Antifading agents for confocal fluorescence microscopy

Small-molecule photostabilizing agents are modifiers of lipid bilayer properties

Small-molecule photostabilizing or protective agents (PAs) provide essential support for the stability demands on fluorescent dyes in single-molecule spectroscopy and fluorescence microscopy. These agents are employed also in studies of cell membranes and model systems mimicking lipid bilayer environments, but there is little information about their possible effects on membrane structure and physical properties. Given the impact of amphipathic small molecules on bilayer properties such as elasticity and intrinsic curvature, we investigated the effects of six commonly used PAs--cyclooctatetraene (COT), para-nitrobenzyl alcohol (NBA), Trolox (TX), 1,4-diazabicyclo[2.2.2]octane (DABCO), para-nitrobenzoic acid (pNBA), and n-propyl gallate (nPG)--on bilayer properties using a gramicidin A (gA)-based fluorescence quench assay to probe for PA-induced changes in the gramicidin monomer↔dimer equilibrium. The experiments were done using fluorophore-loaded large unilamellar vesicles that had been doped with gA, and changes in the gA monomer↔dimer equilibrium were assayed using a gA channel-permeable fluorescence quencher (Tl⁺). Changes in bilayer properties caused by, e.g., PA adsorption at the bilayer/solution interface that alter the equilibrium constant for gA channel formation, and thus the number of conducting gA channels in the large unilamellar vesicle membrane, will be detectable as changes in the rate of Tl⁺ influx-the fluorescence quench rate. Over the experimentally relevant millimolar concentration range, TX, NBA, and pNBA, caused comparable increases in gA channel activity. COT, also in the millimolar range, caused a slight decrease in gA channel activity. nPG increased channel activity at submillimolar concentrations. DABCO did not alter gA activity. Five of the six tested PAs thus alter lipid bilayer properties at experimentally relevant concentrations, which becomes important for the design and analysis of fluorescence studies in cells and model membrane systems. We therefore tested combinations of COT, NBA, and TX; the combinations altered the fluorescence quench rate less than would be predicted assuming their effects on bilayer properties were additive. The combination of equimolar concentrations of COT and NBA caused minimal changes in the fluorescence quench rate.

An oxygen scavenging system for improvement of dye stability in single-molecule fluorescence experiments

The application of single-molecule fluorescence techniques to complex biological systems places demands on the performance of single fluorophores. We present an enzymatic oxygen scavenging system for improved dye stability in single-molecule experiments. We compared the previously described protocatechuic acid/protocatechuate-3,4-dioxygenase system to the currently employed glucose oxidase/catalase system. Under standardized conditions, we observed lower dissolved oxygen concentrations with the protocatechuic acid/protocatechuate-3,4-dioxygenase system. Furthermore, we observed increased initial lifetimes of single Cy3, Cy5, and Alexa488 fluorophores. We further tested the effects of chemical additives in this system. We found that biological reducing agents increase both the frequency and duration of blinking events of Cy5, an effect that scales with reducing potential. We observed increased stability of Cy3 and Alexa488 in the presence of the antioxidants ascorbic acid and n-propyl gallate. This new O(2)-scavenging system should have wide application for single-molecule fluorescence experiments.

Immunofluorescence and confocal laser scanning microscopy of chronic myeloproliferative disorders on archival formaldehyde-fixed bone marrow

Spatial analysis of the histoarchitecture and photographic documentation at high resolution are the principal advantages of confocal laser scanning microscopy (CLSM) over conventional fluorescence microscopy (CFM) if combined with appropriate software. Restrictions for the use of CFM and CLSM, on the other hand, include nonspecific background fluorescence, fading of photolabile fluorochromes, and both tissue-specific and fixation-induced autofluorescence. Most of those shortcomings can now be avoided. Autofluorescence, the most limiting factor of high-resolution CLSM, was recently controlled also for paraffin sections of archival formaldehyde-fixed tissues. This allowed the present study on cytoskeletal fibers and extracellular matrix proteins in both neoplastic cells of myeloproliferative disorders and in medullary stromal cells using CLSM under proper autofluorescence control. By multiple fluorescence labeling, we found that the intracellular smooth muscle alpha-actin (SMA) fibers and the two extracellular adhesive matrix proteins tenascin and fibronectin vary in their presence in stromal and/or myeloid cells according to the degree of bone marrow fibrosis in chronic myeloproliferative disorders (CMPDs). CLSM offers further insight in our attempts to understand a complex interplay between the two cellular compartments.

Separation of lipid transport functions by mutations in the extracellular domain of scavenger receptor class B, type I

Scavenger receptor class B, type I (SR-BI) shows a variety of effects on cellular cholesterol metabolism, including increased selective uptake of high density lipoprotein (HDL) cholesteryl ester, stimulation of free cholesterol (FC) efflux from cells to HDL and phospholipid vesicles, and changes in the distribution of plasma membrane FC as evidenced by increased susceptibility to exogenous cholesterol oxidase. Previous studies showed that these multiple effects require the extracellular domain of SR-BI, but not the transmembrane and cytoplasmic domains. To test whether 1) the extracellular domain of SR-BI mediates multiple activities by virtue of discrete functional subdomains, or 2) the multiple activities are, in fact, secondary to and driven by changes in cholesterol flux, the extracellular domain of SR-BI was subjected to insertional mutagenesis by strategically placing an epitope tag into nine sites. These experiments identified four classes of mutants with disruptions at different levels of function. Class 4 mutants showed a clear separation of function between HDL binding, HDL cholesteryl ester uptake, and HDL-dependent FC efflux on one hand and FC efflux to small unilamellar vesicles and an increased cholesterol oxidase-sensitive pool of membrane FC on the other. Selective disruption of the latter two functions provides evidence for multiple functional subdomains in the extracellular receptor domain. Furthermore, these findings uncover a difference in the SR-BI-mediated efflux pathways for FC transfer to HDL acceptors versus phospholipid vesicles. The loss of the cholesterol oxidase-sensitive FC pool and FC efflux to small unilamellar vesicle acceptors in Class 4 mutants suggests that these activities may be mechanistically related.

Identification of protein disulfide isomerase as an endothelial hypoxic stress protein

Endothelial cells (EC) exposed to hypoxia upregulate a unique set of five stress proteins. These proteins are upregulated in human and bovine aortic and pulmonary artery EC and are distinct from heat shock or glucose-regulated proteins. We previously identified two of these proteins as the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase and enolase and postulated that the remaining proteins were also glycolytic enzymes. Using SDS-PAGE, tryptic digestion, and NH(2)-terminal amino acid sequencing, we report here the identification of the 56-kDa protein as protein disulfide isomerase (PDI). PDI is upregulated by hypoxia at the mRNA level and follows a time course similar to that of the protein, with maximal upregulation detected after exposure to 18 h of 0% O(2). Neither smooth muscle cells nor fibroblasts upregulate PDI to the same extent as EC, which correlates with their decreased hypoxia tolerance. Upregulation of PDI specifically in EC may contribute to their ability to tolerate hypoxia and may occur through PDI's functions as a prolyl hydroxylase subunit, protein folding catalyst, or molecular chaperone.