Time-resolved fluorescence polarization from ordered biological assemblies

Three-dimensional spatio-angular fluorescence microscopy with a polarized dual-view inverted selective-plane illumination microscope (pol-diSPIM)

Polarized fluorescence microscopy is a valuable tool for measuring molecular orientations, but techniques for recovering three-dimensional orientations and positions of fluorescent ensembles are limited. We report a polarized dual-view light-sheet system for determining the three-dimensional orientations and diffraction-limited positions of ensembles of fluorescent dipoles that label biological structures, and we share a set of visualization, histogram, and profiling tools for interpreting these positions and orientations. We model our samples, their excitation, and their detection using coarse-grained representations we call orientation distribution functions (ODFs). We apply ODFs to create physics-informed models of image formation with spatio-angular point-spread and transfer functions. We use theory and experiment to conclude that light-sheet tilting is a necessary part of our design for recovering all three-dimensional orientations. We use our system to extend known two-dimensional results to three dimensions in FM1-43-labelled giant unilamellar vesicles, fast-scarlet-labelled cellulose in xylem cells, and phalloidin-labelled actin in U2OS cells. Additionally, we observe phalloidin-labelled actin in mouse fibroblasts grown on grids of labelled nanowires and identify correlations between local actin alignment and global cell-scale orientation, indicating cellular coordination across length scales.

Fluorescence polarization: a novel indicator of cardiomyocyte contraction

The changes measured in intracellular fluorescein fluorescence polarization (IFFP) are used as a new tool for tracing cytoplasmic effects during contractile cycles of cardiac myocytes (1-2-day-old rat hearts), in addition to the established Ca(2+) monitoring and/or videometric methods of tracking cell-shortening. This novel method was found to be non-intrusive to the contraction cycles. The decay of the transient IFFP signal (from 0.220+/-0.01 to 0.170+/-0.013) seems to be closely related to the extended phase of contractile activation. This fact was further supported when Ca(2+) exchanger inhibitor was introduced and significantly decreased (90%) the rate of beats of contraction and IFFP, but not the Ca(2+) beat rate changes. This result suggests that the IFFP indicator is probably associated with the physiological activation, rather than with Ca(2+) alterations. The IFFP measure monitors the average of effective changes in the micro-viscosity of the cytoplasm protein matrix, associated with cellular activation.

Steady-state polarization from cylindrically symmetric fluorophores undergoing rapid restricted motion

Steady-state fluorescence polarization measurements provide a relatively simple method for investigating the orientation of molecular components in ordered biological systems. However, the observed fluorescence polarization ratios also depend on any mobility of the fluorophores on the time scale of the fluorescence lifetime. Such mobility commonly arises from incomplete immobilization of a fluorescent probe on the macromolecule of interest. A theoretical formalism is presented for the interpretation of steady-state fluorescence polarization ratios in the presence of such rapid fluorophore motion. It is assumed that the fluorophores move freely within a cone between absorption and emission of a photon. Only one new parameter is introduced to describe fluorophore motion, the semi-angle of the cone, and this can be measured in separate experiments on an isotropic system. The fluorophore orientations are assumed to have cylindrical symmetry, but the symmetry axis need not be in the same plane as the center axes of the absorption and emission cones. This geometry applies to muscle and other biological fibers.

Quantitation of liquid-crystalline ordering in F-actin solutions

Actin filaments (F-actin) are important determinants of cellular shape and motility. These functions depend on the collective organization of numerous filaments with respect to both position and orientation in the cytoplasm. Much of the orientational organization arises spontaneously through liquid crystal formation in concentrated F-actin solutions. In studying this phenomenon, we found that solutions of purified F-actin undergo a continuous phase transition, from the isotropic state to a liquid crystalline state, when either the mean filament length or the actin concentration is increased above its respective threshold value. The phase diagram representing the threshold filament lengths and concentrations at which the phase transition occurs is consistent with that predicted by Flory's theory on solutions of noninteracting, rigid cylinders (Flory, 1956b). However, in contrast to other predictions based on this model, we found no evidence for the coexistence of isotropic and anisotropic phases. Furthermore, the phase transition proved to be temperature dependent, which suggests the existence of orientation-dependent interfilament interactions or of a temperature-dependent filament flexibility. We developed a simple method for growing undistorted fluorescent acrylodan-labeled F-actin liquid crystals; and we derived a simple theoretical treatment by which polarization-of-fluorescence measurements could be used to quantitate, for the first time, the degree of spontaneous filament ordering (nematic order parameter) in these F-actin liquid crystals. This order parameter was found to increase monotonically with both filament length and concentration. Actin liquid crystals can readily become distorted by a process known as "texturing." Zigzaging and helicoidal liquid crystalline textures which persisted in the absence of ATP were observed through the polarizing microscope. Possible texturing mechanisms are discussed.

Fluorescence quenching dynamics of tryptophan in proteins. Effect of internal rotation under potential barrier

In many proteins fluorescence from single tryptophan exhibits a nonexponential decay function. To elucidate the origin of this nonexponential decay, we have examined the fluorescence decay function and time-resolved fluorescence anisotropy of a fluorophore covalently bound to a macromolecule by solving a rotational analogue of the Smoluchowski equation. An angular-dependent quenching constant and potential energy for the fluorophore undergoing internal rotation were introduced into the equation of motion for fluorophore. Results of numerical calculations using the equations thus obtained predict that both the fluorescence decay function and time-resolved anisotropy are dependent on rotational diffusion coefficients of fluorophore and potential energy for the internal rotation. The method was applied to the observed fluorescence decay curve of the single tryptophan in apocytochrome c from horse heart. The calculated decay curves fit the observed ones well.

Fraction of myosin cross-bridges bound to actin in active muscle fibers: estimation by fluorescence anisotropy measurements

Time-resolved and steady-state fluorescence anisotropy measurements from fluorescence-labeled myosin cross-bridges in single glycerinated skeletal muscle fibers in rigor, relaxed, MgADP-induced, and contracting states have been made in order to estimate the fraction of actin-bound cross-bridges in active muscle. When the plane of polarization of the excitation light is perpendicular to the fiber axis and its propagation vector has a component parallel to this axis, actin-bound cross-bridge states, such as rigor and MgADP-induced, have time-zero and steady-state anisotropies that are substantially lower than has the relaxed state. This difference provides a means of determining the fraction of cross-bridges bound to actin in active isometric fibers, by comparing the fluorescence anisotropy from active fibers with the anisotropy from bound and unbound cross-bridges in static states. By assuming that the active cross-bridges are either bound (in the manner of rigor or MgADP-induced states) or relaxed, we estimate that greater than 80% of the cross-bridges are actin-bound in active isometric fibers.