From random walk to single-file diffusion

We report an experimental study of diffusion in a quasi-one-dimensional (q1D) colloid suspension which behaves like a Tonks gas. The mean squared displacement as a function of time is described well with an ansatz encompassing a time regime that is both shorter and longer than the mean time between collisions. This ansatz asserts that the inverse mean squared displacement is the sum of the inverse mean squared displacement for short time normal diffusion (random walk) and the inverse mean squared displacement for asymptotic single-file diffusion (SFD). The dependence of the single-file 1D mobility on the concentration of the colloids agrees quantitatively with that derived for a hard rod model, which confirms for the first time the validity of the hard rod SFD theory. We also show that a recent SFD theory by Kollmann leads to the hard rod SFD theory for a Tonks gas.Comment: 4 pages, 4 figure

Optogenetic Control of Molecular Motors and Organelle Distributions in Cells

SummaryIntracellular transport and distribution of organelles play important roles in diverse cellular functions, including cell polarization, intracellular signaling, cell survival, and apoptosis. Here, we report an optogenetic strategy to control the transport and distribution of organelles by light. This is achieved by optically recruiting molecular motors onto organelles through the heterodimerization of Arabidopsis thaliana cryptochrome 2 (CRY2) and its interacting partner CIB1. CRY2 and CIB1 dimerize within subseconds upon exposure to blue light, which requires no exogenous ligands and low intensity of light. We demonstrate that mitochondria, peroxisomes, and lysosomes can be driven toward the cell periphery upon light-induced recruitment of kinesin, or toward the cell nucleus upon recruitment of dynein. Light-induced motor recruitment and organelle movements are repeatable, reversible, and can be achieved at subcellular regions. This light-controlled organelle redistribution provides a new strategy for studying the causal roles of organelle transport and distribution in cellular functions in living cells

Divergence of the Long Wavelength Collective Diffusion Coefficient in Quasi-one and Quasi-two Dimensional Colloid Suspensions

We report the results of experimental studies of the short time-long wavelength behavior of collective particle displacements in quasi-one-dimensional and quasi-two-dimensional colloid suspensions. Our results are represented by the behavior of the hydrodynamic function H(q) that relates the effective collective diffusion coefficient, D_e(q) with the static structure factor S(q) and the self-diffusion coefficient of isolated particles D_0: H(q)=D_e(q)S(q)/D_0. We find an apparent divergence of H(q) as q->0 with the form H(q) proportional to q^-gamma, 1.7<gamma<1.9, for both q1D and q2D colloid suspensions. Given that S(q) does not diverge as q=>0 we infer that D_e(q) does. We provide evidence that this divergence arises from the interplay of boundary conditions on the flow of the carrier liquid and many-body hydrodynamic interactions between colloid particles that affect the long wavelength behavior of the particle collective diffusion coefficient in the suspension. We speculate that in the q1D and q2D systems studied the divergence of H(q) might be associated with a q-dependent partial slip boundary condition, specifically an effective slip length that increases with decreasing q. We also verify, using data from the work of Lin, Rice and Weitz (J. Chem. Phys. 99, 9585 (1993)), the prediction by Bleibel et al (arXiv:1305.3715), that D_e(q) for a monolayer of colloid particles constrained to lie in the interface between two fluids diverges as 1/q as q->0. The verification of that prediction, which is based on an analysis that allows two-dimensional colloid motion embedded in three-dimensional suspending fluid motion, supports the contention that the boundary conditions that define a q2D system play a very important role in determining the long wavelength behavior of the collective diffusion coefficient

Nanoparticle-assisted optical tethering of endosomes reveals the cooperative function of dyneins in retrograde axonal transport

Dynein-dependent transport of organelles from the axon terminals to the cell bodies is essential to the survival and function of neurons. However, quantitative knowledge of dyneins on axonal organelles and their collective function during this long-distance transport is lacking because current technologies to do such measurements are not applicable to neurons. Here, we report a new method termed nanoparticle-assisted optical tethering of endosomes (NOTE) that made it possible to study the cooperative mechanics of dyneins on retrograde axonal endosomes in live neurons. In this method, the opposing force from an elastic tether causes the endosomes to gradually stall under load and detach with a recoil velocity proportional to the dynein forces. These recoil velocities reveal that the axonal endosomes, despite their small size, can recruit up to 7 dyneins that function as independent mechanical units stochastically sharing load, which is vital for robust retrograde axonal transport. This study shows that NOTE, which relies on controlled generation of reactive oxygen species, is a viable method to manipulate small cellular cargos that are beyond the reach of current technology.National Institutes of Health (U.S.) (DP2-NS082125)National Science Foundation (U.S.) (Award 1055112)National Science Foundation (U.S.) (Award 1344302

Compact Electrochromic Optical Recording of Bioelectric Potentials

Electrochromic optical recording (ECORE) is a label-free method that utilizes electrochromism to optically detect electrical signals in biological cells with a high signal-to-noise ratio and is suitable for long-term recording. However, ECORE usually requires a large and intricate optical setup, making it relatively difficult to transport and to study specimens on a large scale. Here, we present a compact ECORE apparatus that drastically reduces the spatial footprint and complexity of the ECORE setup whilst maintaining high sensitivity. An autobalancing differential photodetector automates common-mode noise rejection, removing the need for manually adjustable optics, and a compact laser module conserves space compared to a typical laser mount. The result is a simple, easy-to-use, and relatively low cost system that achieves a sensitivity of 16.7 {\mu}V (within a factor of 5 of the shot noise limit), and reliably detects action potentials from Human-induced pluripotent stem cell (HiPSC) derived cardiomyocytes. This setup can be further improved to within 1.5 dB of the shot noise limit by filtering out power-line interference.Comment: 11 pages, 4 figure