Andrea Lynn Slade

Elimination of host cell PtdIns(4,5)P 2 by bacterial SigD promotes membrane fission during invasion by Salmonella

Salmonella invades mammalian cells by inducing membrane ruffling and macropinocytosis through actin remodelling. Because phosphoinositides are central to actin assembly, we have studied the dynamics of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) in HeLa cells during invasion by Salmonella typhimurium. Here we show that the outermost parts of the ruffles induced by invasion show a modest enrichment in PtdIns(4,5)P2, but that PtdIns(4,5)P2 is virtually absent from the invaginating regions. Rapid disappearance of PtdIns(4,5)P2 requires the expression of the Salmonella phosphatase SigD (also known as SopB). Deletion of SigD markedly delays fission of the invaginating membranes, indicating that elimination of PtdIns(4,5)P2 may be required for rapid formation of Salmonella-containing vacuoles. Heterologous expression of SigD is sufficient to promote the disappearance of PtdIns(4,5)P2, to reduce the rigidity of the membrane skeleton, and to induce plasmalemmal invagination and fission. Hydrolysis of PtdIns(4,5)P2 may be a common and essential feature of membrane fission during several internalization processes including invasion, phagocytosis and possibly endocytosis.

Single molecule imaging of supported planar lipid bilayer—reconstituted human insulin receptors by in situ scanning probe microscopy

A 480-kDa disulfide-linked heterodimer single-pass transmembrane protein, the insulin receptor, is autophosphorylated upon insulin binding to its extracellular domain. Remarkably, the structural basis for this activation process remained largely unknown until the recent cryoelectron microscopy studies of the insulin-insulin receptor complex by Luo et al. [Science 285 (1999) 1077]. We report here the results of an in situ study by high-resolution scanning probe microscopy of the full-length insulin receptor reconstituted within supported planar lipid bilayers. Our preliminary studies confirm that (1) the intact receptor can be reconstituted constitutively within a lipid vesicle and (2) fusion of the receptor-containing vesicles to mica resulted in the formation of molecular flat 5.5-nm-thick supported planar bilayers populated by two populations of protrusions, the shape and size of which are consistent with those of the insulin receptors intra- and extracellular domains as modeled by the cryo-EM data of Ottensmeyer et al. [Biochemistry 39 (2000) 12103]. These results establish a framework for real-time studies of insulin-insulin receptor binding by in situ SPM and single molecule force spectroscopy.

PeakForce Tapping resolves individual microvilli on living cells

Microvilli are a common structure found on epithelial cells that increase the apical surface thus enhancing the transmembrane transport capacity and also serve as one of the cells mechanosensors. These structures are composed of microfilaments and cytoplasm, covered by plasma membrane. Epithelial cell function is usually coupled to the density of microvilli and its individual size illustrated by diseases, in which microvilli degradation causes malabsorption and diarrhea. Atomic force microscopy (AFM) has been widely used to study the topography and morphology of living cells. Visualizing soft and flexible structures such as microvilli on the apical surface of a live cell has been very challenging because the native microvilli structures are displaced and deformed by the interaction with the probe. PeakForce Tapping® is an AFM imaging mode, which allows reducing tip–sample interactions in time (microseconds) and controlling force in the low pico‐Newton range. Data acquisition of this mode was optimized by using a newly developed PeakForce QNM‐Live Cell probe, having a short cantilever with a 17‐µm‐long tip that minimizes hydrodynamic effects between the cantilever and the sample surface. In this paper, we have demonstrated for the first time the visualization of the microvilli on living kidney cells with AFM using PeakForce Tapping. The structures observed display a force dependence representing either the whole microvilli or just the tips of the microvilli layer. Together, PeakForce Tapping allows force control in the low pico‐Newton range and enables the visualization of very soft and flexible structures on living cells under physiological conditions.

Toxin studies using an integrated biophysical and structural biology approach.

Clostridial neurotoxins, such as botulinum and tetanus, are generally thought to invade neural cells through a process of high affinity binding mediated by gangliosides, internalization via endosome formation, and subsequent membrane penetration of the catalytic domain activated by a pH drop in the endosome. This surface recognition and internalization process is still not well understood with regard to what specific membrane features the toxins target, the intermolecular interactions between bound toxins, and the molecular conformational changes that occur as a result of pH lowering. In an effort to elucidate the mechanism of tetanus toxin binding and permeation through the membrane a simple yet representative model was developed that consisted of the ganglioside G{sub tlb} incorporated in a bilayer of cholesterol and DPPC (dipalmitoylphosphatidyl choline). The bilayers were stable over time yet sensitive towards the binding and activity of whole toxin. A liposome leakage study at constant pH as well as with a pH gradient, to mimic the processes of the endosome, was used to elucidate the effect of pH on the toxins membrane binding and permeation capability. Topographic imaging of the membrane surface, via in situ tapping mode AFM, provided nanoscale characterization of the toxins binding location and pore formation activity.

Nanomaterials for directed energy transfer.

An extremely desirable functionality for nanostructured materials is the ability to efficiently activate or interrogate structures within a nanoscale device using optical energy. However, given the packing densities obtainable with nanofabrication, direct focusing of incident optical energy onto individual nanostructures is impractical. One approach to developing such functionality involves the use of “energy transfer”. Energy transfer is the process by which excitation energy resulting from absorption of photons can become delocalized and move from the site where the photon was absorbed. Energy transfer is often associated with organized arrays, or networks, of polarizable structures. In the current context, energy transfer networks can loosely be thought of as “lenses” that “focus” optical energy onto a desired site. The research program described in this report investigated several approaches toward the fabrication of both organic and inorganic energy transfer networks. For the production of metallic nanoparticle chains and arrays which are expected to act as “plasmonic” waveguides, we investigated strain-induced self assembly of metal particles on insulating substrates. We found that up to 6 monolayers of Ag on MgO(001) produced self-assembled island structures, but that thicker films with larger island sizes no longer produced monomodal distributions or periodic ordering. Growth of metallic nanoparticles on templated MgO substrates was also investigated. In a separate endeavor, ultrathin amorphous Si and Ge films composed of nanoscale holes and islands on oxidized Si were discovered to exhibit anomalously low reflectivity over the visible spectrum. For a number of distinct material systems, the low reflectivity regime coincided with

Correlated Single Molecule Fluorescence and Scanning Probe Microscopies: Applications to the Study of Soft Materials

We recently developed an integrated imaging platform that combines single molecule evanescent wave fluorescence imaging (and spectroscopy) with in situ scanning probe microscopy. The advantages, challenges, and potential represented by this coupled tool will be described in the context of the structure-function characteristics of nanostructured biomaterials and thin lipid films.