Real-time measurements of actin filament polymerization by total internal reflection fluorescence microscopy

Imaging of dynamic secretory vesicles in living pollen tubes of Picea meyeri using evanescent wave microscopy

Evanescent wave excitation was used to visualize individual, FM4-64-labeled secretory vesicles in an optical slice proximal to the plasma membrane of Picea meyeri pollen tubes. A standard upright microscope was modified to accommodate the optics used to direct a laser beam at a variable angle. Under evanescent wave microscopy or total internal reflection fluorescence microscopy, fluorophores localized near the surface were excited with evanescent waves, which decay exponentially with distance from the interface. Evanescent waves with penetration depths of 60 to 400 nm were generated by varying the angle of incidence of the laser beam. Kinetic analysis of vesicle trafficking was made through an approximately 300-nm optical section beneath the plasma membrane using time-lapse evanescent wave imaging of individual fluorescently labeled vesicles. Two-dimensional trajectories of individual vesicles were obtained from the resulting time-resolved image stacks and were used to characterize the vesicles in terms of their average fluorescence and mobility, expressed here as the two-dimensional diffusion coefficient D2. The velocity and direction of vesicle motions, frame-to-frame displacement, and vesicle trajectories were also calculated. Analysis of individual vesicles revealed for the first time, to our knowledge, that two types of motion are present, and that vesicles in living pollen tubes exhibit complicated behaviors and oscillations that differ from the simple Brownian motion reported in previous investigations. Furthermore, disruption of the actin cytoskeleton had a much more pronounced effect on vesicle mobility than did disruption of the microtubules, suggesting that actin cytoskeleton plays a primary role in vesicle mobility.

Actin polymerisation at the cytoplasmic face of eukaryotic nuclei

Background

There exists abundant molecular and ultra-structural evidence to suggest that cytoplasmic actin can physically interact with the nuclear envelope (NE) membrane system. However, this interaction has yet to be characterised in living interphase cells.

Results

Using a fluorescent conjugate of the actin binding drug cytochalasin D (CD-BODIPY) we provide evidence that polymerising actin accumulates in vicinity to the NE. In addition, both transiently expressed fluorescent actin and cytoplasmic micro-injection of fluorescent actin resulted in accumulation of actin at the NE-membrane. Consistent with the idea that the cytoplasmic phase of NE-membranes can support this novel pool of perinuclear actin polymerisation we show that isolated, intact, differentiated primary hepatocyte nuclei support actin polymerisation in vitro. Further this phenomenon was inhibited by treatments hindering steric access to outer-nuclear-membrane proteins (e.g. wheat germ agglutinin, anti-nesprin and anti-nucleoporin antibodies).

Conclusion

We conclude that actin polymerisation occurs around interphase nuclei of living cells at the cytoplasmic phase of NE-membranes.

Model of formin-associated actin filament elongation

Formin FH2 domains associate processively with actin-filament barbed ends and modify their rate of growth. We modeled how the elongation rate depends on the concentrations of profilin and actin for four different formins. We assume that (1) FH2 domains are in rapid equilibrium among conformations that block or allow actin addition and that (2) profilin-actin is transferred rapidly to the barbed end from multiple profilin binding sites in formin FH1 domains. In agreement with previous experiments discussed below, we find an optimal profilin concentration with a maximal elongation rate that can exceed the rate of actin alone. High profilin concentrations suppress elongation, largely because free profilin displaces profilin-actin from FH1. The model supports a common polymerization mechanism for the four formin FH1FH2 constructs with differences attributed to varying parameter values. The mechanism does not require ATP hydrolysis by polymerized actin, but we cannot exclude that formins accelerate hydrolysis.

Twinfilin is an actin-filament-severing protein and promotes rapid turnover of actin structures in vivo

Working in concert, multiple actin-binding proteins regulate the dynamic turnover of actin networks. Here, we define a novel function for the conserved actin-binding protein twinfilin, which until now was thought to function primarily as a monomer-sequestering protein. We show that purified budding yeast twinfilin (Twf1) binds to and severs actin filaments in vitro at pH below 6.0 in bulk kinetic and fluorescence microscopy assays. Further, we use total internal reflection fluorescence (TIRF) microscopy to demonstrate that Twf1 severs individual actin filaments in real time. It has been shown that capping protein directly binds to Twf1 and is required for Twf1 localization to cortical actin patches in vivo. We demonstrate that capping protein directly inhibits the severing activity of Twf1, the first biochemical function ascribed to this interaction. In addition, phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] inhibits Twf1 filament-severing activity. Consistent with these biochemical activities, a twf1Δ mutation causes reduced rates of cortical actin patch turnover in living cells. Together, our data suggest that twinfilin coordinates filament severing and monomer sequestering at sites of rapid actin turnover and is controlled by multiple regulatory inputs.

Reconstitution of the transition from lamellipodium to filopodium in a membrane-free system

The cellular cytoskeleton is a complex dynamical network that constantly remodels as cells divide and move. This reorganization process occurs not only at the cell membrane, but also in the cell interior (bulk). During locomotion, regulated actin assembly near the plasma membrane produces lamellipodia and filopodia. Therefore, most in vitro experiments explore phenomena taking place in the vicinity of a surface. To understand how the molecular machinery of a cell self-organizes in a more general way, we studied bulk polymerization of actin in the presence of actin-related protein 2/3 complex and a nucleation promoting factor as a model for actin assembly in the cell interior separate from membranes. Bulk polymerization of actin in the presence of the verprolin homology, cofilin homology, and acidic region, domain of Wiskott-Aldrich syndrome protein, and actin-related protein 2/3 complex results in spontaneous formation of diffuse aster-like structures. In the presence of fascin these asters transition into stars with bundles of actin filaments growing from the surface, similar to star-like structures recently observed in vivo. The transition from asters to stars depends on the ratio [fascin]/[G actin]. The polarity of the actin filaments during the transition is preserved, as in the transition from lamellipodia to filopodia. Capping protein inhibits star formation. Based on these experiments and kinetic Monte Carlo simulations, we propose a model for the spontaneous self-assembly of asters and their transition into stars. This mechanism may apply to the transition from lamellipodia to filopodia in vivo.

Mechanistic differences in actin bundling activity of two mammalian formins, FRL1 and mDia2

Formin proteins are regulators of actin dynamics, mediating assembly of unbranched actin filaments. These multidomain proteins are defined by the presence of a Formin Homology 2 (FH2) domain. Previous work has shown that FH2 domains bind to filament barbed ends and move processively at the barbed end as the filament elongates. Here we report that two FH2 domains, from mammalian FRL1 and mDia2, also bundle filaments, whereas the FH2 domain from mDia1 cannot under similar conditions. The FH2 domain alone is sufficient for bundling. Bundled filaments made by either FRL1 or mDia2 are in both parallel and anti-parallel orientations. A novel property that might contribute to bundling is the ability of the dimeric FH2 domains from both FRL1 and mDia2 to dissociate and recombine. This property is not observed for mDia1. A difference between FRL1 and mDia2 is that FRL1-mediated bundling is competitive with barbed end binding, whereas mDia2-mediated bundling is not. Mutation of a highly conserved isoleucine residue in the FH2 domain does not inhibit bundling by either FRL1 or mDia2, but inhibits barbed end activities. However, the severity of this mutation varies between formins. For mDia1 and mDia2, the mutation strongly inhibits all effects of barbed end binding, but affects FRL1 much less strongly. Furthermore, our results suggest that the Ile mutation affects processivity. Taken together, our data suggest that the bundling activities of FRL1 and mDia2, while producing phenotypically similar bundles, differ in mechanistic detail.

Profilin: emerging concepts and lingering misconceptions

Conflicting data suggest that profilin might function to promote either actin polymerization or depolymerization in cells. There are theoretical reasons and supportive data to suggest that profilin might do both. Perhaps the most accurate description of profilin emphasizes its ability to augment actin-filament dynamics, both in polymerization and in depolymerization. The effect of profilin on the critical concentration of actin, its ability to depolymerize filaments at the barbed end and the formation of a ternary complex with thymosin beta(4) all need to be accurately represented in any attempt to determine a model for profilin function.

Control of the assembly of ATP- and ADP-actin by formins and profilin

Formin proteins nucleate actin filaments, remaining processively associated with the fast-growing barbed ends. Although formins possess common features, the diversity of functions and biochemical activities raised the possibility that formins differ in fundamental ways. Further, a recent study suggested that profilin and ATP hydrolysis are both required for processive elongation mediated by the formin mDia1. We used total internal reflection fluorescence microscopy to observe directly individual actin filament polymerization in the presence of two mammalian formins (mDia1 and mDia2) and two yeast formins (Bni1p and Cdc12p). We show that these diverse formins have the same basic properties: movement is processive in the absence or presence of profilin; profilin accelerates elongation; and actin ATP hydrolysis is not required for processivity. These results suggest that diverse formins are mechanistically similar, but the rates of particular assembly steps vary.

Waveguide excitation fluorescence microscopy: A new tool for sensing and imaging the biointerface

A novel biosensing and imaging technique, the waveguide excitation fluorescence microscope, has been developed for the dynamic and quantitative investigation of bio-interfacial events in situ, ranging from ligand-receptor binding to focal adhesion formation in cell-surface interactions. The technique makes use of the evanescent field created when light travels in a mono-mode, planar optical waveguide to excite fluorescence in the near interface region. Advantages of the technique include high target sensitivity for fluorescence detection (femtomolar range), high surface specificity (ca. 100 nm perpendicular to the waveguide), large area analysis with submicron resolution, 'built-in' calibration of fluorescent light gain, and the capability to perform multi-colour imaging in situ and in real time. In this work, the sensitivity of the system has already been demonstrated through dynamic measurements of the streptavidin-biotin binding event to below 20 pM concentrations, signal to noise comparisons with conventional fluorescence microscopy have shown more than a 10-fold improvement, and surface specificity of the technique has also been illustrated in a comparison of fibroblast focal adhesion images. Thus, this new tool can be used to illuminate processes occurring at the interface between biology and synthetic surfaces in a unique manner.

Purified Integrin Adhesion Complexes Exhibit Actin-Polymerization Activity

BACKGROUND

Cell adhesion and motility are accomplished through a functional linkage of the extracellular matrix with the actin cytoskeleton via adhesion complexes composed of integrin receptors and associated proteins. To determine whether this linkage is attained actively or passively, we isolated integrin complexes from nonadherent hematopoietic cells and determined their influence on the polymerization of actin.

RESULTS

We observed that alpha(V)beta3 complexes are capable of dramatically accelerating the rate of actin assembly, resulting in actin fibers tethered at their growing ends by clustered integrins. The ability to enhance actin polymerization was dependent upon Arg-Gly-Asp-ligand-induced beta3 tyrosine phosphorylation, agonist-induced cellular activation, sequestration of Diaphanous formins, and clustering of the receptor.

CONCLUSIONS

These results suggest that adhesion complexes actively promote actin assembly from their cytosolic face in order to establish a mechanical linkage with the extracellular matrix.

ATP hydrolysis stimulates large length fluctuations in single actin filaments

Polymerization dynamics of single actin filaments is investigated theoretically using a stochastic model that takes into account the hydrolysis of ATP-actin subunits, the geometry of actin filament tips, and the lateral interactions between the monomers as well as the processes at both ends of the polymer. Exact analytical expressions are obtained for the mean growth velocity, for the dispersion in the length fluctuations, and the nucleotide composition of the actin filaments. It is found that the ATP hydrolysis has a strong effect on dynamic properties of single actin filaments. At high concentrations of free actin monomers, the mean size of the unhydrolyzed ATP-cap is very large, and the dynamics is governed by association/dissociation of ATP-actin subunits. However, at low concentrations the size of the cap becomes finite, and the dissociation of ADP-actin subunits makes a significant contribution to overall dynamics. Actin filament length fluctuations reach a sharp maximum at the boundary between two dynamic regimes, and this boundary is always larger than the critical concentration for the actin filament's growth at the barbed end, assuming the sequential release of phosphate. Random and sequential mechanisms of hydrolysis are compared, and it is found that they predict qualitatively similar dynamic properties at low and high concentrations of free actin monomers with some deviations near the critical concentration. The possibility of attachment and detachment of oligomers in actin filament's growth is also discussed. Our theoretical approach is successfully applied to analyze the latest experiments on the growth and length fluctuations of individual actin filaments.

Biochemical analysis of mammalian formin effects on actin dynamics

Formins are members of a conserved family of proteins, present in all eukaryotes, that regulate actin dynamics. Mammals have 15 distinct formin genes. From studies to date, surprising variability between these isoforms has been uncovered. All formins examined have several common effects on actin dynamics in that they: (1) accelerate nucleation rate; (2) alter filament barbed end elongation/depolymerization rates; and (3) antagonize capping protein. However, the potency of each effect can vary greatly between formins. In addition, a subset of formins binds tightly to filament sides and bundle filaments. Even isoforms that are closely related phylogenetically can display marked differences in their effects on actin. This chapter discusses several methods for examining formin function in vitro. We also discuss pitfalls associated with these assays. As one example, the effect of profilin on formin function is difficult to interpret by "pyrene-actin" polymerization assays commonly used in the field and requires assays that can distinguish between filament nucleation and filament elongation. The regulatory mechanisms for formins are not clear and certainly vary between isoforms. A subset of formins is regulated by Rho GTPases, and the assays described in this chapter have been used for characterization of this regulation.

Formin proteins: a domain-based approach

Formin proteins are potent regulators of actin dynamics. Most eukaryotes have multiple formin isoforms, suggesting diverse cellular roles. Formins are modular proteins, containing a series of domains and functional motifs. The Formin homology 2 (FH2) domain binds actin filament barbed ends and moves processively as these barbed ends elongate or depolymerize. The FH1 domain influences FH2 domain function through binding to the actin monomer-binding protein, profilin. Outside of FH1 and FH2, amino acid similarity between formins decreases, suggesting diverse mechanisms for regulation and cellular localization. Some formins are regulated by auto-inhibition through interaction between the diaphanous inhibitory domain (DID) and diaphanous auto-regulatory domain (DAD), and activated by Rho GTPase binding to GTPase-binding domains (GBD). Other formins lack DAD, DID and GBD, and their regulatory mechanisms await elucidation.

Actin polymerization kinetics, cap structure, and fluctuations

Polymerization of actin proteins into dynamic structures is essential to eukaryotic cell life, motivating many in vitro experiments measuring polymerization kinetics of individual filaments. Here, we model these kinetics, accounting for all relevant steps revealed by experiment: polymerization, depolymerization, random ATP hydrolysis, and release of phosphate (P(i)). We relate filament growth rates to the dynamics of ATP-actin and ADP-P(i)-actin caps that develop at filament ends. At the critical concentration of the barbed end, c(crit), we find a short ATP cap and a long fluctuation-stabilized ADP-P(i) cap. We show that growth rates and the critical concentration at the barbed end are intimately related to cap structure and dynamics. Fluctuations in filament lengths are described by the length diffusion coefficient, D. Recently Fujiwara et al. [Fujiwara, I., Takahashi, S., Takaduma, H., Funatsu, T. & Ishiwata, S. (2002) Nat. Cell Biol. 4, 666-673] and Kuhn and Pollard [Kuhn, J. & Pollard, T. D. (2005) Biophys. J. 88, 1387-1402] observed large length fluctuations slightly above c(crit), provoking speculation that growth may proceed by oligomeric rather than monomeric on-off events. For the single-monomer growth process, we find that D exhibits a pronounced peak below c(crit), due to filaments alternating between capped and uncapped states, a mild version of the dynamic instability of microtubules. Fluctuations just above c(crit) are enhanced but much smaller than those reported experimentally. Future measurements of D as a function of concentration can help identify the origin of the observed fluctuations.

Actin Dynamics: Growth from Dendritic Branches

The dendritic nucleation model was devised to explain the cycle of actin dynamics resulting in actin filament network assembly and disassembly in two contexts--at the leading edge of motile cells and in the actin comet tails of intracellular pathogenic bacteria and viruses. Due to the detailed nature of its biochemical predictions, the model has provided an excellent focus for subsequent experimentation. This review summarizes recent work on actin dynamics in the context of the dendritic nucleation model. One outcome of this research is the possibility that additional proteins, as well as the six proteins included in the original model, might increase the efficiency of dendritic nucleation or modify the resulting actin network. In addition, actin dynamics at the leading edge might be influenced by a second actin filament network, independent of dendritic nucleation.

Total internal reflection fluorescence microscopy: technical innovations and novel applications

Recent years have seen the introduction of novel techniques and applications of total internal reflection fluorescence microscopy (TIRFM). Key technical achievements include miniaturization, enhanced depth resolution, reduction of detection volumes and the combination of TIRFM with other microscopic techniques. Novel applications have concentrated on single-molecule detection (e.g. of cellular receptors), imaging of exocytosis or endocytosis, measurements of adhesion foci of microtubules, and studies of the localization, activity and structural arrangement of specific ion channels. In addition to conventional fluorescent dyes, genetically engineered fluorescent proteins are increasingly being used to measure molecular conformations or intermolecular distances by fluorescence resonance energy transfer.