https://hdl.handle.net/1721.1/18155 2026-04-06T04:44:36Z https://hdl.handle.net/1721.1/26696 A Mechanistic Model of the Actin Cycle Bindschadler, M.; Osborn, E. A.; Dewey, C. F. Jr; McGrath, J. L. We have derived a broad, deterministic model of the steady-state actin cycle that includes its major regulatory mechanisms. Ours is the first model to solve the complete nucleotide profile within filaments, a feature that determines the dynamics and geometry of actin networks at the leading edges of motile cells, and one that has challenged investigators developing models to interpret steady-state experiments. We arrived at the nucleotide profile through analytic and numerical approaches that completely agree. Our model reproduces behaviors seen in numerous experiments with purified proteins, but allows a detailed inspection of the concentrations and fluxes that might exist in these experiments. These inspections provide new insight into the mechanisms that determine the rate of actin filament treadmilling. Specifically, we find that mechanisms for enhancing Pi release from the ADP-Pi intermediate on filaments, for increasing the off rate of ADP-bound subunits at pointed ends, and the multiple, simultaneous functions of profilin, make unique and essential contributions to increased treadmilling. In combination, these mechanisms have a theoretical capacity to increase treadmilling to levels limited only by the amount of available actin. This limitation arises because as the cycle becomes more dynamic, it tends toward the unpolymerized state. Biophysical Journal, 2004 2004-05-01T00:00:00Z https://hdl.handle.net/1721.1/26695 Simultaneous Measurements of Actin Filament Turnover, Filament Fraction, and Monomer Diffusion in Endothelial Cells McGrath, J. L.; Tardy, Y.; Dewey, C. F. Jr; Meister, J. J.; Hartwig, J. H. The analogous techniques of photoactivation of fluorescence (PAF) and fluorescence recovery after photobleaching (FRAP) have been applied previously to the study of actin dynamics in living cells. Traditionally, separate experiments estimate the mobility of actin monomer or the lifetime of actin filaments. A mathematical description of the dynamics of the actin cytoskeleton, however, predicts that the evolution of fluorescence in PAF and FRAP experiments depends simultaneously on the diffusion coefficient of actin monomer, D, the fraction of actin in filaments, FF, and the lifetime of actin filaments, t (Tardy et al., 1995, Biophys. J. 69:1674–1682). Here we report the application of this mathematical model to the interpretation of PAF and FRAP experiments in subconfluent bovine aortic endothelial cells (BAECs). The following parameters apply for actin in the bulk cytoskeleton of subconfluent BAECs. PAF: D 5 3.1 6 0.4 3 1028 cm2/s, FF 5 0.36 6 0.04, t 5 7.5 6 2.0 min. FRAP: D 5 5.8 6 1.2 3 1028 cm2/s, FF 5 0.5 6 0.04, t 5 4.8 6 0.97 min. Differences in the parameters are attributed to differences in the actin derivatives employed in the two studies and not to inherent differences in the PAF and FRAP techniques. Control experiments confirm the modeling assumption that the evolution of fluorescence is dominated by the diffusion of actin monomer, and the cyclic turnover of actin filaments, but not by filament diffusion. The work establishes the dynamic state of actin in subconfluent endothelial cells and provides an improved framework for future applications of PAF and FRAP. Biophysical Journal, 1998 1998-09-01T00:00:00Z https://hdl.handle.net/1721.1/26694 Theoretical Estimates of Mechanical Properties of the Endothelial Cell Cytoskeleton Satcher, Robert L. Jr.; Dewey, C. Forbes Jr. Current modeling of endothelial cell mechanics does not account for the network of F-actin that permeates the cytoplasm. This network, the distributed cytoplasmic structural actin (DCSA), extends from apical to basal membranes, with frequent attachments. Stress fibers are intercalated within the network, with similar frequent attachments. The microscopic structure of the DCSA resembles a foam, so that the mechanical properties can be estimated with analogy to these well-studied systems. The moduli of shear and elastic deformations are estimated to be on the order of 10^5 dynes/cm^2 . This prediction agrees with experimental measurements of the properties of cytoplasm and endothelial cells reported elsewhere. Stress fibers can potentially increase the modulus by a factor of 2-10, depending on whether they act in series or parallel to the network in transmitting surface forces. The deformations produced by physiological flow fields are of insufficient magnitude to disrupt cell-to-cell or DCSA cross-linkages. The questions raised by this paradox, and the ramifications of implicating the previously unreported DCSA as the primary force transmission element are discussed. Biophysical Journal, 1996 1996-01-01T00:00:00Z https://hdl.handle.net/1721.1/26693 Interpreting Photoactive Fluorescence Microscopy Measurements of Steady-State Actin Dynamics Tardy, Y.; McGrath, J.L.; Hartwig, J.H.; Dewey, C.F. A continuum model describing the steady-state actin dynamics of the cytoskeleton of living cells Has been developed to aid in the interpretation of photoactivated fluorescence experiments. In a simplified cell geometry, the model assumes uniform concentrations of cytosolic and cytoskeletal actin throughout the cell and no net growth of either pool. The spatiotemporal evolution of the fluorescent actin population is described by a system of two coupled linear partial-differential equations. An analytical solution is found using a Fourier-Laplace transform and important limiting cases relevant to the design of experiments are discussed. The results demonstrate that, despite being a complex function of the parameters, the fluorescence decay in photoactivated fluorescence experiments has a biphasic behavior featuring a short-term decay controlled by monomer diffusion and a long-term decay governed by the monomer exchange rate between the polymerized and unpolymerized actin pools. This biphasic behavior suggests a convenient mechanism for extracting the parameters governing the fluorescence decay from data records. These parameters include the actin monomer diffusion coefficient, filament turnover rate, and ratio of polymerized to unpolymerized actin. Biophysical Journal, 1995 1995-01-01T00:00:00Z https://hdl.handle.net/1721.1/26692 Regulation of Adenine Nucleotide Concentration at Endothelium-Fluid Interface by Viscous Shear Flow Shen, Jian; Gimbrone, Michael A. Jr.; Luscinskas, Francis W.; Dewey, C. Forbes Jr. The action of adenine nucleotides on vascular endothelial cells is apparently mediated by the local flow conditions. Because nucleotides are sequentially degraded from ATP - ADP - AMP -- adenosine by ecto-enzymes at the endothelial surface, it has been hypothesized that the observed flow effect is caused by the flow-dependent change of nucleotide concentration at the cell surface. In this study, we have calculated the concentration profiles of adenine nucleotides at the cell surface under flow conditions encountered in an in vitro parallel-plate flow system, as has been used in several related experimental studies. When medium containing uniformly distributed ATP is perfused over endothelial monolayers, our results show that ATP concentration in the cell vicinity gradually decreases in the streamwise direction as a result of enzymatic degradation. This hydrolysis of ATP results in the generation of ADP, and ADP concentration in turn gradually increases at the cell surface. The concentration profiles of nucleotides are dependent on the levels of applied wall shear rate. As the corresponding shear stress increases from 0.1 to 30 dynes/cm^2, ATP concentration at the cell surface at the center of coverslip increases from 0.66 to 0.93. Under no-flow conditions, our model predicts a steady decline of ATP concentration and a transient increase of ATP-derived ADP, comparable to the published results of previous experiments. These numerical results, combined with our recent experimental data, provide insights into the cellular mechanisms by which hemodynamic flow modulates the effects of vasoactive agents on endothelium. Biophysical Journal, 1993 1993-01-01T00:00:00Z https://hdl.handle.net/1721.1/18534 Improved Temporal Stability of Polymethine Laser Dyes in Aqueous Solutions Hackett, C.E.; Dewey, C.F. Jr Polymethine laser dyes which fluoresce in the near infrared in organic solvents generally exhibit poor temporal stability at room temperature. Two typical laser-pumped polymethine dyes in aqueous solutions containing nonionic surfactants are investigated. Temporal stability was improved 20-fold, as compared to organic solutions, with no significant changes in laser-pumping threshold or tuning range. 1973-01-01T00:00:00Z https://hdl.handle.net/1721.1/18535 Lasing Efficiency and Photochemical Stability of IR Laser Oettinger, Peter E.; Dewey, C. Forbes Jr The lasing efficiencies and photochemical stabilities of laser dyes useful in the 710-1080-nm spectral region have been investigated using a Q-switched ruby laser pumping source. The measured bleaching rates P defined as the probability of irreversible decomposition of a dye molecule per absorbed photon, varied from <= 1 exp-5 to 3 exp-4 for the different dye-solvent combinations investigated. Broad-band lasing efficiencies (the ratios of dye laser output to ruby radiation input) ranged from 4 to 43 percent. Shifts of wavelength tuning range with variations in solvent, dye concentration, and dye laser cavity geometry are reported. 1976-01-01T00:00:00Z https://hdl.handle.net/1721.1/18536 Pressure Dependence of the Infrared Laser Lines in Barium Vapor Madigan, M.; Hocker, L.O.; Flint, J.H.; Dewey, C.F. Jr We have studied nine laser transitions in Ba vapor at wavelengths between 1.13 and 3.05 micrometer. Three of these laser lines have not been reported previously. The relative intensities of the four strongest transitions (1.13, 1.50, 2.55, and 2.92 micrometer) vary with buffer gas pressure, suggesting that collisional deexcitation of the 6p 1^P_1 state occurs at higher pressure and is the main mechanism populating the upper states of the 2.55 and 2.33 micrometer lines. 1980-01-01T00:00:00Z https://hdl.handle.net/1721.1/18507 American Society for Echocardiography Software Suite for Verification and Claesen, S; Heart, G; Li, W; Bharath, A; Chandra, S; Kitney, R; Dewey, CF Jr; Thomas, JD The imminent balloting of the draft DICOM 3.0 module on ultrasound represents a major step forward in the progress of digital storage formats for echocardiography. In this paper, we discuss a software suite soon to be released into the public domain for reading, parsing and verifying DICOM 3.0 ultrasonic objects. The software is designed to allow the user to verify that a Jle consisting of digital ultrasound data, claiming DlCOM 3.0 conformance, does in fact adhere to the standard. A graphical user-interface for the software suite is also provided, allowing the playback of cin& mode images in real-time on standard computer hardware, and across a TCP/IP network. 1995-01-01T00:00:00Z