Plasma Membrane Of Mammalian Cells Research Articles

Up‐regulation of atrial and neuronal Kir3 activity by cholesterol

Cholesterol is one of the major lipid components of the plasma membrane in mammalian cells, and its excess is associated with multiple pathological conditions including cardiovascular and neurodegenerative disease. In recent years, cholesterol has been emerging as a major regulator of ion channel function. Here we focus on G protein‐gated inwardly rectifying K+ (GIRK or Kir3) channels that play a key role in the regulation of membrane excitability in neuronal and cardiac cells.The most common effect of cholesterol on ion channels is a decrease in channel activity. In contrast, we have recently shown that unexpectedly cholesterol enrichment up‐regulates Kir3 activity in atrial myocytes. Notably, we show here that also in freshly isolated CA1 pyramidal neurons from hippocampus, in‐vitro cholesterol enrichment results in a strong increase in physiological Kir3 currents.To obtain mechanistic insights into the up‐regulation of cardiac and neuronal Kir3 channels by cholesterol, we employed planar lipid bilayers, mutagenesis studies using the Xenopus oocytes heterologous expression system, and molecular modeling. We first investigated the biophysical properties of the channel. Our data showed that the increase in channel activity following an increase in cholesterol levels is a result of an increase in the open probability of the channel and not in the unitary current amplitude and channel conductance. Next, we investigated why and how cholesterol up‐regulates Kir3 channels. Our studies traced the unique impact of cholesterol on cardiac Kir3 channels (up‐regulation and not down‐regulation) to a transmembrane region that includes residues in both the inner and outer alpha helices. Moreover, we identified putative cholesterol binding sites that overlap with the gating hinge region in the transmembrane domains of both cardiac and neuronal channels. Together, our studies provide novel insights into the molecular mechanism that underlies cholesterol‐driven up‐regulation of Kir3 currents in the heart and in the brain.Support or Funding InformationScientist Development Grant 11SDG5190025 from the American Heart Association (to ARD)

MTOR complex 1 activity is required to maintain the canonical endocytic recycling pathway against lysosomal delivery

The plasma membrane of mammalian cells undergoes constitutive endocytosis, endocytic sorting, and recycling, which delivers nutrients to the lysosomes. The receptors, along with membrane lipids, are normally returned to the plasma membrane to sustain this action. It is not known, however, whether this process is influenced by metabolic conditions. Here we report that endocytic recycling requires active mechanistic target of rapamycin (aka mammalian target of rapamycin) (mTORC1), a master metabolic sensor. Upon mTORC1 inactivation, either by starvation or by inhibitor, recycling receptors and plasma membrane lipids, such as transferrin receptors and sphingomyelin, are delivered to the lysosomes. This lysosomal targeting is independent of canonical autophagy: both WT and Atg5-/- mouse embryonic fibroblasts responded similarly. Furthermore, we identify hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), an endosomal sorting complexes required for transport (ESCORT-0) component, as a downstream target of mTORC1. Hrs requires mTORC1 activity to maintain its protein expression level. Silencing Hrs without decreasing mTORC1 activity is sufficient to target transferrin and sphingomyelin to the lysosomes. It is thus evident that the canonical recycling pathway is under the regulation of mTORC1 and likely most predominant in proliferating cells where mTORC1 is highly active.

Translocation of the non‐receptor protein GIV/Girdin to the plasma membrane activates heterotrimeric G proteins

Receptor tyrosine kinases (RTKs) and integrins have been traditionally considered to trigger signaling cascades different from those activated by G protein‐coupled receptors (GPCRs). However, GIV (aka Girdin) is a Guanine nucleotide Exchange Factor (GEF) that activates heterotrimeric G protein signaling downstream of RTKs and integrins. GIV is recruited to the cytoplasmic tail of RTKs and integrins upon stimulation, but the mechanism of activation of its GEF function is not well understood. We combined assays in yeast and BRET‐based biosensors in mammalian cells to investigate if association of GIV with the plasma membrane was sufficient to drive G protein activation. We found that GIV does not co‐fractionate with its substrate G protein Gαi3 on cell membranes of unstimulated cells. Anchoring of GIV to the plasma membrane of yeast cells in a constitutive manner or its rapid translocation to the plasma membrane of mammalian cells via chemically‐induced dimerization (CID) leads to robust G protein activation. GIV's Gα‐Binding and Activating (GBA) motif is necessary and sufficient for G protein activation upon membrane translocation. Furthermore, we engineered a synthetic protein to show that recruitment of GIV's GBA motif to membranes via association with active RTKs upon stimulation with a natural ligand, instead of via CID, is also sufficient for G protein activation. These results reveal that recruitment of GIV to membranes in close proximity to its substrate G protein is a mayor mechanism responsible for the activation of its G protein regulatory function.Support or Funding InformationThis work was supported by NIH grants (R01GM108733, R01GM112631) and American Cancer Society (RSG‐13‐362‐01‐TBE)

Naturally occurring anti-glycan antibodies binding to Globo H-expressing cells identify ovarian cancer patients

BackgroundGlycosphingolipids are important compounds of the plasma membrane of mammalian cells and a number of them have been associated with malignant transformation and progression, reinforcing tumour aggressiveness and metastasis. Here we investigated the levels of naturally occurring anti-glycan antibodies to Globo H in blood plasma obtained from high-grade serous ovarian cancer patients (SOC) and women without gynaecological malignancies (control) using suspension glycan array technology employing chemically synthesized glycans as antibody targets.ResultsWe found that anti-human Globo H IgG antibodies were able to significantly discriminate SOC from controls (P < 0.05). A combination with the clinically used tumour marker CA125 increased the diagnostic performance (AUC 0.8711). We next compared suspension array with standard flow cytometry in plasma samples and found that the level of anti-Globo H antibodies highly correlated (r = 0.992). The incubation of plasma-derived anti-glycan antibodies with chemically synthesized (presented on fluorescence microspheres) and native Globo H (expressed on Globo H-positive cell lines) revealed strong reactivity of naturally occurring human anti-Globo H antibodies towards its antigen expressed on ovarian cancer cells.ConclusionsOur data demonstrate that human plasma-derived antibodies to Globo H as well as the presence of the antigen might be considered as therapeutic option in ovarian cancer.

High-resolution imaging and quantification of plasma membrane cholesterol by NanoSIMS

Cholesterol is a crucial lipid within the plasma membrane of mammalian cells. Recent biochemical studies showed that one pool of cholesterol in the plasma membrane is "accessible" to binding by a modified version of the cytolysin perfringolysin O (PFO*), whereas another pool is sequestered by sphingomyelin and cannot be bound by PFO* unless the sphingomyelin is destroyed with sphingomyelinase (SMase). Thus far, it has been unclear whether PFO* and related cholesterol-binding proteins bind uniformly to the plasma membrane or bind preferentially to specific domains or morphologic features on the plasma membrane. Here, we used nanoscale secondary ion mass spectrometry (NanoSIMS) imaging, in combination with 15N-labeled cholesterol-binding proteins (PFO* and ALO-D4, a modified anthrolysin O), to generate high-resolution images of cholesterol distribution in the plasma membrane of Chinese hamster ovary (CHO) cells. The NanoSIMS images revealed preferential binding of PFO* and ALO-D4 to microvilli on the plasma membrane; lower amounts of binding were detectable in regions of the plasma membrane lacking microvilli. The binding of ALO-D4 to the plasma membrane was virtually eliminated when cholesterol stores were depleted with methyl-β-cyclodextrin. When cells were treated with SMase, the binding of ALO-D4 to cells increased, largely due to increased binding to microvilli. Remarkably, lysenin (a sphingomyelin-binding protein) also bound preferentially to microvilli. Thus, high-resolution images of lipid-binding proteins on CHO cells can be acquired with NanoSIMS imaging. These images demonstrate that accessible cholesterol, as judged by PFO* or ALO-D4 binding, is not evenly distributed over the entire plasma membrane but instead is highly enriched on microvilli.

Nanometer-Scale Lipid Clusters in Model Membranes Revealed by Atomic Recombination in Nanosims

Much of what is known about the lateral organization of biological membranes is inferred from the analogy between phase diagrams of ternary mixtures of lipids and the plasma membrane of mammalian cells. However, the relevance of phase diagrams of simple lipid mixtures to the compositionally complex and dynamic plasma membrane has not yet been established. Additionally, the fluorescence and spin labels used to build phase diagrams often interact with lipid bilayers, and detecting nanometer-scale phases remains challenging. We have previously introduced atomic recombination in dynamic SIMS as a technique for detecting lateral heterogeneities that occur on the nanometer length scale, below the resolution of both fluorescence microscopy and conventional NanoSIMS imaging. In this method, the formation of 13C15N- secondary ions from 13C and 15N atoms installed on different lipids only occurs if the isotopically-labeled lipids are within approximately 3 nm of each other. Isotope labels are desirable because they avoid problems caused by fluorescent dyes and because they directly report on the organization of the lipids of interest. Facile isotope-labeling chemistry allows the labels to be placed on any lipid of interest, allowing us to determine which lipids are clustering in complex mixtures. Here, we expand this technique to study the formation and composition of nanodomains of lipids in more complex mixtures for which phase diagrams have not been determined and would be too laborious to fully solve. Furthermore, we explore the formation of lipid clusters outside of phase boundaries, where lipids may still be inhomogenously distributed but display no first order phase transition. These more complex model membranes may be better models for lipid rafts given that lipids in the plasma membrane show no micrometer scale phase separation and no first order phase transition.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It.

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Modular assembly of synthetic proteins that span the plasma membrane in mammalian cells.

BackgroundTo achieve synthetic control over how a cell responds to other cells or the extracellular environment, it is important to reliably engineer proteins that can traffic and span the plasma membrane. Using a modular approach to assemble proteins, we identified the minimum necessary components required to engineer such membrane-spanning proteins with predictable orientation in mammalian cells.ResultsWhile a transmembrane domain (TM) fused to the N-terminus of a protein is sufficient to traffic it to the endoplasmic reticulum (ER), an additional signal peptidase cleavage site downstream of this TM enhanced sorting out of the ER. Next, a second TM in the synthetic protein helped anchor and accumulate the membrane-spanning protein on the plasma membrane. The orientation of the components of the synthetic protein were determined through measuring intracellular Ca2+ signaling using the R-GECO biosensor and through measuring extracellular quenching of yellow fluorescent protein variants by saturating acidic and salt conditions.ConclusionsThis work forms the basis of engineering novel proteins that span the plasma membrane to potentially control intracellular responses to extracellular conditions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12896-016-0320-7) contains supplementary material, which is available to authorized users.

Subcellular Fate of a Fluorescent Cholesterol-Poly(ethylene glycol) Conjugate: An Excellent Plasma Membrane Imaging Reagent.

Cholesterol-containing molecules or nanoparticles play a significant role in achieving favorable plasma membrane imaging and efficient cellular uptake of drugs by the excellent membrane anchoring capability of the cholesterol moiety. By linking cholesterol to a water-soluble component (such as poly(ethylene glycol), PEG), the resulting cholesterol-PEG conjugate can form micelles in aqueous solution through self-assembly, and such a micellar structure represents an important drug delivery vehicle in which hydrophobic drugs can be encapsulated. However, the understanding of the subcellular fate and cytotoxicity of cholesterol-PEG conjugates themselves remains elusive. Herein, by using cholesterol-PEG2000-fluorescein isothiocyanate (Chol-PEG-FITC) as a model system, we found that the Chol-PEG-FITC molecules could attach to the plasma membranes of mammalian cells within 10 min and such a firm membrane attachment could last at least 1 h, displaying excellent plasma membrane staining performance that surpassed that of commonly used commercial membrane dyes such as DiD and CellMask. Besides, we systematically studied the endocytosis pathway and intracellular distribution of Chol-PEG-FITC and found that the cell surface adsorption and endocytosis processes of Chol-PEG-FITC molecules were lipid-raft-dependent. After internalization, the Chol-PEG-FITC molecules gradually reached many organelles with membrane structures. At 5 h, they were mainly distributed in lysosomes and the Golgi apparatus, with some in the endoplasmic reticulum (ER) and very few in the mitochondrion. At 12 h, the Chol-PEG-FITC molecules mostly aggregated in the Golgi apparatus and ER close to the nucleus. Finally, we demonstrated that Chol-PEG-FITC was toxic to mammalian cells only at concentrations above 50 μM. In summary, Chol-PEG-FITC can be a promising plasma membrane imaging reagent to avoid the fast cellular internalization and quick membrane detachment problems faced by commercial membrane dyes. We believe that the investigation of the dynamic subcellular fate of Chol-PEG-FITC can provide important knowledge to facilitate the use of cholesterol-PEG conjugates in fields such as cell surface engineering and drug delivery.

A novel mutant Na+/HCO3− cotransporter NBCe1 in a case of compound‐heterozygous inheritance of proximal renal tubular acidosis

The inheritance of two defective alleles of SLC4A4, the gene that encodes the widely-expressed electrogenic sodium bicarbonate cotransporter NBCe1, results in the bicarbonate-wasting disease proximal renal tubular acidosis (pRTA). In the present study, we report the first case of compound-heterozygous inheritance of pRTA (p.Arg510His/p.Gln913Arg) in an individual with low blood pH, blindness and neurological signs that resemble transient ischaemic attacks. We employ fluorescence microscopy on non-polarized (human embryonic kidney) and polarized (Madin-Darby canine kidney) renal cell lines and electrophysiology on Xenopus oocytes to characterize the mutant transporters (R510H and Q913R). Both mutant transporters exhibit enhanced intracellular retention in renal cells, an observation that probably explains the HCO3- transport deficit in the individual. Both mutants retain a close-to-normal per molecule Na+ /HCO3- cotransport activity in Xenopus oocytes, suggesting that they are suitable candidates for folding-correction therapy. However, Q913R expression is uniquely associated with a depolarizing, HCO3- independent, Cl- -conductance in oocytes that could have pathological consequences if expressed in the cells of patients. Proximal renal tubular acidosis (pRTA) is a rare, recessively-inherited disease characterized by abnormally acidic blood, blindness, as well as below average height and weight. pRTA is typically associated with homozygous mutation of the solute carrier 4 family gene SLC4A4. SLC4A4 encodes the electrogenic sodium bicarbonate cotransporter NBCe1, a membrane protein that acts to maintain intracellular and plasma pH. We present the first description of a case of compound-heterozygous inheritance of pRTA. The individual has inherited two mutations in NBCe1: p.Arg510His (R510H) and p.Gln913Arg (Q913R), one from each parent. In addition to the usual features of pRTA, the patient exhibits unusual signs, such as muscle spasms and fever. We have recreated these mutant transporters for expression in model systems. We find that both of the mutant proteins exhibit substantial intracellular retention when expressed in mammalian renal cell lines. When expressed in Xenopus oocytes, we find that the R510H and Q913R-mutant NBCe1 molecules exhibit apparently normal Na+ /HCO3- cotransport activity but that Q913R is associated with an unusual HCO3- independent anion-leak. We conclude that a reduced accumulation of NBCe1 protein in the basolateral membrane of proximal-tubule epithelia is the most probable cause of pRTA in this case. We further note that the Q913R-associated anion-leak could itself be pathogenic if expressed in the plasma membrane of mammalian cells, compromising the benefit of strategies aiming to enhance mutant NBCe1 accumulation in the plasma membrane.

Graphene Oxide Nanosheets Stimulate Ruffling and Shedding of Mammalian Cell Plasma Membranes

Graphene oxide (GO) has attracted intense interest for use in living systems and environmental applications. GO's compatibility with mammalian cells is sometimes inferred from its low cytotoxicity, but such conclusions ignore non-lethal effects that will influence GO's utility. Here we demonstrate, with rat basophilic leukemia (RBL) cells, profound plasma membrane (PM) ruffling and shedding induced by GO using confocal and live cell fluorescence microscopy, as well as scanning electron microscopy. These membrane structures contain immunoglobulin E receptors, are resistant to detergents, and lack detectable fluorescence labeling of F-actin and fibronectin. The formation of these membrane structures correlates with a loss of contact inhibition between RBL cells. We observe similar cellular responses towards GO for NIH-3T3 fibroblast cells and MDA-MB-231 human breast cancer cells. These findings reveal a previously unreported cellular response towards foreign nanomaterials. Membrane ruffling and shedding raise fundamental questions about how GO interacts with the PM, as well as its potential to modulate cellular mechanosensing for tissue engineering, stem cell differentiation, and other biomedical applications.

Abstract 4333: Identification and optimization of the first highly selective GLUT-1 inhibitors

Abstract Increased glycolysis rates combined with an insufficient cellular respiration in tumor cells have been known for decades as the Warburg effect (1930). One of the key players in this metabolic alteration of cancer cells is glucose transporter 1 (or GLUT-1), which facilitates the transport of glucose across the plasma membranes of mammalian cells. Many tumor biopsies underline the crucial importance of GLUT-1 overexpression for the vitality of many tumor entities. Moreover the glucose uptake dependency of cancer cells can be visualized by the widespread clinical application of 18fluorodeoxyglucose (FDG) uptake in PET tumor imaging. Therefore, inhibition of glucose uptake for the treatment of cancer seems to be an interesting approach. Based on our first identified GLUT-1 inhibitors (1H-pyrazolo[3,4-d]pyrimidines) with modest selectivity profile, a high throughput screen (HTS) was performed to identify new potent lead structures with unique selectivity against the other members of the GLUT-1 family (GLUT-2,-3 and -4). Among several lead structures we identified from the HTS the quinoline carboxamides with the most promising selectivity profile. Extensive SAR elaboration by synthesizing more than 1600 new derivatives of the lead structure clearly revealed the essential moieties for high activity, potency, and selectivity. Additionally, we were able to improve the in vitro and in vivo PK properties permitting in vivo animal studies. We are able to report for the first time the preclinical profile and structure of the very potent and highly selective GLUT-1 inhibitor BAY-876 representing an ideal tool to further evaluate the scope and limitations of GLUT-1 inhibition as new therapeutic option. Citation Format: Marcus Bauser, Bernd Buchmann, Holger Siebeneicher, Arwed Cleve, Hartmut Rehwinkel, Roland Neuhaus, Iring Heisler, Thomas Mueller, Charlotte Christine Kopitz. Identification and optimization of the first highly selective GLUT-1 inhibitors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4333.

Universal Cell Surface Imaging for Mammalian, Fungal, and Bacterial Cells

Because of the distinct surface structures of different cells (mammalian cells, fungi, and bacteria), surface labeling for these cells requires a variety of fluorescent dyes. Besides, fluorescent dyes (especially the commercial ones) for staining Gram-negative bacterial cell walls are still lacking. Herein, a conformation-adjustable glycol chitosan (GC) derivative (GC-PEG cholesterol-FITC) with "all-in-one" property was developed to realize universal imaging for plasma membranes of mammalian cells (via hydrophobic interaction) and cell walls of fungal and bacterial cells (via electrostatic interaction). By comparing the different staining behaviors of GC-PEG cholesterol-FITC and three other analogs (GC-PEG-FITC, GC-FITC, and cholesterol-PEG-FITC), we have elucidated the different roles the hydrophobic and electrostatic interactions play in the staining performance of these different cells. Such a simple, noncytotoxic, economic, and universal cell surface staining reagent will be very useful for investigating cell surface-related biological events and advancing cell surface engineering of various types of cells.

Measurement of Rapid Amiloride-Dependent pH Changes at the Cell Surface Using a Proton-Sensitive Field-Effect Transistor.

We present a novel method for the rapid measurement of pH fluxes at close proximity to the surface of the plasma membrane in mammalian cells using an ion-sensitive field-effect transistor (ISFET). In conjuction with an efficient continuous superfusion system, the ISFET sensor was capable of recording rapid changes in pH at the cells’ surface induced by intervals of ammonia loading and unloading, even when using highly buffered solutions. Furthermore, the system was able to isolate physiologically relevant signals by not only detecting the transients caused by ammonia loading and unloading, but display steady-state signals as would be expected by a proton transport-mediated influence on the extracellular proton-gradient. Proof of concept was demonstrated through the use of 5-(N-ethyl-N-isopropyl)amiloride (EIPA), a small molecule inhibitor of sodium/hydrogen exchangers (NHE). As the primary transporter responsible for proton balance during cellular regulation of pH, non-electrogenic NHE transport is notoriously difficult to detect with traditional methods. Using the NHE positive cell lines, Chinese hamster ovary (CHO) cells and NHE3-reconstituted mouse skin fibroblasts (MSF), the sensor exhibited a significant response to EIPA inhibition, whereas NHE-deficient MSF cells were unaffected by application of the inhibitor.

Imaging approaches for analysis of cholesterol distribution and dynamics in the plasma membrane

Cholesterol is an important lipid component of the plasma membrane (PM) of mammalian cells, where it is involved in control of many physiological processes, such as endocytosis, cell migration, cell signalling and surface ruffling. In an attempt to explain these functions of cholesterol, several models have been put forward about cholesterol’s lateral and transbilayer organization in the PM. In this article, we review imaging techniques developed over the last two decades for assessing the distribution and dynamics of cholesterol in the PM of mammalian cells. Particular focus is on fluorescence techniques to study the lateral and inter-leaflet distribution of suitable cholesterol analogues in the PM of living cells. We describe also several methods for determining lateral cholesterol dynamics in the PM including fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), single particle tracking (SPT) and spot variation FCS coupled to stimulated emission depletion (STED) microscopy. For proper interpretation of such measurements, we provide some background in probe photophysics and diffusion phenomena occurring in cell membranes. In particular, we show the equivalence of the reaction-diffusion approach, as used in FRAP and FCS, and continuous time random walk (CTRW) models, as often invoked in SPT studies. We also discuss mass spectrometry (MS) based imaging of cholesterol in the PM of fixed cells and compare this method with fluorescence imaging of sterols. We conclude that evidence from many experimental techniques converges towards a model of a homogeneous distribution of cholesterol with largely free and unhindered diffusion in both leaflets of the PM.

Modular Assembly of Synthetic Proteins that Span the Plasma Membrane in Mammalian Cells

Background To achieve synthetic control over how a cell responds to other cells or the extracellular environment, it is important to reliably engineer proteins that can traffic and span the plasma membrane. Using a modular approach to assemble proteins, we identified the minimum necessary components required to engineer such membrane-spanning proteins with predictable orientation in mammalian cells.

On the puzzling distribution of cholesterol in the plasma membrane

The distribution of cholesterol between the two leaves of the plasma membrane in mammalian cells presents a conundrum; given cholesterol's known affinity for sphingomyelin, which resides predominantly in the exoplasmic leaf, why is it that experiment finds a majority of the cholesterol in the cytoplasmic leaf? This article reviews a recently proposed solution to this puzzle.

Dopamine Transporter Activity Is Modulated by α-Synuclein

The duration and strength of the dopaminergic signal are regulated by the dopamine transporter (DAT). Drug addiction and neurodegenerative and neuropsychiatric diseases have all been associated with altered DAT activity. The membrane localization and the activity of DAT are regulated by a number of intracellular proteins. α-Synuclein, a protein partner of DAT, is implicated in neurodegenerative disease and drug addiction. Little is known about the regulatory mechanisms of the interaction between DAT and α-synuclein, the cellular location of this interaction, and the functional consequences of this interaction on the basal, amphetamine-induced DAT-mediated dopamine efflux, and membrane microdomain distribution of the transporter. Here, we found that the majority of DAT·α-synuclein protein complexes are found at the plasma membrane of dopaminergic neurons or mammalian cells and that the amphetamine-mediated increase in DAT activity enhances the association of these proteins at the plasma membrane. Further examination of the interaction of DAT and α-synuclein revealed a transient interaction between these two proteins at the plasma membrane. Additionally, we found DAT-induced membrane depolarization enhances plasma membrane localization of α-synuclein, which in turn increases dopamine efflux and enhances DAT localization in cholesterol-rich membrane microdomains.

A bioanalytical assay to distinguish cellular uptake routes for liposomes.

Lipid-based nanoparticles are frequently used for drug or DNA delivery into mammalian cells. However it is difficult to determine whether such particles are taken up via endocytosis or fusion to the plasma membrane. Here, we propose a simple and reliable analytical method to do so based on the unique spectral properties of the fluorescent tracer BODIPY FL. At high local concentrations, this dye displays an additional red-shifted emission peak that is absent at low concentrations. In dye-loaded liposomes taken up by endocytosis, the local dye concentration did not significantly change upon internalization. Accordingly, unchanged fluorescence spectra were detected. When cells were incubated with liposomes able to fuse with the plasma membrane of mammalian cells, a reduction of local dye concentration and much weaker emission in the red-shifted peak were observed. The ratio of intensities in both fluorescence channels was shown to be a reliable indicator of the cellular uptake mechanism.

STED-FLCS: An Advanced Tool to Reveal Spatiotemporal Heterogeneity of Molecular Membrane Dynamics.

Heterogeneous diffusion dynamics of molecules play an important role in many cellular signaling events, such as of lipids in plasma membrane bioactivity. However, these dynamics can often only be visualized by single-molecule and super-resolution optical microscopy techniques. Using fluorescence lifetime correlation spectroscopy (FLCS, an extension of fluorescence correlation spectroscopy, FCS) on a super-resolution stimulated emission depletion (STED) microscope, we here extend previous observations of nanoscale lipid dynamics in the plasma membrane of living mammalian cells. STED-FLCS allows an improved determination of spatiotemporal heterogeneity in molecular diffusion and interaction dynamics via a novel gated detection scheme, as demonstrated by a comparison between STED-FLCS and previous conventional STED-FCS recordings on fluorescent phosphoglycerolipid and sphingolipid analogues in the plasma membrane of live mammalian cells. The STED-FLCS data indicate that biophysical and biochemical parameters such as the affinity for molecular complexes strongly change over space and time within a few seconds. Drug treatment for cholesterol depletion or actin cytoskeleton depolymerization not only results in the already previously observed decreased affinity for molecular interactions but also in a slight reduction of the spatiotemporal heterogeneity. STED-FLCS specifically demonstrates a significant improvement over previous gated STED-FCS experiments and with its improved spatial and temporal resolution is a novel tool for investigating how heterogeneities of the cellular plasma membrane may regulate biofunctionality.