The actin cytoskeleton has been implicated in restricting diffusion of plasma membrane components. Here, simultaneous observations of quantum dot-labelled FcεRI motion and GFPtagged actin dynamics provide direct evidence that actin filament bundles define micron-sized domains that confine mobile receptors. Dynamic reorganisation of actin structures occurs over seconds, making the location and dimensions of actin-defined domains time dependent. Multiple FcεRI often maintain extended close proximity without detectable correlated motion, suggesting that they are co-confined within membrane domains. FcεRI signalling is activated by cross-linking with multivalent antigen. We show that receptors become immobilised within seconds of crosslinking. Disruption of the actin cytoskeleton results in delayed immobilisation kinetics and increased diffusion of cross-linked clusters. These results implicate actin in membrane partitioning that not only restricts diffusion of membrane proteins, but also dynamically influences their longrange mobility, sequestration, and response to ligand binding.Signal transduction from the external environment to the cell interior is typically mediated by ligand-bound transmembrane receptors embedded in a lipid bilayer. In many systems, receptor activation is associated with changes in receptor dynamics and membrane topography1 -3 . Among these are the multi-chain immune recognition receptor family members that include the B-cell receptor (BCR) of B-cells, the T-cell receptor (TCR) of Tcells, and the high affinity IgE receptor (FcεRI) of mast cells and basophils, which are crucial to the execution of key events in the immune response. Cross-linking of these transmembrane receptors induces receptor oligomerisation, protein and lipid kinase activation and Ca 2+ mobilisation, leading in turn to cytoskeletal reorganisation, receptor trafficking and cell-specific responses including altered gene expression [4][5][6] . These signalling events have been well studied by biochemical techniques, but the precise mechanism by which oligomerisation initiates these events has remained elusive. Full understanding of these complex signalling cascades will require a more complete description of receptor movements in the membrane, including restrictions that might limit receptor diffusion and accessibility.Correspondence should be addressed to D.S.L. (dlidke@salud.unm.edu). 4 These authors contributed equally to this work. NIH Public Access Author ManuscriptNat Cell Biol. Author manuscript; available in PMC 2011 January 18. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptA rich literature on single particle tracking (SPT) methods to follow the lateral diffusion of transmembrane and membrane-associated proteins7 -10 has revealed nanometer-scale "confinement zones" that restrict lateral diffusion and supports the general notion that plasma membrane organisation is more structured than originally postulated by the fluid mosaic model11. A membrane-skeleton fence (picket fence) model ...

Nature abounds with intricate composite architectures composed of hard and soft materials synergistically intertwined to provide both useful functionality and mechanical integrity. Recent synthetic efforts to mimic such natural designs have focused on nanocomposites, prepared mainly by slow procedures like monomer or polymer infiltration of inorganic nanostructures or sequential deposition. Here we report the self-assembly of conjugated polymer/silica nanocomposite films with hexagonal, cubic or lamellar mesoscopic order using polymerizable amphiphilic diacetylene molecules as both structure-directing agents and monomers. The self-assembly procedure is rapid and incorporates the organic monomers uniformly within a highly ordered, inorganic environment. Polymerization results in polydiacetylene/silica nanocomposites that are optically transparent and mechanically robust. Compared to ordered diacetylene-containing films prepared as Langmuir monolayers or by Langmuir-Blodgett deposition, the nanostructured inorganic host alters the diacetylene polymerization behaviour, and the resulting nanocomposite exhibits unusual chromatic changes in response to thermal, mechanical and chemical stimuli. The inorganic framework serves to protect, stabilize, and orient the polymer, and to mediate its function. The nanocomposite architecture also provides sufficient mechanical integrity to enable integration into devices and microsystems.

Background-Excitation-contraction coupling in striated muscle requires proper communication of plasmalemmal voltage-activated Ca 2ϩ channels and Ca 2ϩ release channels on sarcoplasmic reticulum within junctional membrane complexes. Although previous studies revealed a loss of junctional membrane complexes and embryonic lethality in germ-line junctophilin-2 (JPH2) knockout mice, it has remained unclear whether JPH2 plays an essential role in junctional membrane complex formation and the Ca 2ϩ -induced Ca 2ϩ release process in the heart. Our recent work demonstrated loss-of-function mutations in JPH2 in patients with hypertrophic cardiomyopathy. Methods and Results-To elucidate the role of JPH2 in the heart, we developed a novel approach to conditionally reduce JPH2 protein levels using RNA interference. Cardiac-specific JPH2 knockdown resulted in impaired cardiac contractility, which caused heart failure and increased mortality. JPH2 deficiency resulted in loss of excitationcontraction coupling gain, precipitated by a reduction in the number of junctional membrane complexes and increased variability in the plasmalemma-sarcoplasmic reticulum distance. Conclusions-Loss of JPH2 had profound effects on Ca 2ϩ release channel inactivation, suggesting a novel functional role for JPH2 in regulating intracellular Ca 2ϩ release channels in cardiac myocytes. Thus, our novel approach of cardiac-specific short hairpin RNA-mediated knockdown of junctophilin-2 has uncovered a critical role for junctophilin in intracellular Ca 2ϩ release in the heart. (Circulation. 2011;123:979-988.)Key Words: calcium Ⅲ excitation Ⅲ heart failure Ⅲ junctophilin Ⅲ sarcoplasmic reticulum E xcitation-contraction (EC) coupling is the fundamental mechanism by which depolarization of the voltage-gated Ca 2ϩ channels (VGCCs) in the plasmalemma triggers a much greater release of Ca 2ϩ from the sarcoplasmic reticulum (SR) via type 2 ryanodine receptors (RyR2), a process known as Ca 2ϩ -induced Ca 2ϩ release (CICR). 1 This Ca 2ϩ release amplification depends on the organization of VGCC and RyR2 within junctional membrane complexes (JMCs), also known as calcium release units. 2 Disruption of JMC structure, as seen in heart failure, profoundly affects CICR and thus cardiac muscle contractility. 3 Clinical Perspective on p 988The molecular mechanisms involved in organizing Ca 2ϩ channels within the JMC remain poorly understood. One family of proteins, known as junctophilins (JPHs), has been proposed to provide a structural bridge between the plasmalemma and SR, thereby potentially ensuring approximation of VGCC and RyR2. 4 Junctophilin-2 (JPH2) is the major cardiac isoform among the 4 JPH isoforms, which are expressed within JMCs of all excitable cell types. 5 JPH proteins comprise 8 N-terminal "membrane occupation and recognition nexus" domains, a space-spanning ␣-helix, and a C-terminal transmembrane domain. The membrane occupation and recognition nexus domains mediate binding to the plasmalemma, and the hydrophobic transmembrane domain is anchored into ...

Polydiacetylenes (PDAs) form a unique class of polymeric materials that couple highly aligned and conjugated backbones with tailorable pendant sidegroups and terminal functionalities. They can be structured in the form of bulk materials, multilayer and monolayer films, polymerized vesicles, and even incorporated into inorganic host matrices to form nanocomposites. The resulting materials exhibit an array of spectacular properties, beginning most notably with dramatic chromogenic transitions that can be activated optically, thermally, chemically, and mechanically. Recent studies have shown that these transitions can even be controlled and observed at the nanometre scale. These transitions have been harnessed for the purpose of chemical and biomolecular sensors, and on a more fundamental level have led to new insights regarding chromogenic phenomena in polymers. Other recent studies have explored how the strong structural anisotropy that these materials possess leads to anisotropic nanomechanical behaviour. These recent advances suggest that PDAs could be considered as a potential component in nanostructured devices due to the large number of tunable properties. In this paper, we provide a succinct review of the latest insights and applications involving PDA. We then focus in more detail on our work concerning ultrathin films, specifically structural properties, mechanochromism, thermochromism, and in-plane mechanical anisotropy of PDA monolayers. Atomic force microscopy (AFM) and fluorescence microscopy confirm that films 1-3 monolayers thick can be organized into highly ordered domains,with the conjugated backbones parallel to the substrate. The number of stable layers is controlled by the head-group functionality. Local mechanical stress applied by AFM and near-field optical probes induces the chromogenic transition in the film at the nanometre scale. The transition R680 Topical Review involves substantial optical and structural changes in a highly compressed form. Thermochromism is also studied using spectroscopic ellipsometry and fluorescence intensity measurements, and reveals that ultrathin films can reversibly attain an intermediate phase before irreversibly transforming to a final stable state. Further AFM studies also reveal the relation between the highly anisotropic film structure and its nanomechanical properties. In particular, friction at the nanometre scale depends dramatically upon the angle between the polymer backbone and the sliding direction, with the maximum found when sliding perpendicular to the backbones. The observed threefold anisotropy in mechanical dissipation also leads to contrast in the phase response of intermittent-contact AFM, indicating for the first time that in-plane anisotropy of polymeric systems in general can be investigated using this technique.

Abstractaburns (lj.sandia.gov, tel: 505-844-9642, fa: 505-844-5470 A mechanically-induced color transition ("mechanochromisrn") in polydiacetylene thin films has been generated at the nanometer scale using the tips of two different scanning probe microscopes. A blue-to-red chromatic transition in polydiacetylene molecular trilayer films, ' polymerized horn 10,12-pentacosadiynoic acid (poly-PCDA), was found to result from shear forces acting between the tip and the poly-PCDA molecules, as independently observed with near-field scanning optical microscopy and atomic force microscopy (AFM). Red domains were identified by a fluorescence emission signature. Transformed regio'ns as small as 30 nm in width were observed with AFM. The irreversibly transformed domains preferentially grow along the polymer backbone direction. Significant rearrangement of poly-PCDA bilayer segments is observed by AFM in transformed regions. The removal of these segments appears to be a characteristic feature of the transition. To our knowledge, this is the first observation of nanometer-scale mechanochromism in any material.

Fluorescence correlation spectroscopy (FCS) is used to examine mobility of labeled probes at specific sites in supported bilayers consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid domains in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Those sites are mapped beforehand with simultaneous atomic force microscopy and submicron confocal fluorescence imaging, allowing characterization of probe partitioning between gel DPPC and disordered liquid DOPC domains with corresponding topography of domain structure. We thus examine the relative partitioning and mobility in gel and disordered liquid phases for headgroup- and tailgroup-labeled GM1 ganglioside probes and for headgroup- and tailgroup-labeled phospholipid probes. For the GM1 probes, large differences in mobility between fluid and gel domains are observed; whereas unexpected mobility is observed in submicron gel domains for the phospholipid probes. We attribute the latter to domain heterogeneities that could be induced by the probe. Furthermore, fits to the FCS data for the phospholipid probes in the DOPC fluid phase require two components (fast and slow). Although proximity to the glass substrate may be a factor, local distortion of the probe by the fluorophore could also be important. Overall, we observe nonideal aspects of phospholipid probe mobility and partitioning that may not be restricted to supported bilayers.

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We have investigated the thermochromic transition of an ultrathin poly(diacetylene) film. The Langmuir film is composed of three layers of polymerized 10, 12-pentacosadiynoic acid [CHJCHJ#====C===(CHJ~COOH] (poly-PCDA) organized into crystalline domains on a silicon substrate. Spectroscopic ellipsometry and fluorescence intensity measurements are obtained with in-situ temperature control. Poly-PCDA films exhibit a reversible thermal transition between the initial blue form and an intermediate "purple" form that exists only at elevated temperature" (between 303 -333 K), followed by an irreversible transition to the red form after annealing above 320 K. We propose that the purple form is thermally distorted blue poly-PCDA, and may represent a transitional configuration in the irreversible conversion to red. This hypothesis is supported by the appearance of unique features in the absorption spectra for each form as derived from the ellipsometry measurements. Significant fluorescence emission occurs only with the red form, and is reduced at elevated temperatures while the absorption remains unchanged. Reduced emission is likely related to thermal fluctuations of the hydrocarbon side chains. Time-resolved fluorescence measurements of the irreversible transition have been performed. Using a first-order kinetic analysis of these measurements we deduce an energy barrier of 17.6M. 1 kcal mo~*between the blue and red forms.