Correlation of live-cell imaging with volume scanning electron microscopy.

Live-cell imaging is one of the most widely applied methods in live science. Here we describe two setups for live-cell imaging, which can easily be combined with volume SEM for correlative studies. The first procedure applies cell culture dishes with a gridded glass support, which can be used for any light microscopy modality. The second approach is a flow-chamber setup based on Ibidi µ-slides. Both live-cell imaging strategies can be followed up with serial blockface- or focused ion beam-scanning electron microscopy. Two types of resin embedding after heavy metal staining and dehydration are presented making best use of the particular advantages of each imaging modality: classical en-bloc embedding and thin-layer plastification. The latter can be used only for focused ion beam-scanning electron microscopy, but is advantageous for studying cell-interactions with specific substrates, or when the substrate cannot be removed. En-bloc embedding has diverse applications and can be applied for both described volume scanning electron microscopy techniques. Finally, strategies for relocating the cell of interest are discussed for both embedding approaches and in respect to the applied light and scanning electron microscopy methods.

Bridging microscopes: 3D correlative light and scanning electron microscopy of complex biological structures.

The rationale of correlative light and electron microscopy (CLEM) is to collect data on different information levels--ideally from an identical area on the same sample--with the aim of combining datasets at different levels of resolution to achieve a more holistic view of the hierarchical structural organization of cells and tissues. Modern three-dimensional (3D) imaging techniques in light and electron microscopy opened up new possibilities to expand morphological studies into the third dimension at the nanometer scale and over various volume dimensions. Here, we present two alternative approaches to correlate 3D light microscopy (LM) data with scanning electron microscopy (SEM) volume data. An adapted sample preparation method based on high-pressure freezing for structure preservation, followed by freeze-substitution for multimodal en-bloc imaging or serial-section imaging is described. The advantages and potential applications are exemplarily shown on various biological samples, such as cells, individual organisms, human tissue, as well as plant tissue. The two CLEM approaches presented here are per se not mutually exclusive, but have their distinct advantages. Confocal laser scanning microscopy (CLSM) and focused ion beam-SEM (FIB-SEM) is most suitable for targeted 3D correlation of small volumes, whereas serial-section LM and SEM imaging has its strength in large-area or -volume screening and correlation. The second method can be combined with immunocytochemical methods. Both methods, however, have the potential to extract statistically relevant data of structural details for systems biology.

Comparative analysis of the membrane proteome of closely related metastatic and nonmetastatic tumor cells.

The identification of proteins that are preferentially expressed on the membrane of metastatic tumor cells is of fundamental importance in cancer research. Here, we report the systematic comparison of the membrane proteome of two closely related murine teratocarcinoma cell lines (F9B9 and F9DR), of which only one (F9DR) is capable of forming liver metastases in vivo. The proteomic methodology used in this study featured the surface protein biotinylation on tumor cells followed by protein purification on streptavidin resin and relative quantification of corresponding tryptic peptides by mass spectrometric procedures. The study allowed the identification of 998 proteins and the determination of their relative abundance. Proteins previously known to be associated with metastatic spread were found to be either up-regulated (e.g., synaptojanin-2) or down-regulated (e.g., Ceacam1) in F9DR cells. A dramatic increase in abundance at the cell membrane was observed for a broad variety of proteins (e.g., high-mobility group protein B1), which were mainly thought to reside in intracellular compartments, a finding that was confirmed using confocal laser scanning microscopy and immunochemical analysis of cell cultures. Furthermore, we showed by microautoradiographic analysis that certain target proteins can readily be reached by intravenously administered radiolabeled antibodies. Finally, we showed that the most promising antigens for antibody-based pharmacodelivery approaches are strongly and selectively expressed on the surface of tumor cells in three different syngeneic mouse models of liver metastases. Taken together, our results indicate that the expression of intracellular proteins on the membrane of metastatic cells is a feature much more common than previously expected.

Effect of the modulation of the membrane lipid composition on the localization and function of P-glycoprotein in MDR1-MDCK cells.

Multidrug resistance (MDR) is a major obstacle in cancer therapy. It results from different mechanisms; among them is P-glycoprotein (P-gp)-mediated drug efflux out of cells. The mechanism of action remains elusive. The membrane lipid surrounding of P-gp, especially cholesterol, has been postulated to play an important role. To determine the effect of cholesterol depletion on P-gp, Madin Darby canine kidney (MDCK) cells, transfected with the mdr1 gene (MDR1-MDCK cells), were treated with methyl-beta-cyclodextrin (MbetaCD). The localization and function of P-gp were analyzed using confocal laser scanning microscopy. Treatment with 100 mM MbetaCD did not affect viability but altered the structural appearance of the cells and abolished efflux of rhodamine 123, a P-gp substrate. The MbetaCD treatment released P-gp from intact cells into the supernatant and reduced the amount of P-gp in total membrane preparations. The P-gp was shifted from the raft fractions (1% Triton X-100, 4 degrees C) to higher density fractions in MbetaCD-treated cells. The amount of cholesterol was significantly decreased in the raft fractions. Treatment of cells with 1-phenyl-2-decanoylamino-3-morpholino-1-propanol, a glucosylceramide synthase inhibitor, also led to a shift of P-gp to higher density fractions. These results show that removal of cholesterol modulates the membrane lipid composition, changes the localization of P-gp, and results in loss of P-gp function.

Quantitation of the tumor-targeting properties of antibody fragments conjugated to cell-permeating HIV-1 TAT peptides.

Human monoclonal antibodies are promising agents for the development of more selective anticancer therapeutics. However, the tumor-targeting efficiency of most anticancer antibodies is severely limited by their poor penetration into the tumor mass. Recent studies have shown that a peptide derived from the HIV TAT protein could improve the distribution of cytoplasmic reporter proteins when administered systemically as fusion proteins or cross-linked chimeras. In this article, we tested by quantitative biodistribtution analysis whether conjugation to TAT peptides could improve the tumor targeting properties of scFv(L19)-Cys: an engineered human antibody fragment specific for the ED-B domain of fibronectin, a marker located in the modified extracellular matrix surrounding tumor neovasculature. Our results show that TAT peptides, consisting either of L-amino acids or D-amino acids, can efficiently transduce target cells when conjugated to fluorophores and/or antibody fragments, suggesting a receptor-independent cell entry mechanism. However, conjugation of scFv(L19)-Cys to TAT peptides resulted in a severely reduced tumor targeting performance compared to the unconjugated antibody, as measured in murine F9 teratocarcinoma-bearing mice, after intravenous injection of the radiolabeled antibody preparations. Our results outline the usefulness of TAT peptides for the efficient in vitro transduction of cells with globular proteins. In particular, the use of TAT peptides composed of D-amino acids may significantly reduce proteolytic degradation. At the same time, the poor biodistribution properties of antibody-TAT conjugates cast doubts over the applicability of this methodology for the delivery of biopharmaceuticals in vivo.