in Xenopus oocytes

Kaitocephalin is the first discovered natural toxin with protective properties against excitotoxic-death of cultured neurons induced by N-methyl-d-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainic acid (kainate, KA) receptors. Nevertheless, the effects of kaitocephalin on the function of these receptors were unknown. In this work we report some pharmacological properties of synthetic (-)-kaitocephalin on rat brain glutamate receptors expressed in Xenopus laevis oocytes and, on the homomeric AMPA-type GluR3 and KA-type GluR6 receptors. Kaitocephalin was found to be a more potent antagonist of NMDA receptors (IC(50) = 75 +/- 9 nM) than of AMPA receptors from cerebral cortex (IC(50) = 242 +/- 37 nM) and from homomeric GluR3 subunits (IC(50) = 502 +/- 55 nM). Moreover, kaitocephalin is a weak antagonist of the KA-type receptor GluR6 (IC(50) ~ 100 muM) and of metabotropic (IC(50) > 100 muM) glutamate receptors expressed by rat brain mRNA.

Functional characterization of N-terminally GFP-tagged GLP-1 receptor

The glucagon-like peptide-1 receptor (GLP-1 receptor) mediates important effects on peripheral tissues and the central nervous system. It seems one of the most promising therapeutic targets for treatment of diabetes mellitus type 2. Surprisingly, very little is known about the cellular mechanisms that regulate its function in vivo. One of the approaches to study receptor dynamics, expression, or signaling is using GFP-tagged fluorescent proteins. In this study, we synthesized and characterized N-terminally GFP-tagged GLP-1 (GFP-GLP-1) receptor in CHO cells. We demonstrated that GFP-GLP-1 receptor is weakly expressed in the plasma membranes and is functionally coupled to adenylyl cyclase via heterotrimeric G-proteins, similarly as its wild type.

Constructing glucagon like peptide-1 receptor fused with derivatives of GFP for visualizing protein-protein interaction in living cells

The glucagon-like peptide-1 receptor (GLP-1 receptor) mediates important antidiabetogenic effects on peripheral tissues. It appears to be one of the most promising therapeutic targets for treatment of diabetes mellitus type 2. Surprisingly, very little is known about the cellular mechanisms that regulate receptor function in living cells. One of the approaches how to study receptor dynamics is by using tagged fluorescent proteins. In this study, YFP-tagged GLP-1 (YFP-GLP-1) receptor and CFP-tagged GLP-1 (CFP-GLP-1) receptor for visualizing protein-protein interaction in living cells were constructed and localized in CHO cells. Cells expressing YFP-GLP-1 and CFP-GLP-1 receptor showed characteristic GLP-1 mediated increase in cAMP, similar to cells expressing a wild type GLP-1 receptor. This means that both types of receptors are functional and localized in plasma membrane.

Electrophysiological and fluorescence microscopy studies with HERG channel/EGFP fusion proteins

HERG (human ether-a-go-go-related gene) encodes the Kv11.1 protein alpha-subunit that underlies the rapidly activating delayed rectifier K+ current (IKr) in the heart. Alterations in the functional properties or membrane incorporation of HERG channels, either by genetic mutations or by administration of drugs, play major roles in the development of life-threatening torsades de pointes cardiac arrhythmias. Visualization of ion channel localization is facilitated by enhanced green fluorescent protein (EGFP) tagging, but this process can alter their properties. The aim of the present study was to characterize the electrophysiological properties and the cellular localization of HERG channels in which EGFP was tagged either to the C terminus (HERG/EGFP) or to the N terminus (EGFP/HERG). These fusion constructs were transiently expressed in human embryonic kidney (HEK) 293 cells, and the whole-cell patch-clamp configuration and a confocal laser scanning microscope with primary anti-HERG antibodies and fluorescently labeled secondary antibodies were used. For EGFP/HERG channels the deactivation kinetics were faster and the peak tail current density was reduced when compared to both wild-type HERG channels and HERG/EGFP channels. Laser scanning microscopic studies showed that both fusion proteins were localized in the cytoplasm and on discrete microdomains in the plasma membrane. The extent of labeling with anti-HERG antibodies of HEK 293 cells expressing EGFP/HERG channels was less when compared to HERG/EGFP channels. In conclusion, both electrophysiological and immunocytochemical studies showed that EGFP/HERG channels themselves have a protein trafficking defect. HERG/EGFP channels have similar properties as untagged HERG channels and, thus, might be especially useful for fluorescence microscopy studies.

Distinct subcellular distribution of delta-opioid receptor fused with various tags in PC12 cells

In small dorsal root ganglion neurons, delta-opioid receptors (DORs) have been found to be mainly distributed in the cytoplasm and often associated with the membrane of large dense-core vesicles (LDCVs) that contain neuropeptides. To study the distribution of DORs under various physiological or pharmacological conditions, the receptors fused with different tags are constructed, transfected into cells or animals, and examined with microscopy. In this study, we show that DOR with different tags have distinct patterns of subcellular distribution in neuroendocrine cells, PC12 cells. Both immunostaining and vesicle fraction analysis showed that the native DORs expressed in PC12 cells were mainly associated with LDCVs. In transfected PC12 cells, DOR tagged with Myc or hemagglutinin exhibited LDCV localization. However, DOR fused with GFP at N- or C-terminus was found to be mainly localized on the cell surface, and mediated the function of DOR agonist. Therefore, the distribution of DOR fused with GFP differs from the native DORs. These results suggest that the subcellular distribution of the receptor could be better presented by the fused tag with smaller molecular size.