Mechanical properties of F-actin network

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references.Cells sense, generate and respond to forces in their surroundings through cytoskeletal dynamics. Actin, the most abundant protein found in eukaryotic cells, is organized into various cytoskeletal structures that provide physical support for the cell and play important roles in numerous cellular processes. Assembly of F-actin into higher-order structures is regulated by over 100 actin binding proteins (ABPs). Although extensive measurements to estimate the mechanical properties of ABP/F-actin networks showed that they are nonlinear and viscoelastic, a full understanding of the origin of such fascinating behaviors is lacking. This thesis presents a multi-scale approach to identify the factors that determine the mechanical properties of F-actin networks from the macroscopic level to the single-molecule level. The mechanical properties of F-actin networks were probed by passive and active methods using optical tweezers. For the passive approach the thermal fluctuations of colloidal spheres are monitored to estimate the frequency-dependent complex shear modulus of an F-actin network. In the active approach, the response of an embedded microsphere to a driving force is tracked to obtain the strain-dependent viscoelasticity. The developed methods were applied to F-actin networks cross-linked with various ABPs such as filamin and a -actinin, with and without gelsolin to control filament length. Microstructures of those networks were also characterized in terms of filament length, mesh size, and degree of bundling.(cont.) Comparison between cross-linked F-actin with two different length scales of actin filament suggested that network connectivity is another critical parameter in determining mechanical properties. To better understand how the cross-linking protein responds to an external force, a single molecule assay was used to measure the rupture force of a complex formed by an ABP filamin linking two actin filaments. Both force-induced unbinding and unfolding of filamin were observed at the critical force of 70 ± 23pN and 57 ± 19pN, respectively, although unbinding occurred more frequently. Similar pulling experiments were also performed on cross-linked F-actin networks and an abrupt transition was observed in the force trace indicating network rupture. The critical forces at transitions exhibited a similar loading-rate dependence to that observed for rupture forces in the single molecule measurements. Nonlinear behavior observed in strain-dependent microrheology was found to be irreversible. Combined results of molecular unbinding, network rupture, and irreversible network properties suggest that unbinding rather than unfolding is a dominant mechanism governing the mechanical properties of cross-linked F-actin networks. In addition, the mechanical behavior of F-actin networks subjected to an external prestress was investigated using a shear device. Visualization of sheared F-actin networks showed the structural evolution including mesh deformation, filament alignment, and network rupture.(cont.) Measurement of mechanical properties as a function of external strain demonstrated that some regions exhibited strain-hardening while the others showed strain-softening. Aligned stretching of actin filaments observed at high strain seemed to play a role in strain-stiffening. By comparing the behaviors of an F-actin network cross-linked with wildtype and mutant FLNa, it was demonstrated how molecular structure of the ABP alters the mechanical behavior of F-actin network.by Hyungsuk Lee.Ph.D

Genetic polymorphisms in monoamine neurotransmitter systems show only weak association with acute post-surgical pain in humans

BACKGROUND: Candidate gene studies on the basis of biological hypotheses have been a practical approach to identify relevant genetic variation in complex traits. Based on previous reports and the roles in pain pathways, we have examined the effects of variations of loci in the genes of monoamine neurotransmitter systems including metabolizing enzymes, receptors and transporters on acute clinical pain responses in humans. RESULTS: Variations in the catecholamine metabolizing enzyme genes (MAOA and COMT) showed significant associations with the maximum post-operative pain rating while the serotonin transporter gene (SLC6A4) showed association with the onset time of post-operative pain. Analgesic onset time after medication was significantly associated with the norepinephrine transporter gene (SLC6A2). However, the association between COMT genetic variation and pain sensitivity in our study differ from previous studies with small sample sizes, population stratification and pain phenotype derived from combining different types of pain stimuli. Correcting for multiple comparisons did not sustain these genetic associations between monoamine neurotransmitter systems and pain sensitivity even in this large and homogeneous sample. CONCLUSION: These results suggest that the previously reported associations between genetic polymorphisms in the monoamine neurotransmitter systems and the interindividual variability in pain responses cannot be replicated in a clinically relevant pain phenotype

PROTECTION FROM OXYGEN AND MOISTURE VIA THIN OXIDE BARRIER COATING FOR ORGANIC ELECTRONICS

ABSTRACT Protection from oxygen and moisture is crucial for organic light emitting diodes (OLED) used in flexible display applications. The surface coating is an effective way to protect the enclosed functional materials and organic substrates in order to ensure their device functionality. Parylene films, which can be vapor deposited at room temperature, are known as a superior conformal polymeric material. In fact, a few attempts have shown that the parylene can be a good barrier coating for the OLED devices. This polymer film cannot, however, provide a long-term reliability due to nature of the polymer degradation in the presence of oxygen and moisture. In order to compensate such a drawback, we have explored the 'biomimetic' solution processing to deposit the oxide films on the organic substrate. Oxide films can provide a better protection and more robust surface. In this study, polyethylene terephthalate (PET) commonly used for an organic substrate, is deposited with the TiO 2 films with or without the parylene underlying layer. Importantly, the oxide coating is processed at very low temperatures (< 60˚C) in aqueous solution, so the process can avoid premature failure due to high-temperature processes, and is applicable to organic structures, cheap and environmentfriendly. In addition, hermeticity tests are devised to measure the moisture and oxygen permeation. Interfacial structure and mechanical properties of the resultant coatings are tested via scanning electron microscope (SEM), atomic force microscope (AFM), optical microscope (OM) and nanoindentation

A tissue-engineered jellyfish with biomimetic propulsion

Reverse engineering of biological form and function requires hierarchical design over several orders of space and time. Recent advances in the mechanistic understanding of biosynthetic compound materials, computer-aided design approaches in molecular synthetic biology and traditional soft robotics, and increasing aptitude in generating structural and chemical microenvironments that promote cellular self-organization have enhanced the ability to recapitulate such hierarchical architecture in engineered biological systems. Here we combined these capabilities in a systematic design strategy to reverse engineer a muscular pump. We report the construction of a freely swimming jellyfish from chemically dissociated rat tissue and silicone polymer as a proof of concept. The constructs, termed 'medusoids', were designed with computer simulations and experiments to match key determinants of jellyfish propulsion and feeding performance by quantitatively mimicking structural design, stroke kinematics and animal-fluid interactions. The combination of the engineering design algorithm with quantitative benchmarks of physiological performance suggests that our strategy is broadly applicable to reverse engineering of muscular organs or simple life forms that pump to survive

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements