Dynamics of single potassium channel proteins in the plasma membrane of migrating cells

Functional expression of mitochondrial KCa3.1 channels in non-small cell lung cancer cells

Lung cancer is one of the leading causes of cancer-related deaths worldwide. The Ca²⁺-activated K⁺ channel K_Ca3.1 contributes to the progression of non-small cell lung cancer (NSCLC). Recently, K_Ca3.1 channels were found in the inner membrane of mitochondria in different cancer cells. Mitochondria are the main sources for the generation of reactive oxygen species (ROS) that affect the progression of cancer cells. Here, we combined Western blotting, immunofluorescence, and fluorescent live-cell imaging to investigate the expression and function of K_Ca3.1 channels in the mitochondria of NSCLC cells. Western blotting revealed K_Ca3.1 expression in mitochondrial lysates from different NSCLC cells. Using immunofluorescence, we demonstrate a co-localization of K_Ca3.1 channels with mitochondria of NSCLC cells. Measurements of the mitochondrial membrane potential with TMRM reveal a hyperpolarization following the inhibition of K_Ca3.1 channels with the cell-permeable blocker senicapoc. This is not the case when cells are treated with the cell-impermeable peptidic toxin maurotoxin. The hyperpolarization of the mitochondrial membrane potential is accompanied by an increased generation of ROS in NSCLC cells. Collectively, our results provide firm evidence for the functional expression of K_Ca3.1 channels in the inner membrane of mitochondria of NSCLC cells.

Single-Molecule Imaging in Living Plant Cells: A Methodological Review

Single-molecule imaging is emerging as a revolutionary approach to studying fundamental questions in plants. However, compared with its use in animals, the application of single-molecule imaging in plants is still underexplored. Here, we review the applications, advantages, and challenges of single-molecule fluorescence imaging in plant systems from the perspective of methodology. Firstly, we provide a general overview of single-molecule imaging methods and their principles. Next, we summarize the unprecedented quantitative details that can be obtained using single-molecule techniques compared to bulk assays. Finally, we discuss the main problems encountered at this stage and provide possible solutions.

pH-Channeling in Cancer: How pH-Dependence of Cation Channels Shapes Cancer Pathophysiology

Tissue acidosis plays a pivotal role in tumor progression: in particular, interstitial acidosis promotes tumor cell invasion, and is a major contributor to the dysregulation of tumor immunity and tumor stromal cells. The cell membrane and integral membrane proteins commonly act as important sensors and transducers of altered pH. Cell adhesion molecules and cation channels are prominent membrane proteins, the majority of which is regulated by protons. The pathophysiological consequences of proton-sensitive ion channel function in cancer, however, are scarcely considered in the literature. Thus, the main focus of this review is to highlight possible events in tumor progression and tumor immunity where the pH sensitivity of cation channels could be of great importance.

Rhomboid distorts lipids to break the viscosity-imposed speed limit of membrane diffusion

Enzymes that cut proteins inside membranes regulate diverse cellular events, including cell signaling, homeostasis, and host-pathogen interactions. Adaptations that enable catalysis in this exceptional environment are poorly understood. We visualized single molecules of multiple rhomboid intramembrane proteases and unrelated proteins in living cells (human and Drosophila) and planar lipid bilayers. Notably, only rhomboid proteins were able to diffuse above the Saffman-Delbrück viscosity limit of the membrane. Hydrophobic mismatch with the irregularly shaped rhomboid fold distorted surrounding lipids and propelled rhomboid diffusion. The rate of substrate processing in living cells scaled with rhomboid diffusivity. Thus, intramembrane proteolysis is naturally diffusion-limited, but cells mitigate this constraint by using the rhomboid fold to overcome the "speed limit" of membrane diffusion.

Single Quantum Dot Tracking Illuminates Neuroscience at the Nanoscale

The use of nanometer-sized semiconductor crystals, known as quantum dots, allows us to directly observe individual biomolecular transactions through a fluorescence microscope. Here, we review the evolution of single quantum dot tracking over the past two decades, highlight key biophysical discoveries facilitated by quantum dots, briefly discuss biochemical and optical implementation strategies for a single quantum dot tracking experiment, and report recent accomplishments of our group at the interface of molecular neuroscience and nanoscience.

Exploring the Spatiotemporal Organization of Membrane Proteins in Living Plant Cells

Plasma membrane proteins have important roles in transport and signal transduction. Deciphering the spatiotemporal organization of these proteins provides crucial information for elucidating the links between the behaviors of different molecules. However, monitoring membrane proteins without disrupting their membrane environment remains difficult. Over the past decade, many studies have developed single-molecule techniques, opening avenues for probing the stoichiometry and interactions of membrane proteins in their native environment by providing nanometer-scale spatial information and nanosecond-scale temporal information. In this review, we assess recent progress in the development of labeling and imaging technology for membrane protein analysis. We focus in particular on several single-molecule techniques for quantifying the dynamics and assembly of membrane proteins. Finally, we provide examples of how these new techniques are advancing our understanding of the complex biological functions of membrane proteins.

Limitations of Qdot labelling compared to directly-conjugated probes for single particle tracking of B cell receptor mobility

Single-particle tracking (SPT) is a powerful method for exploring single-molecule dynamics in living cells with nanoscale spatiotemporal resolution. Photostability and bright fluorescence make quantum dots (Qdots) a popular choice for SPT. However, their large size could potentially alter the mobility of the molecule of interest. To test this, we labelled B cell receptors on the surface of B-lymphocytes with monovalent Fab fragments of antibodies that were either linked to Qdots via streptavidin or directly conjugated to the small organic fluorophore Cy3. Imaging of receptor mobility by total internal reflection fluorescence microscopy (TIRFM), followed by quantitative single-molecule diffusion and confinement analysis, definitively showed that Qdots sterically hinder lateral mobility regardless of the substrate to which the cells were adhered. Qdot labelling also drastically altered the frequency with which receptors transitioned between apparent slow- and fast-moving states and reduced the size of apparent confinement zones. Although we show that Qdot-labelled probes can detect large differences in receptor mobility, they fail to resolve subtle differences in lateral diffusion that are readily detectable using Cy3-labelled Fabs. Our findings highlight the utility and limitations of using Qdots for TIRFM and wide-field-based SPT, and have significant implications for interpreting SPT data.

Immature human dendritic cells enhance their migration through KCa3.1 channel activation

Migration capacity is essential for dendritic cells (DCs) to present antigen to T cells for the induction of immune response. The DC migration is supposed to be a calcium-dependent process, while not fully understood. Here, we report a role of the KCa3.1/IK1/SK4 channels in the migration capacity of both immature (iDC) and mature (mDC) human CD14(+)-derived DCs. KCa3.1 channels were shown to control the membrane potential of human DC and the Ca(2+) entry, which is directly related to migration capacities. The expression of migration marker such as CCR5 and CCR7 was modified in both types of DCs by TRAM-34 (100nM). But, only the migration of iDC was decreased by use of both TRAM-34 and KCa3.1 siRNA. Confocal analyses showed a close localization of CCR5 with KCa3.1 in the steady state of iDC. Finally, the implication of KCa3.1 seems to be limited to the migration capacities as T cell activation of DCs appeared unchanged. Altogether, these results demonstrated that KCa3.1 channels have a pro-migratory effect on iDC migration. Our findings suggest that KCa3.1 in human iDC play a major role in their migration and constitute an attractive target for the cell therapy optimization.

New light on ion channel imaging by total internal reflection fluorescence (TIRF) microscopy

Ion channels play pivotal roles in a wide variety of cellular functions; therefore, their physiological characteristics, pharmacological responses, and molecular structures have been extensively investigated. However, the mobility of an ion channel itself in the cell membrane has not been examined in as much detail. A total internal reflection fluorescence (TIRF) microscope allows fluorophores to be imaged in a restricted region within an evanescent field of less than 200 nm from the interface of the coverslip and plasma membrane in living cells. Thus the TIRF microscope is useful for selectively visualizing the plasmalemmal surface and subplasmalemmal zone. In this review, we focused on a single-molecule analysis of the dynamic movement of ion channels in the plasma membrane using TIRF microscopy. We also described two single-molecule imaging techniques under TIRF microscopy: fluorescence resonance energy transfer (FRET) for the identification of molecules that interact with ion channels, and subunit counting for the determination of subunit stoichiometry in a functional channel. TIRF imaging can also be used to analyze spatiotemporal Ca(2+) events in the subplasmalemma. Single-molecule analyses of ion channels and localized Ca(2+) signals based on TIRF imaging provide beneficial pharmacological and physiological information concerning the functions of ion channels.

CLAVATA3 dodecapeptide modified CdTe nanoparticles: a biocompatible quantum dot probe for in vivo labeling of plant stem cells

CLAVATA3 (CLV3) dodecapeptides function in plant stem cell maintenance, but CLV3 function in cell-cell communication remains less clear. Here, we coupled CLV3 dodecapeptides to synthesized CdTe nanoparticles to track their bioactivity on stem cells in the root apical meristem. To achieve this, we first synthesized CdTe quantum dots (QDs) using a one-pot method, and then evaluated the cytotoxicity of the QDs in BY-2 cells. The results showed that QDs in plant cells must be used at low concentrations and for short treatment time. To make biocompatible probes to track stem cell fate, we conjugated CLV3 dodecapeptides to the QDs by the zero-coupling method; this modification greatly reduced the cytotoxicity of the QDs. Furthermore, we detected CLV3-QDs localized on the cell membrane, consistent with the known localization of CLV3. Our results indicate that using surface-modified QDs at low concentrations and for short time treatment can improve their utility for plant cell imaging.

Interaction of tumour cells with their microenvironment: ion channels and cell adhesion molecules. A focus on pancreatic cancer

Cancer must be viewed as a 'tissue', constituted of both transformed cells and a heterogeneous microenvironment, the 'tumour microenvironment' (TME). The TME undergoes a complex remodelling during the course of multistep tumourigenesis, hence strongly contributing to tumour progression. Ion channels and transporters (ICTs), being expressed on both tumour cells and in the different cellular components of the TME, are in a strategic position to sense and mediate signals arising from the TME. Often, this transmission is mediated by integrin adhesion receptors, which are the main cellular receptors capable of mediating cell-to-cell and cell-to-matrix bidirectional signalling. Integrins can often operate in conjunction with ICT because they can behave as functional partners of ICT proteins. The role of integrin receptors in the crosstalk between tumour cells and the TME is particularly relevant in the context of pancreatic cancer (PC), characterized by an overwhelming TME which actively contributes to therapy resistance. We discuss the possibility that this occurs through integrins and ICTs, which could be exploited as targets to overcome chemoresistance in PC.

Assembling an ion channel: ORF 3a from SARS-CoV

Protein 3a is a 274 amino acid polytopic channel protein with three putative transmembrane domains (TMDs) encoded by severe acute respiratory syndrome corona virus (SARS-CoV). Synthetic peptides corresponding to each of its three individual transmembrane domains (TMDs) are reconstituted into artificial lipid bilayers. Only TMD2 and TMD3 induce channel activity. Reconstitution of the peptides as TMD1 + TMD3 as well as TMD2 + TMD3 in a 1 : 1 mixture induces membrane activity for both mixtures. In a 1 : 1 : 1 mixture, channel like behavior is almost restored. Expression of full length 3a and reconstitution into artificial lipid bilayers reveal a weak cation selective (PK ≈ 2 PCl ) rectifying channel. In the presence of nonphysiological concentration of Ca-ions the channel develops channel activity.

Encapsulating quantum dots into enveloped virus in living cells for tracking virus infection

Utilization of quantum dots (QDs) for single virus tracking has attracted growing interest. Through modification of viral surface proteins, viruses can be labeled with various functionalized QDs and used for tracking the routes of viral infections. However, incorporation of QDs on the viral surface may affect the efficiency of viral entry and alter virus-cell interactions. Here, we describe that QDs can be encapsulated into the capsid of vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped lentivirus (PTLV) in living cells without modification of the viral surface. QDs conjugated with modified genomic RNAs (gRNAs), which contain a packaging signal (Psi) sequence for viral genome encapsulation, can be packaged into virions together with the gRNAs. QD-containing PTLV demonstrated similar entry efficiency as the wild-type PTLV. After infection, QD signals entered the Rab5+ endosome and then moved to the microtubule organizing center of the infected cells in a microtubule-dependent manner. Findings in this study are consistent with previously reported infection routes of VSV and VSV-G pseudotyped lentivirus, indicating that our established QD packaging approach can be used for enveloped virus labeling and tracking.

A critical survey of methods to detect plasma membrane rafts

The plasma membrane is still one of the enigmatic cellular structures. Although the microscopic structure is getting clearer, not much is known about the organization at the nanometre level. Experimental difficulties have precluded unambiguous approaches, making the current picture rather fuzzy. In consequence, a variety of different membrane models has been proposed over the years, on the basis of different experimental strategies. Recent data obtained via high-resolution single-molecule microscopy shed new light on the existing hypotheses. We thus think it is a good time for reviewing the consistency of the existing models with the new data. In this paper, we summarize the available models in ten propositions, each of which is discussed critically with respect to the applied technologies and the strengths and weaknesses of the approaches. Our aim is to provide the reader with a sound basis for his own assessment. We close this chapter by exposing our picture of the membrane organization at the nanoscale.

Membrane protein dynamics and functional implications in mammalian cells

The organization of the plasma membrane is both highly complex and highly dynamic. One manifestation of this dynamic complexity is the lateral mobility of proteins within the plane of the membrane, which is often an important determinant of intermolecular protein-binding interactions, downstream signal transduction, and local membrane mechanics. The mode of membrane protein mobility can range from random Brownian motion to immobility and from confined or restricted motion to actively directed motion. Several methods can be used to distinguish among the various modes of protein mobility, including fluorescence recovery after photobleaching, single-particle tracking, fluorescence correlation spectroscopy, and variations of these techniques. Here, we present both a brief overview of these methods and examples of their use to elucidate the dynamics of membrane proteins in mammalian cells-first in erythrocytes, then in erythroblasts and other cells in the hematopoietic lineage, and finally in non-hematopoietic cells. This multisystem analysis shows that the cytoskeleton frequently governs modes of membrane protein motion by stably anchoring the proteins through direct-binding interactions, by restricting protein diffusion through steric interactions, or by facilitating directed protein motion. Together, these studies have begun to delineate mechanisms by which membrane protein dynamics influence signaling sequelae and membrane mechanical properties, which, in turn, govern cell function.

Mutations of the EPHB6 receptor tyrosine kinase induce a pro-metastatic phenotype in non-small cell lung cancer

Alterations of Eph receptor tyrosine kinases are frequent events in human cancers. Genetic variations of EPHB6 have been described but the functional outcome of these alterations is unknown. The current study was conducted to screen for the occurrence and to identify functional consequences of EPHB6 mutations in non-small cell lung cancer. Here, we sequenced the entire coding region of EPHB6 in 80 non-small cell lung cancer patients and 3 tumor cell lines. Three potentially relevant mutations were identified in primary patient samples of NSCLC patients (3.8%). Two point mutations led to instable proteins. An in frame deletion mutation (del915-917) showed enhanced migration and accelerated wound healing in vitro. Furthermore, the del915-917 mutation increased the metastatic capability of NSCLC cells in an in vivo mouse model. Our results suggest that EPHB6 mutations promote metastasis in a subset of patients with non-small cell lung cancer.

Total internal reflection fluorescence imaging of Ca(2+)-induced Ca(2+) release in mouse urinary bladder smooth muscle cells

In smooth muscles (SMs), cytosolic Ca(2+) ([Ca(2+)](cyt)) dynamics during an action potential are triggered by Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs) in the plasma membrane. The physiological significance of Ca(2+) amplification by subsequent Ca(2+) release through ryanodine receptors (RyRs) from the sarcoplasmic reticulum (SR) is still a matter of topics in SMs. In the present study, depolarization-evoked local Ca(2+) dynamics in Ca(2+) microdomain were imaged using total internal reflection fluorescence (TIRF) microscopy in mouse urinary bladder SM cells (UBSMCs). Upon depolarization under whole-cell voltage-clamp, the rapid and local elevation of [Ca(2+)](cyt) was followed by larger [Ca(2+)](cyt) increase with propagation occurred in a limited TIRF zone within ~200nm from cell surface. The depolarization-evoked [Ca(2+)](cyt) increase in a TIRF zone was abolished or greatly reduced by the pretreatment with Cd(2+) or ryanodine, respectively. The initial local [Ca(2+)](cyt) increases were mediated by Ca(2+) influx through single or clustered VDCCs as Ca(2+) sparklets, and the following step was elicited by Ca(2+)-induced Ca(2+) release (CICR) through RyR from SR. The depolarization-induced outward currents, mainly due to large-conductance Ca(2+)-activated K(+) channel activation, were also markedly reduced by Cd(2+) and ryanodine. In addition, TIRF analyses showed that the fluorescent signals of individual or clustered VDCC distributed in relatively uniform fashion and that a subset of RyRs in the subplasmalemmal SR also located in TIRF zone. In conclusion, fast TIRF imaging successfully demonstrated two step Ca(2+) events upon depolarization in Ca(2+) microdomain of UBSMCs; the initial Ca(2+) influx as Ca(2+) sparklets through discrete VDCC or their clusters and the following CICR via the activation of loosely coupled RyRs in SR located in the Ca(2+) microdomains.

Molecular assembly and dynamics of fluorescent protein-tagged single KCa1.1 channel in expression system and vascular smooth muscle cells

The large-conductance Ca(2+)-activated K(+) (K(Ca)1.1, BK) channel has pivotal roles in the regulation of vascular tone. To clarify the molecular dynamics of BK channels and their functionally coupled protein on the membrane surface, we examined single-molecule imaging of fluorescent-labeled BK subunits in the plasma membrane using total internal reflection fluorescence (TIRF) microscopy. The dynamic mobility of yellow fluorescent protein (YFP)-tagged BKα subunit (BKα-YFP) expressed in human embryo kidney 293 (HEK) cells was detected in TIRF regions at the level of individual channels and their clusters on the plasma membrane with a diffusion coefficient of 6.7 × 10(3) nm(2)/s. When BKα-YFP was coexpressed with cyan fluorescent protein (CFP)-tagged BKβ1 subunit (BKβ1-CFP) in HEK cells, the mobility was reduced by ∼50%. Fluorescent image analyses suggest that green fluorescent protein (GFP)-tagged BKα subunit (BKα-GFP) expressed in vascular smooth muscle cells (VSMCs), at low density, preferentially formed a heterotetrameric molecular assembly with native BKα subunits, rather than homotetrameric BKα-GFP. Movement of BKα-YFP in VSMCs (0.29 × 10(3) nm(2)/s) was far more restricted than BKα-YFP/BKβ1-CFP in HEK cells (2.5 × 10(3) nm(2)/s). Actin disruption by pretreatment with cytochalasin D in VSMCs appeared to increase the mobile behavior of BKα-YFP, which was then significantly reduced by addition of jasplakinolide. Most BKα-YFP colocalized with caveolin 1 (Cav1)-CFP in VSMCs, but unexpectedly not frequently in HEK cells. Fluorescence resonance energy transfer analyses showed the direct interaction between BKα-YFP and Cav1-CFP, particularly in VSMCs. These results, obtained by single molecule imaging in living cells, indicate that the dynamics of BKα molecules on the membrane surface are strongly restricted or regulated by its auxiliary β-subunit, cytoskeleton, and direct interaction with Cav1 in VSMCs.

Dynamic redistribution of calcium sensitive potassium channels (hK(Ca)3.1) in migrating cells

Calcium-sensitive potassium channels (K(Ca)3.1) are expressed in virtually all migrating cells. Their activity is required for optimal cell migration so that their blockade leads to slowing down. K(Ca)3.1 channels must be inserted into the plasma membrane in order to elicit their physiological function. However, the plasma membrane of migrating cells is subject to rapid recycling by means of endo- and exocytosis. Here, we focussed on the endocytic internalization and the intracellular transport of the human isoform hK(Ca)3.1. A hK(Ca)3.1 channel construct with an HA-tag in the extracellularly located S3-S4 linker was transfected into migrating transformed renal epithelial MDCK-F cells. Channel internalization was visualized and quantified with immunofluorescence and a cell-based ELISA. Movement of hK(Ca)3.1 channel containing vesicles as well as migration of MDCK-F cells were monitored by means of time lapse video microscopy. hK(Ca)3.1 channels are endocytosed during migration. Most of the hK(Ca)3.1 channel containing vesicles are moving at a speed of up to 2 µm/sec in a microtubule-dependent manner towards the front of MDCK-F cells. Our experiments indicate that endocytosis of hK(Ca)3.1 channels is clathrin-dependent since they colocalize with clathrin adaptor proteins and since it is impaired when a C-terminal dileucine motif is mutated. The C-terminal dileucine motif is also important for the subcellular localization of hK(Ca)3.1 channels in migrating cells. Mutated channels are no longer concentrated at the leading edge. We therefore propose that recycling of hK(Ca)3.1 channels contributes to their characteristic subcellular distribution in migrating cells.

Connecting rare DNA conformations and surface dynamics using single-molecule resonance energy transfer

A mechanistic understanding of single-stranded DNA (ssDNA) behavior in the near-surface environment is critical to advancing DNA-directed self-assembled nanomaterials. A new approach is described that uses total internal reflection fluorescence microscopy to measure resonance energy transfer at the single-molecule level, providing a mechanistic understanding of the connection between molecular conformation and interfacial dynamics near amine-modified surfaces. Large numbers (>10(5)) of ssDNA trajectories were observed, permitting dynamic correlation of molecular conformation with desorption and surface mobility. On the basis of dynamic behavior, molecules could be designated as members of the more common coiled population or a rare, weakly bound conformation. Molecules in the coiled state generally exhibited slow diffusion and conformational fluctuations that decreased with increasing average end-to-end distance. Lattice simulations of adsorbed self-avoiding polymers successfully predicted these trends. In contrast, the weakly bound conformation, observed in about 5% of molecules, had a large end-to-end distance but demonstrated conformational fluctuations that were much higher than predicted by simulations for adsorbed flexible chains. This conformation correlated positively with desorption events and led to fast diffusion, indicating weak surface associations. Understanding the role of the weakly bound conformation in DNA hybridization, and how solution conditions and surface properties may favor it, could lead to improved self-assembled nanomaterials.

Movements of individual BKCa channels in live cell membrane monitored by site-specific labeling using quantum dots

The movements of BK(Ca) channels were investigated in live cells using quantum dots (QDs). The extracellular N-terminus was metabolically tagged with biotin, labeled with streptavidin-conjugated QDs and then monitored using real-time time-lapse imaging in COS-7 cells and cultured neurons. By tracking hundreds of channels, we were able to determine the characteristics of channel movements quantitatively. Channels in COS-7 cells exhibited a confined diffusion in an area of 1.915 μm(2), with an initial diffusion coefficient of 0.033 μm(2)/s. In neurons, the channel movements were more heterogeneous and highly dependent on subcellular location. While the channels in soma diffused slowly without clear confinement, axodendritic channels showed more rapid and pseudo-one-dimensional movements. Intriguingly, the channel movement in somata was drastically increased by the neuronal β4 subunit, in contrast to the channels in the axodendritic area where the mobility were significantly decreased. Thus, our results demonstrate that the membrane mobility of BK(Ca) channels can be greatly influenced by the expression system used, subunit composition, and subcellular location. This QD-based, single-molecule tracking technique can be utilized to investigate the cellular mechanisms that determine the mobility as well as the localization of various membrane proteins in live cells.

Covalent monofunctionalization of peptide-coated quantum dots for single-molecule assays

Fluorescent probes for biological imaging of single molecules (SM) have many stringent design requirements. In the case of quantum dot (QD) probes, it remains a challenge to control their functional properties with high precision. Here, we describe the simple preparation of QDs with reduced size and monovalency. Our approach combines a peptide surface coating, stable covalent conjugation of targeting units and purification by gel electrophoresis. We precisely characterize these probes by ensemble and SM techniques and apply them to tracking individual proteins in living cells.

Direct observation of individual KCNQ1 potassium channels reveals their distinctive diffusive behavior

We have directly observed the trafficking and fusion of ion channel containing vesicles and monitored the release of individual ion channels at the plasma membrane of live mammalian cells using total internal reflection fluorescence microscopy. Proteins were fused in-frame with green or red fluorescent proteins and expressed at low level in HL-1 and HEK293 cells. Dual color imaging revealed that vesicle trafficking involved motorized movement along microtubules followed by stalling, fusion, and subsequent release of individual ion channels at the plasma membrane. We found that KCNQ1-KCNE1 complexes were released in batches of about 5 molecules per vesicle. To elucidate the properties of ion channel complexes at the cell membrane we tracked the movement of individual molecules and compared the diffusive behavior of two types of potassium channel complex (KCNQ1-KCNE1 and Kir6.2-SUR2A) to that of a G-protein coupled receptor, the A1 adenosine receptor. Plots of mean squared displacement against time intervals showed that mobility depended on channel type, cell type, and temperature. Analysis of the mobility of wild type KCNQ1-KCNE1 complexes showed the existence of a significant immobile subpopulation and also a significant number of molecules that demonstrated periodic stalling of diffusive movements. This behavior was enhanced in cells treated with jasplakinolide and was abrogated in a C-terminal truncated form (KCNQ1(R518X)-KCNE1) of the protein. This mutant has been identified in patients with the long QT syndrome. We propose that KCNQ1-KCNE1 complexes interact intermittently with the actin cytoskeleton via the C-terminal region and this interaction may have a functional role.

Subdiffusion in time-averaged, confined random walks

Certain techniques characterizing diffusive processes, such as single-particle tracking or molecular dynamics simulation, provide time averages rather than ensemble averages. Whereas the ensemble-averaged mean-squared displacement (MSD) of an unbounded continuous time random walk (CTRW) with a broad distribution of waiting times exhibits subdiffusion, the time-averaged MSD, delta2, does not. We demonstrate that, in contrast to the unbounded CTRW, in which delta2 is linear in the lag time Delta, the time-averaged MSD of the CTRW of a walker confined to a finite volume is sublinear in Delta, i.e., for long lag times delta2 approximately Delta1-alpha. The present results permit the application of CTRW to interpret time-averaged experimental quantities.

Total internal reflection fluorescence microscopy of cell adhesion on patterned self-assembled monolayers on gold

We report the use of total internal reflection fluorescence microscopy (TIRFM) to study cell adhesion on patterned self-assembled monolayers (SAMs) on gold surfaces. Microcontact printing was used to pattern hydrophobic features to which the extracellular protein fibronectin was adsorbed, while dip-pen nanolithography was used to produce electroactive nanoarrays of hydroquinone-terminated alkanethiol on gold-coated quartz substrates. The hydroquinone was electrochemically oxidized to the corresponding quinone, and an oxyamine-tethered linear Arg-Gly-Asp (RGD) peptide was chemoselectively immobilized. A prism-based method of TIRFM was used to examine adhered cells on both the microscale and nanoscale features. We also demonstrate that, following imaging with TIRFM, the substrates can be visualized using conventional fluorescence microscopy.

Versatile analysis of single-molecule tracking data by comprehensive testing against Monte Carlo simulations

We propose here an approach for the analysis of single-molecule trajectories which is based on a comprehensive comparison of an experimental data set with multiple Monte Carlo simulations of the diffusion process. It allows quantitative data analysis, particularly whenever analytical treatment of a model is infeasible. Simulations are performed on a discrete parameter space and compared with the experimental results by a nonparametric statistical test. The method provides a matrix of p-values that assess the probability for having observed the experimental data at each setting of the model parameters. We show the testing approach for three typical situations observed in the cellular plasma membrane: i), free Brownian motion of the tracer, ii), hop diffusion of the tracer in a periodic meshwork of squares, and iii), transient binding of the tracer to slowly diffusing structures. By plotting the p-value as a function of the model parameters, one can easily identify the most consistent parameter settings but also recover mutual dependencies and ambiguities which are difficult to determine by standard fitting routines. Finally, we used the test to reanalyze previous data obtained on the diffusion of the glycosylphosphatidylinositol-protein CD59 in the plasma membrane of the human T24 cell line.

The primary cilium coordinates signaling pathways in cell cycle control and migration during development and tissue repair

Cell cycle control and migration are critical processes during development and maintenance of tissue functions. Recently, primary cilia were shown to take part in coordination of the signaling pathways that control these cellular processes in human health and disease. In this review, we present an overview of the function of primary cilia and the centrosome in the signaling pathways that regulate cell cycle control and migration with focus on ciliary signaling via platelet-derived growth factor receptor alpha (PDGFRalpha). We also consider how the primary cilium and the centrosome interact with the extracellular matrix, coordinate Wnt signaling, and modulate cytoskeletal changes that impinge on both cell cycle control and cell migration.