Electron microscopical enzyme histochemistry on unfixed tissues and cells. Bridging the gap between LM and EM enzyme histochemistry

Imaging enzymes at work: metabolic mapping by enzyme histochemistry

For the understanding of functions of proteins in biological and pathological processes, reporter molecules such as fluorescent proteins have become indispensable tools for visualizing the location of these proteins in intact animals, tissues, and cells. For enzymes, imaging their activity also provides information on their function or functions, which does not necessarily correlate with their location. Metabolic mapping enables imaging of activity of enzymes. The enzyme under study forms a reaction product that is fluorescent or colored by conversion of either a fluorogenic or chromogenic substrate or a fluorescent substrate with different spectral characteristics. Most chromogenic staining methods were developed in the latter half of the twentieth century but still find new applications in modern cell biology and pathology. Fluorescence methods have rapidly evolved during the last decade. This review critically evaluates the methods that are available at present for metabolic mapping in living animals, unfixed cryostat sections of tissues, and living cells, and refers to protocols of the methods of choice.

Rapid combined light and electron microscopy on large frozen biological samples

The use of large unfixed frozen tissue samples (10 x 10 x 5 mm(3)) for combined light microscopy (LM) and electron microscopy (EM) is described. First, cryostat sections are applied for various LM histochemical approaches including in situ hybridization, immunohistochemistry and metabolic mapping (enzyme histochemistry). When EM inspection is needed, the tissue blocks that were used for cryostat sectioning and are stored at -80 degrees C, are then fixed at 4 degrees C with glutaraldehyde/paraformaldehyde and prepared for EM according to standard procedures. Ultrastructurally, most morphological aspects of normal and pathological tissue are retained whereas cryostat sectioning at -25 degrees C does not have serious damaging effects on the ultrastructure. This approach allows simple and rapid combined LM and EM of relatively large tissue specimens with acceptable ultrastructure. Its use is demonstrated with the elucidation of transdifferentiated mouse stromal elements in human pancreatic adenocarcinoma explants grown subcutaneously in nude mice. Combined LM and EM analysis revealed that these elements resemble cartilage showing enchondral mineralization and aberrant muscle fibres with characteristics of skeletal muscle cells.

NADPH production by the pentose phosphate pathway in the zona fasciculata of rat adrenal gland

Biosynthesis of steroid hormones in the cortex of the adrenal gland takes place in smooth endoplasmic reticulum and mitochondria and requires NADPH. Four enzymes produce NADPH: glucose-6-phosphate dehydrogenase (G6PD), the key regulatory enzyme of the pentose phosphate pathway, phosphogluconate dehydrogenase (PGD), the third enzyme of that pathway, malate dehydrogenase (MDH), and isocitrate dehydrogenase (ICDH). However, the contribution of each enzyme to NADPH production in the cortex of adrenal gland has not been established. Therefore, activity of G6PD, PGD, MDH, and ICDH was localized and quantified in rat adrenocortical tissue using metabolic mapping, image analysis, and electron microscopy. The four enzymes have similar localization patterns in adrenal gland with highest activities in the zona fasciculata of the cortex. G6PD activity was strongest, PGD, MDH, and ICDH activity was approximately 60%, 15%, and 7% of G6PD activity, respectively. The K(m) value of G6PD for glucose-6-phosphate was two times higher than the K(m) value of PGD for phosphogluconate. As a consequence, virtual flux rates through G6PD and PGD are largely similar. It is concluded that G6PD and PGD provide the major part of NADPH in adrenocortical cells. Their activity is localized in the cytoplasm associated with free ribosomes and membranes of the smooth endoplasmic reticulum, indicating that NADPH-demanding processes related to biosynthesis of steroid hormones take place at these sites. Complete inhibition of G6PD by androsterones suggests that there is feedback regulation of steroid hormone biosynthesis via G6PD.

Improved localization of glucose-6-phosphate dehydrogenase activity in cells with 5-cyano-2,3-ditolyl-tetrazolium chloride as fluorescent redox dye reveals its cell cycle-dependent regulation

Since the introduction of cyano-ditolyl-tetrazolium chloride (CTC), a tetrazolium salt that gives rise to a fluorescent formazan after reduction, it has been applied to quantify activity of dehydrogenases in individual cells using flow cytometry. Confocal laser scanning microscopy (CLSM) showed that the fluorescent formazan was exclusively localized at the surface of individual cells and not at intracellular sites of enzyme activity. In the present study, the technique has been optimized to localize activity of glucose-6-phosphate dehydrogenase (G6PD) intracellularly in individual cells. Activity was demonstrated in cultured fibrosarcoma cells in different stages of the cell cycle. Cells were incubated for the detection of G6PD activity using a medium containing 6% (w/v) polyvinyl alcohol, 5 mM CTC, magnesium chloride, sodium azide, the electron carrier methoxyphenazine methosulphate, NADP, and glucose-6-phosphate. Before incubation, cells were permeabilized with 0.025% glutaraldehyde. Fluorescent formazan was localized exclusively in the cytoplasm of fibrosarcoma cells. The amount of fluorescent formazan in cells increased linearly with incubation time when measured with flow cytometry and CLSM. When combining the Hoechst staining for DNA with the CTC method for the demonstration of G6PD activity, flow cytometry showed that G6PD activity of cells in S phase and G2/M phase is 27 +/- 4% and 43 +/- 4% higher, respectively, than that of cells in G1 phase. CLSM revealed that cells in all phases of mitosis as well as during apoptosis contained considerably lower G6PD activity than cells in interphase. It is concluded that posttranslational regulation of G6PD is responsible for this cell cycle-dependent activity.

Metabolic mapping of proteinase activity with emphasis on in situ zymography of gelatinases: review and protocols

Proteases are essential for protein catabolism, regulation of a wide range of biological processes, and in the pathogenesis of many diseases. Several techniques are available to localize activity of proteases in tissue sections or cell preparations. For localization of the activity of matrix metalloproteinases, in situ zymography was introduced some decades ago. The procedure is based on zymography using SDS polyacrylamide gels containing gelatin, casein, or fibrin as substrate. For in situ zymography, either a photographic emulsion containing gelatin or a fluorescence-labeled proteinaceous macromolecular substrate is brought into contact with a tissue section or cell preparation. After incubation, enzymatic activity is revealed as white spots in a dark background or as black spots in a fluorescent background. However, this approach does not allow precise localization of proteinase activity because of limited sensitivity. A major improvement in sensitivity was achieved with the introduction of dye-quenched (DQ-)gelatin, which is gelatin that is heavily labeled with FITC molecules so that its fluorescence is quenched. After cleavage of DQ-gelatin by gelatinolytic activity, fluorescent peptides are produced that are visible against a weakly fluorescent background. The incubation with DQ-gelatin can be combined with simultaneous immunohistochemical detection of a protein on the same section. To draw valid conclusions from the findings with in situ zymography, specific inhibitors need to be used and the technique has to be combined with immunohistochemistry and zymography. In that case, in situ zymography provides data that extend our understanding of the role of specific proteinases in various physiological and pathological conditions.