Real-time dynamics of label-free single mast cell granules revealed by differential interference contrast microscopy

Image analysis of mast cell degranulation in a concentration gradient of stimuli formed in the channel between a glass plate and a silicon substrate

Measurement of released granule components, popularly used to quantify mast cell exocytosis, does not deliver real-time information about degranulation at the single-cell level nor the ratio of responding/non-responding cells. Rather it provides, only end-point, bulk-population data. Here we studied degranulation of rat peritoneal mast cells dispersed in a narrow horizontal channel between a silicon substrate and a glass plate. Upon exposure to a concentration gradient of a soluble stimulus, degranulation started from those cells facing towards the highest concentration of stimulus. We captured images of exocytosing cells without the need for phase-contrast or differential interference-contrast microscopy. This was achieved using the reflection caused by the silicon substrate. The time-lapse images of cells in the channel were segmented into multiple concentration belts to identify the proportion of degranulated cells in each belt region. Maximum ratios of degranulated cells in the belt regions determined by time-course curve fitting calculations were then plotted against the distance from the stimulus injection site, resulting in a sigmoidal response curve. This method provides a powerful means for real-time analysis of concentration- and stimulus-dependent degranulation of mast cells and allows comparison of cell responses under different conditions. To show its effectiveness, we evaluated the effect of a protein kinase C (PKC) inhibitor, Gö6976, on degranulation induced by various stimuli. In contrast to stimulation with concanavalin A+lysophosphatidylserine (lysoPS) or nerve growth factor+lysoPS (completely inhibited by Gö6976 over the whole range of stimulus concentrations used) or compound 48/80 and mastoparan (no inhibition by Gö6976), stimulation with ionomycin, a known Ca(2+) ionophore, showed a concentration-dependent inhibition by Gö6976, with a major inhibition at low stimulus concentrations and a diminished one at higher ionomycin concentrations. The results indicate that ionomycin-induced degranulation is mainly induced via a PKC-independent signal cascade at high stimulus concentrations, whereas below a certain concentration, degranulation is completely dependent on PKC.

Structurally ordered mesoporous carbon nanoparticles as transmembrane delivery vehicle in human cancer cells

A structurally ordered, CMK-1 type mesoporous carbon nanoparticle (MCN) material was successfully synthesized by using a MCM-48 type mesoporous silica nanoparticle as template. The structure of MCN was analyzed by a series of different techniques, including the scanning and transmission electron microscopy, powder X-ray diffraction, and N2 sorption analysis. To the best of our knowledge, no study has been reported prior to our investigation on the utilization of these structurally ordered mesoporous carbon nanoparticles for the delivery of membrane impermeable chemical agents inside of eukaryotic cells. The cellular uptake efficiency and biocompatibility of MCN with human cervical cancer cells (HeLa) were investigated. Our results show that the inhibitory concentration (IC50) value of MCN is very high (>50 microg/mL per million cells) indicating that MCN is fairly biocompatible in vitro. Also, a membrane impermeable fluorescence dye, Fura-2, was loaded to the mesoporous matrix of MCN. We demonstrated that the MCN material could indeed serve as a transmembrane carrier for delivering Fura-2 through the cell membrane to release these molecules inside of live HeLa cells. We envision that further developments of this MCN material will lead to a new generation of nanodevices for transmembrane delivery and intracellular release applications.

Endocytosis of a single mesoporous silica nanoparticle into a human lung cancer cell observed by differential interference contrast microscopy

The unique structural features of mesoporous silica nanoparticles (MSN) have made them very useful in biological applications, such as gene therapy and drug delivery. Flow cytometry, confocal microscopy, and electron microscopy have been used for observing the endocytosis of MSN. However, flow cytometry cannot directly observe the process of endocytosis. Confocal microscopy requires fluorescence labeling of the cells. Electron microscopy can only utilize fixed cells. In the present work, we demonstrate for the first time that differential interference contrast (DIC) microscopy can be used to observe the entire endocytosis process of MSN into living human lung cancer cells (A549) without fluorescence staining. There are three physical observables that characterize the locations of MSN and the stages of the endocytosis process: motion, shape, and vertical position. When it was outside the cell, the MSN underwent significant Brownian motion in the cell growth medium. When it was trapped on the cell membrane, the motion of the MSN was greatly limited. After the MSN had entered the cell, it resumed motion at a much slower speed because the cytoplasm is more viscous than the cell growth medium and the cellular cytoskeleton networks act as obstacles. Moreover, there were shape changes around the MSN due to the formation of a vesicle after the MSN had been trapped on the cell membrane and prior to entry into the cell. Finally, by coupling a motorized vertical stage to the DIC microscope, we recorded the location of the MSN in three dimensions. Such accurate 3D particle tracking ability in living cells is essential for studies of selectively targeted drug delivery based on endocytosis.

Vesicle trafficking dynamics and visualization of zones of exocytosis and endocytosis in tobacco pollen tubes

Pollen tubes are one of the fastest growing eukaryotic cells. Rapid anisotropic growth is supported by highly active exocytosis and endocytosis at the plasma membrane, but the subcellular localization of these sites is unknown. To understand molecular processes involved in pollen tube growth, it is crucial to identify the sites of vesicle localization and trafficking. This report presents novel strategies to identify exocytic and endocytic vesicles and to visualize vesicle trafficking dynamics, using pulse-chase labelling with styryl FM dyes and refraction-free high-resolution time-lapse differential interference contrast microscopy. These experiments reveal that the apex is the site of endocytosis and membrane retrieval, while exocytosis occurs in the zone adjacent to the apical dome. Larger vesicles are internalized along the distal pollen tube. Discretely sized vesicles that differentially incorporate FM dyes accumulate in the apical, subapical, and distal regions. Previous work established that pollen tube growth is strongly correlated with hydrodynamic flux and cell volume status. In this report, it is shown that hydrodynamic flux can selectively increase exocytosis or endocytosis. Hypotonic treatment and cell swelling stimulated exocytosis and attenuated endocytosis, while hypertonic treatment and cell shrinking stimulated endocytosis and inhibited exocytosis. Manipulation of pollen tube apical volume and membrane remodelling enabled fine-mapping of plasma membrane dynamics and defined the boundary of the growth zone, which results from the orchestrated action of endocytosis at the apex and along the distal tube and exocytosis in the subapical region. This report provides crucial spatial and temporal details of vesicle trafficking and anisotropic growth.