Subcellular optogenetic activation of Cdc42 controls local and distal signaling to drive immune cell migration

Migratory immune cells use intracellular signaling networks to generate and orient spatially polarized responses to extracellular cues. The monomeric G protein Cdc42 is believed to play an important role in controlling the polarized responses, but it has been difficult to determine directly the consequences of localized Cdc42 activation within an immune cell. Here we used subcellular optogenetics to determine how Cdc42 activation at one side of a cell affects both cell behavior and dynamic molecular responses throughout the cell. We found that localized Cdc42 activation is sufficient to generate polarized signaling and directional cell migration. The optically activated region becomes the leading edge of the cell, with Cdc42 activating Rac and generating membrane protrusions driven by the actin cytoskeleton. Cdc42 also exerts long-range effects that cause myosin accumulation at the opposite side of the cell and actomyosin-mediated retraction of the cell rear. This process requires the RhoA-activated kinase ROCK, suggesting that Cdc42 activation at one side of a cell triggers increased RhoA signaling at the opposite side. Our results demonstrate how dynamic, subcellular perturbation of an individual signaling protein can help to determine its role in controlling polarized cellular responses

Self-organized metal nanostructures through laser driven thermocapillary convection

When ultrathin metal films are subjected to multiple cycles of rapid melting and resolidification by a ns pulsed laser, spatially correlated interfacial nanostructures can result from a competition among several possible thin film self-organizing processes. Here we investigate self-organization and the ensuing length scales when Co films (1-8 nm thick) on SiO_{\text{2}} surfaces are repeatedly and rapidly melted by non-uniform (interference) laser irradiation. Pattern evolution produces nanowires, which eventually break-up into nanoparticles exhibiting spatial order in the nearest neighbor spacing, \lambda_{NN2}.The scaling behavior is consistent with pattern formation by thermocapillary flow and a Rayleigh-like instability. For h_{0}\leq2 nm, a hydrodynamic instability of a spinodally unstable film leads to the formation of nanoparticles.Comment: 10 pages, 3 figure

High-capacity electrode materials for electrochemical energy storage: Role of nanoscale effects

This review summarizes the current state-of-the art electrode materials used for high-capacity lithium-ion-based batteries and their significant role towards revolutionizing the elec- trochemical energy storage landscape in the area of consumer electronics, transportation and grid storage application. We discuss the role of nanoscale effects on the electrochemical performance of high-capacity battery electrode materials. Decrease in the particle size of the primary electrode materials from micron to nanometre size improves the ionic and electronic diffusion rates signifi- cantly. Nanometre-thick solid electrolyte (such as lithium phosphorous oxynitride) and oxides (such as Al 2 O 3 ,ZnO,TiO 2 etc.) material coatings also improve the interfacial stability and rate capability of a number of battery chemistries. We elucidate these effects in terms of different high-capacity battery chemistries based on intercalation and conversion mechanism

When Can Limited Randomness Be Used in Repeated Games?

The central result of classical game theory states that every finite normal form game has a Nash equilibrium, provided that players are allowed to use randomized (mixed) strategies. However, in practice, humans are known to be bad at generating random-like sequences, and true random bits may be unavailable. Even if the players have access to enough random bits for a single instance of the game their randomness might be insufficient if the game is played many times. In this work, we ask whether randomness is necessary for equilibria to exist in finitely repeated games. We show that for a large class of games containing arbitrary two-player zero-sum games, approximate Nash equilibria of the $n$-stage repeated version of the game exist if and only if both players have $\Omega(n)$ random bits. In contrast, we show that there exists a class of games for which no equilibrium exists in pure strategies, yet the $n$-stage repeated version of the game has an exact Nash equilibrium in which each player uses only a constant number of random bits. When the players are assumed to be computationally bounded, if cryptographic pseudorandom generators (or, equivalently, one-way functions) exist, then the players can base their strategies on "random-like" sequences derived from only a small number of truly random bits. We show that, in contrast, in repeated two-player zero-sum games, if pseudorandom generators \emph{do not} exist, then $\Omega(n)$ random bits remain necessary for equilibria to exist

Robust nanopatterning by laser-induced dewetting of metal nanofilms

We have observed nanopattern formation with robust and controllable spatial ordering by laser-induced dewetting in nanoscopic metal films. Pattern evolution in Co film of thickness 1\leq h\leq8 nm on SiO_{2} was achieved under multiple pulse irradiation using a 9 ns pulse laser. Dewetting leads to the formation of cellular patterns which evolve into polygons that eventually break up into nanoparticles with monomodal size distribution and short range ordering in nearest-neighbour spacing R. Spatial ordering was attributed to a hydrodynamic thin film instability and resulted in a predictable variation of R and particle diameter D with h. The length scales R and D were found to be independent of the laser energy. These results suggest that spatially ordered metal nanoparticles can be robustly assembled by laser-induced dewetting

Thermodynamic approach to the dewetting instability in ultrathin films

The fluid dynamics of the classical dewetting instability in ultrathin films is a non-linear process. However, the physical manifestation of the instability in terms of characteristic length and time scales can be described by a linearized form of the initial conditions of the films's dynamics. Alternately, the thermodynamic approach based on equating the rate of free energy decrease to the viscous dissipation [de Gennes, C. R. Acad. Paris.v298, 1984] can give similar information. Here we have evaluated dewetting in the presence of thermocapillary forces arising from a film-thickness (h) dependent temperature. Such a situation can be found during pulsed laser melting of ultrathin metal films where nanoscale effects lead to a local h-dependent temperature. The thermodynamic approach provides an analytical description of this thermocapillary dewetting. The results of this approach agree with those from linear theory and experimental observations provided the minimum value of viscous dissipation is equated to the rate of free energy decrease. The flow boundary condition that produces this minimum viscous dissipation is when the film-substrate tangential stress is zero. The physical implication of this finding is that the spontaneous dewetting instability follows the path of minimum rate of energy loss.Comment: 8 pages, 3 figures. Under revie