Detection of gold probes with video-enhanced contrast microscopy: nanovid microscopy

Molecular stretching modulates mechanosensing pathways

For individual cells in tissues to create the diverse forms of biological organisms, it is necessary that they must reliably sense and generate the correct forces over the correct distances and directions. There is considerable evidence that the mechanical aspects of the cellular microenvironment provide critical physical parameters to be sensed. How proteins sense forces and cellular geometry to create the correct morphology is not understood in detail but protein unfolding appears to be a major component in force and displacement sensing. Thus, the crystallographic structure of a protein domain provides only a starting point to then analyze what will be the effects of physiological forces through domain unfolding or catch-bond formation. In this review, we will discuss the recent studies of cytoskeletal and adhesion proteins that describe protein domain dynamics. Forces applied to proteins can activate or inhibit enzymes, increase or decrease protein-protein interactions, activate or inhibit protein substrates, induce catch bonds and regulate interactions with membranes or nucleic acids. Further, the dynamics of stretch-relaxation can average forces or movements to reliably regulate morphogenic movements. In the few cases where single molecule mechanics are studied under physiological conditions such as titin and talin, there are rapid cycles of stretch-relaxation that produce mechanosensing signals. Fortunately, the development of new single molecule and super-resolution imaging methods enable the analysis of single molecule mechanics in physiologically relevant conditions. Thus, we feel that stereotypical changes in cell and tissue shape involve mechanosensing that can be analyzed at the nanometer level to determine the molecular mechanisms involved.

Self-assembling nanoclusters in living systems: application for integrated photothermal nanodiagnostics and nanotherapy

Nanotechnologies represent an unprecedented recent advance that may revolutionize many areas of medicine and biology, including cancer diagnostics and treatment. Nanoparticle-based technologies have demonstrated especially high potential for medical purposes, ranging from diagnosing diseases to providing novel therapies. However, to be clinically relevant, the existing nanoparticle-based technologies must overcome several challenges, including selective nanoparticle delivery, potential cytotoxicity, imaging of nanoparticles, and real-time assessment of their therapeutic efficacy. This review addresses these issues by summarizing the recent advances in medical diagnostics and therapy with a focus on the self-assembly of gold nanoparticles into nanoclusters in live cells, in combination with their detection using photothermal (PT) techniques.

In vivo plasma membrane organization: results of biophysical approaches

In the last two decades, various biophysical techniques have been used to investigate the organization of the plasma membrane in live cells. This review describes some of the most important experimental findings and summarizes the characteristics and limitations of a few frequently used biophysical techniques. In addition, the current knowledge about three membrane organizational elements: the membrane-associated cytoskeleton, caveolae and lipid microdomains, is described in detail. Unresolved issues, experimental contradictions and future directions to integrate the variety of experimental data into a revised model of the plasma membrane of eukaryotic cells are discussed in the last section.

Tracking movements of lipids and Thy1 molecules in the plasmalemma of living fibroblasts by fluorescence video microscopy with nanometer scale precision

The lateral diffusion of 100 nm fluorescent latex microspheres (FS) bound to either N-biotinyl-phosphatidyl-ethanolamine or the glycosylphosphatidylinositol-linked protein Thy1 were monitored in the plasmalemma of primary rat fibroblasts by single particle tracking of FS centroids from digital fluorescence micrographs. A silicon intensified target camera was found to be superior to slow scan cooled CCD and intensified interline transfer CCD cameras for monitoring lateral diffusion of rapidly moving FS with nanometer level precision. To estimate the maximum tracking precision, a 4 sec-sequence comprising 120 images of FS fixed to a cover glass was obtained. The mean distance of the centroids from the origin was 7.5 +/- 0.4 nm, and no centroids were beyond 16 nm from the origin. The SIT camera was then used to track FS attached to lipids and Thy1 molecules on the surface of fibroblasts. The lateral diffusion of lipid-bound FS was unconstrained, and the ensemble averaged diffusion coefficient was 0.80 x 10(-9) cm2/sec. Thy1-bound FS existed in two mobility populations, both of which demonstrated constrained mobility. The rapidly moving population, comprising 61% of the total, had an ensemble diffusion coefficient of 6.1 x 10(-10) cm2/sec, and appeared to be restricted to domains with a mean length of about 700 nm. The slowly moving population, comprising about 39% of the total, had a diffusion coefficient of 5.7 x 10(-12) cm2/sec. These results demonstrate that nanovid can be extended to the realm of fluorescence microscopy and support previous studies indicating that while the lateral mobilities of at least some lipids are not constrained to small domains by barriers to lateral diffusion in the fibroblast plasmalemma, a peripheral membrane protein which is bound only by a lipid anchor can be prevented from diffusing freely.

Two-photon fluorescence correlation spectroscopy: method and application to the intracellular environment

We report on the application of two photon molecular excitation to fluorescence correlation spectroscopy. We demonstrate the first fluorescence correlation spectroscopy measurements of translational mobility in the cytoplasm of living cells. Two-photon excitation inherently excites small sample volumes in three dimensions, providing depth discrimination similar to confocal microscopy, without emission pinholes. We demonstrated accurate measurements of the diffusion constant, D, for particles of several different known sizes, in bulk solutions of different viscosity. We then showed measurements of translational diffusion for 7- and 15-nm radius latex beads in the cytoplasm of mouse fibroblast cells. We measured time-dependent diffusion coefficients. When first injected in the cells, the spheres moved from two to five times slower than in water, with average rates of 18 x 10(-8) cm2/s for the 7 nm and 5 x 10(-8) cm2/s for the 15 nm radius spheres. After a few hours, spheres stick to the cells, and the motion slows down 10 to 100 times.

Tracking of single fluorescent particles in three dimensions: use of cylindrical optics to encode particle position

We present a novel optical technique for three-dimensional tracking of single fluorescent particles using a modified epifluorescence microscope containing a weak cylindrical lens in the detection optics and a microstepper-controlled fine focus. Images of small, fluorescent particles were circular in focus but ellipsoidal above and below focus; the major axis of the ellipsoid shifted by 90 degrees in going through focus. Particle z position was determined from the image shape and orientation by applying a peak detection algorithm to image projections along the x and y axes; x, y position was determined from the centroid of the particle image. Typical spatial resolution was 12 nm along the optical axis and 5 nm in the image plane with a maximum sampling rate of 3-4 Hz. The method was applied to track fluorescent particles in artificial solutions and living cells. In a solution of viscosity 30 cP, the mean squared distance (MSD) traveled by a 264 nm diameter rhodamine-labeled bead was linear with time to 20 s. The measured diffusion coefficient, 0.0558 +/- 0.001 micron2/s (SE, n = 4), agreed with the theoretical value of 0.0556 micron2/s. Statistical variability of MSD curves for a freely diffusing bead was in quantitative agreement with Monte Carlo simulations of three-dimensional random walks. In a porous glass matrix, the MSD data was curvilinear and showed reduced bead diffusion. In cytoplasm of Swiss 3T3 fibroblasts, bead diffusion was restricted. The water permeability in individual Chinese Hamster Ovary cells was measured from the z movement of a fluorescent bead fixed at the cell surface in response osmotic gradients; water permeability was increased by > threefold in cells expressing CHIP28 water channels. The simplicity and precision of this tracking method may be useful to quantify the complex trajectories of fluorescent particles in living cells.

Reflectance in situ hybridization (RISH): detection, by confocal reflectance laser microscopy, of gold-labelled riboprobes in breast cancer cell lines and histological specimens

A method for reflectance in situ hybridization (RISH) is presented. The importance of the method is demonstrated by results obtained on cytological and histological breast cancer specimens. Scattering reflectance signals from 1-nm colloidal-gold particles after RNA/RNA in situ hybridization, using digoxigenin-labelled riboprobes, were detected by confocal scanning laser microscopy. The mRNA expression of two ras-related genes, rho B and rho C, was analysed in human histological breast cancer specimens and in human breast cancer cell lines. Horizontal (x, y) and vertical (z) optical sections after three-dimensional imaging were used for visualization. A marked heterogeneity (between individual cells and between specimens) was noted for the expression of the rho B gene, both in cytological and in histological samples. On the other hand, rho C was always expressed and showed no heterogeneity. This method allows the identification of several cellular constituents in an heterogeneous tissue structure, as demonstrated by the simultaneous detection of rho B (or rho C) by reflectance and of DNA, cytokeratin and/or vimentin by fluorescence.

Lateral diffusion and retrograde movements of individual cell surface components on single motile cells observed with Nanovid microscopy

A recently introduced extension of video-enhanced light microscopy, called Nanovid microscopy, documents the dynamic reorganization of individual cell surface components on living cells. 40-microns colloidal gold probes coupled to different types of poly-L-lysine label negative cell surface components of PTK2 cells. Evidence is provided that they bind to negative sialic acid residues of glycoproteins, probably through nonspecific electrostatic interactions. The gold probes, coupled to short poly-L-lysine molecules (4 kD) displayed Brownian motion, with a diffusion coefficient in the range 0.1-0.2 micron2/s. A diffusion coefficient in the 0.1 micron2/s range was also observed with 40-nm gold probes coupled to an antibody against the lipid-linked Thy-1 antigen on 3T3 fibroblasts. Diffusion of these probes is largely confined to apparent microdomains of 1-2 microns in size. On the other hand, the gold probes, coupled to long poly-L-lysine molecules (240 kD) molecules and bound to the leading lamella, were driven rearward, toward the boundary between lamelloplasm and perinuclear cytoplasm at a velocity of 0.5-1 micron/min by a directed ATP-dependent mechanism. This uniform motion was inhibited by cytochalasin, suggesting actin microfilament involvement. A similar behavior on MO cells was observed when the antibody-labeled gold served as a marker for the PGP-1 (GP-80) antigen. These results show that Nanovid microscopy, offering the possibility to observe the motion of individual specific cell surface components, provides a new and powerful tool to study the dynamic reorganization of the cell membrane during locomotion and in other biological contexts as well.