Intracellular Cell Motility

Nanoparticles have been used to study and characterize motion in the cytoplasm or cellular membrane of live cells. The nanoparticles have been designed and conjugated with a specific antigen or antibody to monitor the motion of specific components on the membrane of live cells [24-26]. The movement of these nanoparticles was detected and followed using a video camera as the nanoparticles moved along with the components of interest and left their discerning motility path [40, 41]. Colloidal Au nanoparticles were very popular for such studies because individual nanoparticles could be followed for an extended length of time to obtain realtime mobility using dark-field optical microscopy. Single Au nanoparticles were directly visualized by optical dark-field microscopy using a video camera [49, 50]. The mobility of Au nanoparticles has been studied in PTK-2 cells [42].

The motion of single Au nanoparticles has been determined using algorithms and image processing hardware

[42]. Typical patterns of saltatory and Brownian motion were taken into account for statistical analysis. Saltatory movement was characterized by long jumps (>1 /m) and long stop times (2-5 s), whereas Brownian motion was characterized by small elongations and no stop times [51]. These criteria allow one to continuously update the distributions of the jump time, jump velocity, jump direction, stop time, and orientation on the coordination matrix. If the next direction lay within 30° of the former direction, it was considered to be linear saltatory motion, whereas if that was not the case, the former motion was considered to be a finished saltatory motion. The jump time was used to determine whether the nanoparticles were participating in energy-demanding [adenotriphosphate (ATP) utilizing movement] saltatory movement. If the motion was not saltatory, then it was determined to be Brownian random motion. The diffusion coefficient was used to assign the movements of the nanoparticles in Brownian random motion.

Bare metal nanoparticles or metal nanoparticles labeled with fluorescent dyes have been used as probes for the study of intracellular cell motility [40-42, 49, 50, 52]. Furthermore, these nanoparticles conjugated with antibodies, receptors, and proteins have been employed to determine the motil-ity characteristics of the corresponding components on the surface of live cells or inside the cells [24-26]. This technique provides the mobility features of individual nanopar-ticles and allows a large group of single nanoparticles to be monitored in real time simultaneously. Unlike classical fluorescence recovery after photobleaching (FRAP) techniques that offer the average motion of a number of components [53], nanoparticles were used to study individual cellular components. Thus, the subtle and heterogeneous spatial differentiation of the motion is unmasked.

Recently, FluoSphere beads (20 nm) labeled with a Fab fragment of monoclonal antibody (W6/32) have been developed and used for the study and characterization of the motion of MHC (major histocompatibility complex) class I molecules on the surface of HeLa cells [52]. The labeled FluoSphere beads were tracked in real time using high-sensitivity fluorescence microscopy. The spots recorded at 4-s intervals by fluorescence microscopy were connected to deduce the path of mobility (Fig. 6). Typically, a higher mobility was observed from bare metal nanoparticles and a lower mobility was observed from nanoparticles incorporated with fluorescence dyes or polymer based because bare nanoparticles have less interaction with biological entities.

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