Microscopic imaging of cells

Circular polarization interferometry: circularly polarized modes of cholesteric liquid crystals

We describe a novel polarization interferometer which permits the determination of the refractive indices for circularly-polarized light. It is based on a Jamin-Lebedeff interferometer, modified with waveplates, and permits us to experimentally determine the refractive indices nL and nR of the respectively left- and right-circularly polarized modes in a cholesteric liquid crystal. Whereas optical rotation measurements only determine the circular birefringence, i.e. the difference (nL - nR), the interferometer also permits the determination of their absolute values. We report refractive indices of a cholesteric liquid crystal in the region of selective (Bragg) reflection as a function of temperature.

Mapping of adherens junction components using microscopic resonance energy transfer imaging

Quantitative microscopic imaging of resonance energy transfer (RET) was applied for immunological high resolution proximity mapping of several cytoskeletal components of cell adhesions. To conduct this analysis, a microscopic system was developed, consisting of a highly stable field illuminator, computer-controlled filter wheels for rapid multiple-color imaging and a sensitive, high resolution CCD camera, enabling quantitative data recording and processing. Using this system, we have investigated the spatial inter-relationships and organization of four adhesion-associated proteins, namely vinculin, talin, alpha-actinin and actin. Cultured chick lens cells were double labeled for each of the junctional molecules, using fluorescein- and rhodamine-conjugated antibodies or phalloidin. RET images were acquired with fluorescein excitation and rhodamine emission filter setting, corrected for fluorescein and rhodamine fluorescence, and normalized to the fluorescein image. The results pointed to high local densities of vinculin, talin and F-actin in focal adhesions, manifested by mean RET values of 15%, 12% and 10%, respectively. On the other hand, relatively low values (less than 1%) were observed following double immunofluorescence labeling of the same cells for alpha-actinin. Double indirect labeling for pairs of these four proteins (using fluorophore-conjugated antibodies or phalloidin) resulted in RET values of 5% or lower, except for the pair alpha-actinin and actin, which yielded significantly higher values (13-15%). These results suggest that despite their overlapping staining patterns, at the level of resolution of the light microscope, the plaque proteins vinculin and talin are not homogeneously interspersed at the molecular level but form segregated clusters. alpha-Actinin, on the other hand, does not appear to form such clusters but, rather, closely interacts with actin. We discuss here the conceptual and applicative aspects of RET measurements and the implications of the results on the subcellular molecular organization of adherens-type junctions.

Video microscopy of intracellular pH in primary cultures of rabbit proximal and early distal tubules

The purpose of this study was to investigate intracytoplasmic pH (pHi) regulation in primary cultures of proximal (PCT) and distal bright (DCTb) convoluted tubules. PCT and DCTb segments were microdissected from rabbit kidney cortex and cultured in a hormonally defined medium. The cultured epithelia were grown on semi-transparent permeable supports. The pHi was determined by video microscopy and digital image processing using 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) and measuring the ratio of BCECF fluorescence excited by two successive wavelengths (490 nm and 450 nm). Resting pHi values, determined in bicarbonate-free medium (extracellular pH: 7.40), were 7.25 +/- 0.02 (n = 23) and 7.17 +/- 0.04 (n = 30) for cultured PCT and DCTb respectively. After the acid-loading procedure, cultured proximal cells recovered their pHi by means of the classic Na+/H+ antiporter, sensitive to amiloride and located in the apical membrane only. In cultured DCTb part of the pHi recovery was mediated by a Na+/H+ exchange present in the basolateral side. Moreover, at physiological initial pHi values, chloride removal from the apical solution caused the pHi to increase in the presence of bicarbonate. In acidified cultured DCTb cells, a partial pHi recovery was induced in sodium-free media by 15 mM HCO(-3) in the presence of an outward chloride gradient. This pHi change was completely abolished by 4,4'-diisothiocyanostilbene 2,2'-disulfonic acid (1 mM). These data suggest that DCTb cells possess in apical anion/base exchanger that resembles the Na(+)-independent Cl-/HCO(-3) exchanger.

Iterative fractionation of recycling receptors from lysosomally destined ligands in an early sorting endosome

To study the fusion and separation of endocytic compartments, we have used digital image analysis to quantify the accumulation of fluorescent ligands in endosomes during continuous endocytosis for periods of 1-20 min. Fluorescently labeled transferrin (Tf) and low density lipoproteins (LDL) were used as markers of recycling receptors and lysosomally directed ligands respectively. By measuring the intensity of individual endosomes, we found that the amount of LDL per endosome increases 30-40-fold between 1 and 10 min and then plateaus. In contrast, the amount of Tf per endosome reaches a steady state within 2 min at a level that is only three to four times that at 1 min. We used pulse-chase double label methods to demonstrate that Tf cycles through the compartment in which the LDL accumulates. When both Tf and LDL are added to cells simultaneously for 2 min, nearly all endosomes contain both labels. With 2-4 min further incubation in the absence of external ligands, LDL-containing compartments become depleted of Tf as Tf is directed to para-Golgi recycling endosomes. However, if Tf is added to the medium 2-4 min after a pulse with LDL, most of the LDL-containing endosomes become labeled with Tf. The data indicate that at least 30-40 endocytic vesicles containing both Tf and LDL fuse with an endosomal compartment over a period of 5-10 min. LDL accumulates within this compartment and Tf is simultaneously removed. Simple mathematical models suggest that this type of iterative fractionation can lead to very high efficiency sorting.

Characterization of internalization and endosome formation of epidermal growth factor in transfected NIH-3T3 cells by computerized image-intensified three-dimensional fluorescence microscopy

Computerized image-intensified fluorescence microscopy has been used to quantify routing and subcellular concentrations of rhodaminated EGF (Rh-EGF) during its receptor-mediated endocytosis in two transfected NIH-3T3 cell lines expressing 2 X 10(5) and 1.5 X 10(6) receptors per cell, respectively. A series of images were digitized by focusing at different depths through the volume of a single cell. The digitized pictures were corrected for fluorescence photobleaching, and removal of out-of-focus fluorescence contributions by deconvolution using the point spread function of the microscope optics (Agard, D. A., and J. W. Sedat. 1980. Proc. Soc. Photo-Opt. Instr. Eng. 264:110-117) allowed automatic computer analysis of the time dependence of endosomal vesicle size and fluorescence intensity in a live cell and also enabled the study of isolated vesicles. An increase in the amount of fluorescence bound to the cell surface, either by increasing the number of receptors expressed per cell or the concentration of Rh-EGF in the incubation drop, yielded an increase in the total fluorescence of internalized vesicles without an increase in their number and area. The linear relation between fluorescence intensity and area for vesicles at different times indicates that EGF concentration is conserved. This is compatible with fusion of small vesicles to form larger ones. However, as endocytosis proceeds, a twofold increase in the slope of the fluorescence vs. area plots is observed for larger vesicles, suggesting that active sorting causes the EGF to be concentrated. Alternatively, this factor could be produced by cumulative fluorescence contributions from stacked membranes. Since coated pits are internalized independent of their occupancy with EGF receptor, we propose that endocytosis does not involve a mechanism specifically recognizing occupied receptor but is rather triggered by a global intracellular event.