The TAU biological and soft matter seminar gathers
researchers from physics, chemistry, biology, and medicine, who are interested
in the behavior of biomaterials and complex fluids. The aim is to get
acquainted with the work and research interests of colleagues across the
different disciplines. The seminar is very informal and student-oriented.
Meetings take about two hours, allowing for many questions and lively
discussions.
Time and Location
Unless otherwise noted, the seminar takes place on Wednesdays,
13:30-15:00, at Orenstein Building (Chemistry), Room #205.
E-Mail List
If you wish to join the seminar e-mail list, please send a message to biosoft@post.tau.ac.il.
November 16th 2011: Oren Regev (BGU)
Carbon nanotube-liposome conjugate for efficient drug transportation
Carbon nanotubes (CNT) are widely explored as carriers for drug delivery due to their facile transport through cellular membranes. However, the amount of loaded drug on a CNT is rather small. Liposomes on the other hand are employed as carrier of a large amount of drug. We developed a new drug delivery system, in which drug-loaded liposomes are covalently attached to CNT to form a CNT-liposomes conjugate (CLC). The advantage of this novel approach is the large amount of drug that can be delivered into cells by the CLC system, thus preventing potential adverse systemic effects of CNT when administered at high doses. This system is expected to provide versatile and controlled means for enhanced delivery of one or more agents stably associated with the liposomes.
November 30th 2011: Dan Ben-Yaakov (TAU)
Non-Electrostatic Interactions and Ion-Specific Effects in Ionic Solutions
Franz Hofmeister and co-workers reported already during the 1880's and 1890's on monovalent ionic species (such as fluoride and chloride) that are more effective at precipitating proteins ("salting out") than others (such as bromide and iodide). This pioneering observation of ion-specific effects was followed by a huge amount of experimental studies, reporting on ion-specific interactions emerging in various experimental systems. Examples include bulk properties such as activity and viscosity of ionic solutions, as well as interfacial properties such as air/water surface tension and interaction of surfactant micelles, lipid-bilayer membranes, proteins, DNA molecules and more.
Although being an old observation, the theoretical origin for these effects is still not fully formulated. Dispersion forces, solvation, ionic size and polarizability are several examples of the mechanisms suggested during the recent decades as possible candidates to explain the specificity, and to interpret a large body of experimental evidence. However, the complexity of the inter-constituents interactions is a major difficulty of constructing a complete theoretical model.
By using a simplified phenomenological approach we try to obtain a more intuitive understanding of non-electrostatic and ion-specific effects. Several such models will be discussed in the seminar. In particular, I will elaborate on the preferential solvation of ions in a binary mixture, and the ionic hydration shell, as sources of an ion-solvent interaction. The influence of these additional effects on the density profiles and the inter-surface force will be discussed. For inter-surface separations at the nanometers range a significant effect on the force is found. The change of the force (relative to the prediction of the standard Poisson-Boltzmann model) may be either positive or negative, depending on the nature of the non-electrostatic interactions. I will discuss the results and their consequences to the interpretation of experimental data.
December 14th 2011: Anne Bernheim (BGU)
Releasing the brakes: how cortactin enhances actin-based motility
The polymerization of actin is directed to the surface of the cell membrane or vesicles, by localizing to the surface nucleating molecules which then activates the branching of the filaments using the Arp2/3 complex. The actin network that forms at the surface produces an elastic pushing force on the surface. However, the same nucleating molecules that initiates actin polymerization, also inherently inhibits the translation of the protrusive force into motion, by binding to the same actin network. This is an inherent problem, as in order to localize the branching process to the membrane, the nucleating factor has to make contact with the network during the formation of the new branch. In order to address this problem we have used a bottom-up approach in which beads coated with actin nucleators are pushed by the polymerization of an actin network at their surface. We found that cortactin (known to play an important role in cell movement), plays a critical role in translating actin polymerization at a bead surface into motion by releasing the network-bound nucleaor from the new branching site. This enhanced release has two major effects: it increases the turnover rate of branching per nucleator molecule, and it decreases the friction-like force caused by the binding of the moving surface with respect to the growing actin network.
December 28th 2011: Lia Addadi (WIS)
Structural Organization and Localization of Mixed Cholesterol: Lipid Domains in Cell Membranes
Lipid microdomains, or rafts, consisting of sphingolipids and cholesterol, play important roles in membrane trafficking and in signaling. Despite years of study on these domains, many open questions remain about their precise characteristics. To address the combined issues of composition, structure and location of these domains, we have developed new experimental approaches, based on the use of specific monoclonal antibodies as recognition tools, combined with direct structure determinations on single hydrated lipid bilayers by X-ray diffraction. One such 'structural' antibody was raised against a mixed phase of cholesterol and ceramide of known structure, and has been used to demonstrate the existence and location of ceramide/cholesterol domains in cultured cells. I'll discuss the possible implications of these findings, and in particular the relevance of understanding the role of lipid lateral organization in biological membranes.
January 11th 2012: Mario Feingold (BGU)
Towards Single Cell Optical Tomography
Using a single-beam, oscillating Optical Tweezers we demonstrate trapping and rotation of rod-shaped bacterial cells with respect to the optical axis. The angle of rotation is determined by the amplitude of the oscillation. This technique allows imaging fluorescently labeled 3D sub-cellular structures from different, optimized viewpoints. To illustrate our method we analyze the Z-ring of E. coli. We use cells that express FtsZ-GFP and have their cytoplasmic membrane stained with FM4-64. In a vertically oriented cell, both the Z-ring and the cytoplasmic membrane images appear as symmetric circular structures that lend themselves to quantitative analysis.
Scanning the cell alignment and using 3D image reconstruction from the corresponding images of a fluorescently labeled 3D sub-cellular structure, would make our approach analogous to that of cryo-electron tomography.
January 25th 2012: Diego Krapf (CSU)
Anomalous diffusion and ergodicity breaking in the plasma membrane: the role of endocytosis
Kv2.1 is unusual among voltage-gated K+ channels in that it localizes to micron-sized clusters on the cell surface of neurons. Within these clusters, Kv2.1 is non-conducting. I will discuss single-molecule tracking experimental results showing that these surface structures are specialized platforms involved in the trafficking of membrane proteins to and from the cell surface. This study is the first to identify stable cell surface platforms dedicated to ion channel trafficking. Multi-color TIRF-based studies indicate that fluorescently labeled K+ channel containing vesicles directly tether to and deliver cargo in a discrete fashion to the Kv2.1 surface clusters. We find that retrieval of Kv2.1 from the membrane occurs also at the cluster perimeter, via a clathrin-mediated endocytic pathway.
The internalization of channels is often aborted because the channel escapes from the endocytic pit. However, when a channel is captured by a clathrin-coated pit it is temporarily immobilized. These stalling events introduce an anomalous subdiffusion process that can be modeled by a continuous time random walk (CTRW). Transient immobilization not only induces anomalous subdiffusion but also weak ergodicity breaking, that is, the ensemble and time averages do not coincide. We find evidence showing that the ensemble and temporal distributions are different. Interestingly, ergodicity is recovered in the presence of actin inhibitors. By performing simultaneous TIRF imaging of quantum-dot-tagged Kv2.1 and RFP-tagged clathrin light chain, we find that stalling events mainly take place when the channel is captured within a forming clathrin coated pit. These results show that abortive endocytic events are responsible for the maintenance of a CTRW with a power law distribution of stalling times.
