Human neutrophil motility: time-dependent three-dimensional shape and granule diffusion

Cytoplasmic Flow and Mixing Due to Deformation of Motile Cells

The cytoplasm of a living cell is a dynamic environment through which intracellular components must move and mix. In motile, rapidly deforming cells such as human neutrophils, bulk cytoplasmic flow couples cell deformation to the transport and dispersion of cytoplasmic particles. Using particle-tracking measurements in live neutrophil-like cells, we demonstrate that fluid flow associated with the cell deformation contributes to the motion of small acidic organelles, dominating over diffusion on timescales above a few seconds. We then use a general physical model of particle dispersion in a deforming fluid domain to show that transport of organelle-sized particles between the cell periphery and the bulk can be enhanced by dynamic deformation comparable to that observed in neutrophils. Our results implicate an important mechanism contributing to organelle transport in these motile cells: cytoplasmic flow driven by cell shape deformation.

Disentangling Random Motion and Flow in a Complex Medium

We describe a technique for deconvolving the stochastic motion of particles from large-scale fluid flow in a dynamic environment such as that found in living cells. The method leverages the separation of timescales to subtract out the persistent component of motion from single-particle trajectories. The mean-squared displacement of the resulting trajectories is rescaled so as to enable robust extraction of the diffusion coefficient and subdiffusive scaling exponent of the stochastic motion. We demonstrate the applicability of the method for characterizing both diffusive and fractional Brownian motion overlaid by flow and analytically calculate the accuracy of the method in different parameter regimes. This technique is employed to analyze the motion of lysosomes in motile neutrophil-like cells, showing that the cytoplasm of these cells behaves as a viscous fluid at the timescales examined.

On the computational modeling of the innate immune system

In recent years, there has been an increasing interest in the mathematical and computational modeling of the human immune system (HIS). Computational models of HIS dynamics may contribute to a better understanding of the relationship between complex phenomena and immune response; in addition, computational models will support the development of new drugs and therapies for different diseases. However, modeling the HIS is an extremely difficult task that demands a huge amount of work to be performed by multidisciplinary teams. In this study, our objective is to model the spatio-temporal dynamics of representative cells and molecules of the HIS during an immune response after the injection of lipopolysaccharide (LPS) into a section of tissue. LPS constitutes the cellular wall of Gram-negative bacteria, and it is a highly immunogenic molecule, which means that it has a remarkable capacity to elicit strong immune responses. We present a descriptive, mechanistic and deterministic model that is based on partial differential equations (PDE). Therefore, this model enables the understanding of how the different complex phenomena interact with structures and elements during an immune response. In addition, the model's parameters reflect physiological features of the system, which makes the model appropriate for general use.

Intracellular trafficking and dynamics of P bodies

RNAs are exported from the nucleus to the cytoplasm, where they undergo translation and produce proteins needed for the cellular life cycle. Some mRNAs are targeted by different RNA decay mechanisms and thereby undergo degradation. The 5'-->3' degradation machinery localizes to cytoplasmic complexes termed P bodies (PBs). They function in RNA turnover, translational repression, RNA-mediated silencing, and RNA storage. A quantitative live-cell imaging approach to study the dynamic aspects of PB trafficking in the cytoplasm revealed that PB movements are rather confined and dependent on an existing microtubule network. Microtubule depolymerization led to a drastic decrease in PB mobility, as well as a release of regulation on PB assembly and a dramatic increase in PB numbers. The different aspects of PB trafficking and encounters with mRNA molecules in the cytoplasm are discussed.

The dynamics of mammalian P body transport, assembly, and disassembly in vivo

Exported mRNAs are targeted for translation or can undergo degradation by several decay mechanisms. The 5'-->3' degradation machinery localizes to cytoplasmic P bodies (PBs). We followed the dynamic properties of PBs in vivo and investigated the mechanism by which PBs scan the cytoplasm. Using proteins of the decapping machinery, we asked whether PBs actively scan the cytoplasm or whether a diffusion-based mechanism is sufficient. Live-cell imaging showed that PBs were anchored mainly to microtubules. Quantitative single-particle tracking demonstrated that most PBs exhibited spatially confined motion dependent on microtubule motion, whereas stationary PB pairs were identified at the centrosome. Some PBs translocated in long-range movements on microtubules. PB mobility was compared with mitochondria, endoplasmic reticulum, peroxisomes, SMN bodies, and stress granules, and diffusion coefficients were calculated. Disruption of the microtubule network caused a significant reduction in PB mobility together with an induction of PB assembly. However, FRAP measurements showed that the dynamic flux of assembled PB components was not affected by such treatments. FRAP analysis showed that the decapping enzyme Dcp2 is a nondynamic PB core protein, whereas Dcp1 proteins continuously exchanged with the cytoplasm. This study reveals the mechanism of PB transport, and it demonstrates how PB assembly and disassembly integrate with the presence of an intact cytoskeleton.

The secretory beta-amyloid precursor protein is a motogen for human epidermal keratinocytes

Cell migration is known to be triggered by constituents of the extracellular matrix such as fibronectin and by soluble mediators commonly summarized as motogens. Many growth factors such as the epidermal growth factor (EGF) have been shown to act as motogens. Recently, the secretory N-terminal portion of the beta-amyloid precursor protein (sAPP) has been identified as a keratinocyte growth factor. Hence, in this study we analysed whether sAPP stimulates also keratinocyte migration employing the stroboscopic cell motility assay. The migration velocity as well as the frequency of lamellipodia protrusion and ruffle formation were increased about two-fold thus corresponding to the effect of EGF. Using a newly developed beta1-integrin migration track assay we observed that sAPP increased the proportion of migrating keratinocytes and their directional persistence. sAPP appeared to operate synergistically with fibronectin with respect to its motogenic effect. Using a modified Boyden chamber assay we showed that sAPP besides its chemokinetic effect functions as a chemoattractant. Like EGF, sAPP exerted its motogenic effect through the activation of Rac kinase but the receptor for sAPP appears to be distinct. The results suggest that sAPP operates as a motogen in the human epidermis, where it may participate in the regulation of reepithelialization during wound healing.

Probing molecular processes in live cells by quantitative multidimensional microscopy

Modern light microscopy has become a most powerful analytical tool for studying molecular processes in live cells. Recent advances in sample preparation, microscope design and image processing allow the generation of "multidimensional" data, simultaneously reporting the three-dimensional distribution and concentrations of several different molecules within cells and tissues at multiple time points with sub-micron spatial resolution and sub-second temporal resolution. Thus, molecular interactions and processes that were approached by biochemical analyses in vitro can now be directly monitored in live cells. Here, we address different aspects of multidimensional microscopy and, in particular, image quantification and the characterization of molecular dynamics, as applied to the study of cell adhesion.

A computer-assisted system for reconstructing and interpreting the dynamic three-dimensional relationships of the outer surface, nucleus and pseudopods of crawling cells

Newly developed software additions to the three-dimensional dynamic image analysis system, 3D-DIAS, are described for simultaneously reconstructing and motion analyzing in three dimensions the outer surface, nucleus and pseudopods of living, crawling cells. This new system is then used to describe for the first time a nuclear behavior cycle in translocating Dictyostelium discoideum amoebae and to investigate the role of pseudopod extension in this process. The nuclear behavior cycle is tuned to the two phases of the general cell behavior cycle [Wessels et al., 1994], and includes nuclear migration both in the z- and in the x,y-axes from the proximal border of the prior anterior pseudopod to the proximal border of a newly expanding anterior pseudopod. Nuclear migration is cued by pseudopod-substratum contact, achieves velocities in excess of 50 microm/min, and is accompanied by characteristic changes in nuclear shape. The rules and characteristics of nuclear behavior are demonstrated to be intact in two mutants affecting pseudopod formation, a myosin IB null mutant (myoB-) and a myosin II heavy chain phosphorylation mutant (3XALA). The rules and characteristics of nuclear migration, however, are disrupted upon dissolution of microtubules by colcemid. Together the above results demonstrate that the newly developed 3D-DIAS system can be used to gain new insights into the dynamic changes in the intracellular 3D architecture associated with cellular translocation.

Clonal tests of conventional kinesin function during cell proliferation and differentiation

Null mutations in the Drosophila Kinesin heavy chain gene (Khc), which are lethal during the second larval instar, have shown that conventional kinesin is critical for fast axonal transport in neurons, but its functions elsewhere are uncertain. To test other tissues, single imaginal cells in young larvae were rendered null for Khc by mitotic recombination. Surprisingly, the null cells produced large clones of adult tissue. The rates of cell proliferation were not reduced, indicating that conventional kinesin is not essential for cell growth or division. This suggests that in undifferentiated cells vesicle transport from the Golgi to either the endoplasmic reticulum or the plasma membrane can proceed at normal rates without conventional kinesin. In adult eye clones produced by null founder cells, there were some defects in differentiation that caused mild ultrastructural changes, but they were not consistent with serious problems in the positioning or transport of endoplasmic reticulum, mitochondria, or vesicles. In contrast, defective cuticle deposition by highly elongated Khc null bristle shafts suggests that conventional kinesin is critical for proper secretory vesicle transport in some cell types, particularly ones that must build and maintain long cytoplasmic extensions. The ubiquity and evolutionary conservation of kinesin heavy chain argue for functions in all cells. We suggest interphase organelle movements away from the cell center are driven by multilayered transport mechanisms; that is, individual organelles can use kinesin-related proteins and myosins, as well as conventional kinesin, to move toward the cell periphery. In this case, other motors can compensate for the loss of conventional kinesin except in cells that have extremely long transport tracks.

Dynein- and microtubule-mediated translocation of adenovirus serotype 5 occurs after endosomal lysis

Modified viruses are used as gene transfer vectors because of their ability to transfer genetic material efficiently to the nucleus of a target cell. To better understand intracellular translocation of adenovirus serotype 5 (Ad), fluorophores were covalently conjugated to Ad capsids, and movement of fluorescent Ad within the cytoplasm was observed during the first hour of infection of a human lung epithelial carcinoma cell line (A549). Ad translocation was characterized with respect to its ability to achieve nuclear envelope localization as well as directed movement in the cytoplasm. Whereas Ad achieved efficient nuclear localization 60 min after infection of A549 cells under control conditions, depolymerization of the microtubule cytoskeleton by addition of 25 microM nocodazole reversibly inhibited development of nuclear localization. In contrast, depolymerization of microfilaments by addition of 1 microM cytochalasin D had no effect on nuclear localization. Direct video observation of Ad motility showed that nocodazole, but not cytochalasin D, caused a reversible decrease in rapid linear translocations of Ad in the cytoplasm of A549 cells. Microinjection of function-blocking antibodies against the microtubule-dependent motor protein, cytoplasmic dynein, but not kinesin, blocked nuclear localization of Ad, consistent with net minus end-directed motility indicated by accumulation of Ad at mitotic spindles. Fluorescence ratio imaging revealed a neutral pH in the environment of translocating Ad, leading to a model in which the interaction of Ad with an intact microtubule cytoskeleton and functional cytoplasmic dynein occurs after escape from endosomes and is a necessary prerequisite to nuclear localization of adenovirus serotype 5.

Cytoarchitecture and physical properties of cytoplasm: volume, viscosity, diffusion, intracellular surface area

Classical biochemistry is founded on several assumptions valid in dilute aqueous solutions that are often extended without question to the interior milieu of intact cells. In the first section of this chapter, we present these assumptions and briefly examine the ways in which the cell interior may depart from the conditions of an ideal solution. In the second section, we summarize experimental evidence regarding the physical properties of the cell cytoplasm and their effect on the diffusion and binding of macromolecules and vesicles. While many details remain to be worked out, it is clear that the aqueous phase of the cytoplasm is crowded rather than dilute, and that the diffusion and partitioning of macromolecules and vesicles in cytoplasm is highly restricted by steric hindrance as well as by unexpected binding interactions. Furthermore, the enzymes of several metabolic pathways are now known to be organized into structural and functional units with specific localizations in the solid phase, and as much as half the cellular protein content may also be in the solid phase.

Quantifying lamella dynamics of cultured cells by SACED, a new computer-assisted motion analysis

Formation of lamellipodia and the retraction of ruffles are essential activities during motility and migration of eukaryotic cells. We have developed a computer-assisted stroboscopic method for the continuous observation of cell dynamics (stroboscopic analysis of cell dynamics, SACED) that allows one to analyze changes in lamellipodia protrusion and ruffle retraction with high resolution in space and time. To demonstrate the potential of this method we analyzed keratinocytes in culture, unstimulated or stimulated with epidermal growth factor (EGF), which is known to induce cell motility and migration. Keratinocytes stimulated with EGF exhibited a 2.6-fold increase in their migration velocity, which coincided with enhanced ruffle retraction velocity (144%) and increased ruffle frequency (135%) compared to control cells. We also recorded an enhanced frequency of lamellipodia (135%), whereas the velocity of lamellipodia protrusion remained unchanged. These results on ruffle and lamellipodia dynamics in epidermal cells show that SACED is at least equal to established methods in terms of accuracy. SACED is, however, advantageous concerning resolution in time and therefore allows one to analyze the activity of lamellipodia and ruffles in as yet unknown detail. Moreover, SACED offers two opportunities that render this technique superior to established methods: First, several parameters relevant to cell motility can be analyzed simultaneously. Second, a large number of cells can conveniently be examined, which facilitates the compilation of statistically significant data.

F-actin network formation in tethers and in pseudopods stimulated by chemoattractant

Micropipets are used either to deliver a given concentration of the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) to a local region of a human neutrophil or to create a membrane tether. Pseudopods, which have a cylindrical shape and grow at a constant rate, are formed in either case. After reaching a maximum extension, they retract, even in the presence of chemoattractant. As a pseudopod grows, cell granules begin to penetrate the pseudopod region to a "boundary" that defines a distance to the pseudopod's leading edge that is almost constant. The exclusion of granules from this domain indicates that it is filled with a dense network. The formation of this network involves the plasma membrane because pseudopod growth ceases when a membrane tether is pulled away from the leading edge. The rate of pseudopod growth depends on fMLP concentration just as the number of occupied N-formyl peptide receptors depends on this concentration. The experimental data are explained by assuming that F-actin network is formed next to the plasma membrane. The newly formed network displaces the membrane and the dominant process in the network region then becomes F-actin depolymerization. The rate of pseudopod growth is determined by the rate of the process leading to network formation. This process is apparently an enzymatic type of reaction. It has a positive enthalpy change and, therefore, is endothermic.

Locomotion of neutrophil fragments occurs by graded radial extension

The only kinematic description of cell locomotion that relates the dynamics of actin filaments to whole cell movement is the graded radial extension (GRE) model for fish keratocytes, which glide without changing their shapes or sizes. To test whether the GRE model is applicable to other cell types, we analyzed the detailed shape changes during locomotion of heat-induced motile fragments of human polymorphonuclear leukocytes (PMNs). These fragments, called cytokineplasts, were loaded uniformly with a fluorescent cytoplasm-staining dye and their motility and shape changes were analyzed by fluorescence-video microscopy and digital image processing. Two-dimensional (2-D) analysis showed that cytokineplasts only changed their shapes and sizes slightly and apparently maintained their roughly circular shapes, whereas fluorescence-intensity analysis revealed distinct changes in their cytoplasmic thickness profiles. Furthermore, small structures on the cytoplasmic margins behaved as predicted by the GRE model, which therefore is probably also applicable to the parental PMNs, which show complex shape changes. This is the first indication that the GRE model operates in non-fish-keratocyte cells and may, therefore, be a universal model of cell locomotion.