In the last decade, the long-standing biologist's dream of seeing the molecular events within the living cell came true. This technological achievement is largely due to the development of fluorescence microscopy technologies and the advent of green fluorescent protein as a fluorescent probe. Such imaging technologies allowed us to determine the subcellular localization, mobility and transport pathways of specific proteins and even visualize protein-protein interactions of single molecules in living cells. Direct observation of such molecular dynamics can provide important information about cellular events that cannot be obtained by other methods. Thus, imaging of protein dynamics in living cells becomes an important tool for cell biology to study molecular and cellular functions. In this special issue of review articles, we review various imaging technologies of microscope hardware and fluorescent probes useful for cell biologists, with a focus on recent development of live cell imaging.

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The use of fluorescence imaging methods, most recently based on fluorescent protein technology, and the availability of high quality fluorescence imaging systems have driven a revolution in cell and molecular biology. Live cell imaging, especially using fluorescence, is now used in a wide variety of assays in academic and commercial laboratories. The use of this technology requires particular attention to be paid to cell engineering, the design of the image acquisition system, the imaging protocol, and subsequent processing and analytic methods. In this review, we discuss each of these steps, highlighting practical techniques developed by us and others.

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Green fluorescent protein (GFP) from the bioluminescent jellyfish Aequorea victoria has become an important tool in molecular and cellular biology as a transcriptional reporter, fusion tag, and biosensor. Most significantly, it encodes a chromophore intrinsically within its protein sequence, obviating the need for external substrates or cofactors and enabling the genetic encoding of strong fluorescence. Mutagenesis studies have generated GFP variants with new colors, improved fluorescence and other biochemical properties. In parallel, GFPs and GFP-like molecules have been cloned from other organisms, including the bioluminescent sea pansy Renilla reniformis and other non-bioluminescent Anthozoa animals. In the jellyfish and sea pansy, the GFPs are coupled to their chemoluminescence. Instead of emitting the blue light generated by aequorin and luciferase, the GFPs absorb their energy of primary emission and emit green light, which travels farther in the sea. In contrast, GFP-like proteins in reef Anthozoa are thought to play a role in photoprotection of their symbiotic zooxanthellae in shallow water; they transform absorbed UV radiation contained in sunlight into longer fluorescence wavelengths (Salih, A., Larkum, A., Cox, G., Kuhl, M., and Hoegh-Guldberg, O. 2000. Nature, 408: 850-853). In this review, I will describe both the biological and practical aspects of Anthozoan GFP-like proteins, many of which will be greatly improved in utility and commercially available before long. The ubiquity of these molecular tools makes it important to appreciate the interplay between sunlight and GFP-like proteins of Anthozoan animals, and to consider the optimal use of these unique proteins in biological studies.

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Confocal laser scanning microscopy is experiencing a revolution in speed from the world of seconds to that of milliseconds. The spinning Nipkow disk method with microlenses has made this remarkable innovation possible. In combination with the ultrahigh-sensitivity, high-speed and high-resolution camera system based on avalanche multiplication of photoconduction, we are now able to observe the extremely dynamic movement of small vesicles in living cells in real time.

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Marvelous background rejection in total internal reflection fluorescence microscopy (TIR-FM) has made it possible to visualize single-fluorophores in living cells. Cell signaling proteins including peptide hormones, membrane receptors, small G proteins, cytoplasmic kinases as well as small signaling compounds have been conjugated with single chemical fluorophore or tagged with green fluorescent proteins and visualized in living cells. In this review, the reasons why single-molecule analysis is essential for studies of intracellular protein systems such as cell signaling system are discussed, the instrumentation of TIR-FM for single-molecule imaging in living cells is explained, and how single molecule visualization has been used in cell biology is illustrated by way of two examples: signaling of epidermal growth factor in mammalian cells and chemotaxis of Dictyostelium amoeba along a cAMP gradient. Single-molecule analysis is an ideal method to quantify the parameters of reaction dynamics and kinetics of unitary processes within intracellular protein systems. Knowledge of these parameters is crucial for the understanding of the molecular mechanisms underlying intracellular events, thus single-molecule imaging in living cells will be one of the major technologies in cellular nanobiology.

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Multispectral imaging technologies have been widely used in fields of astronomy and remote sensing. Interdisciplinary approaches developed in, for example, the National Aeronautics and Space Administration (NASA, USA), the Jet Propulsion Laboratory (JPL, USA), or the Communications Research Laboratory (CRL, Japan) have extended the application areas of these technologies from planetary systems to cellular systems. Here we overview multispectral imaging systems that have been devised for microscope applications. We introduce these systems with particular interest in live cell imaging. Finally we demonstrate examples of spectral imaging of living cells using commercially available systems with no need for user engineering.

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Caveolin, a 20-24 kDa integral membrane protein, is a principal component of caveolar domains. Caveolin-1 is expressed predominantly in endothelial cells, fibroblasts, and adipocytes, while the expression of caveolin-3 is confined to muscle cells. However, their localization in various muscles has not been well documented. Using double-immunofluorescence labeling and confocal laser microscopy, we examined the localization of caveolins-1 and 3 in adult monkey skeletal, cardiac and uterine smooth muscles and the co-immunolocalization of these caveolins with dystrophin, which is a product of the Duchenne muscular dystrophy gene. In the skeletal muscle tissue, caveolin-3 was localized along the sarcolemma except for the transverse tubules, and co-immunolocalized with dystrophin, whereas caveolin-1 was absent except in the blood vessels of the muscle tissue. In cardiac muscle cells, caveolins-1 and -3 and dystrophin were co-immunolocalized on the sarcolemma and transverse tubules. In uterine smooth muscle cells, caveolin-1, but not caveolin-3, was co-immunolocalized with dystrophin on the sarcolemma.

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Arp2/3 protein complex consists of seven subunits (Arp2, Arp3, p41-Arc, p34-Arc, p21-Arc, p20-Arc and p16-Arc) in apparent 1:1 stoichiometry. This complex has been shown to promote the formation of Y-branch structures of F-actin in cultured cells. We generated specific antibodies against chicken Arp2, Arp3, and p34-Arc to analyze the distribution of these subunits in chicken tissues.In whole samples of brain and gizzard, antibodies against each recombinant protein reacted with single bands of predicted molecular mass based on their cDNA sequences of the antigens. Anti-p34-Arc antibody detected at least two neighboring spots in 2D-PAGE, which might suggest the existence of isoforms or modified forms. Arp2/3 complex bound to an F-actin affinity column from gizzard extract. However, Arp2/3 complex did not tightly bind major actin cytoskeleton because the complex was extracted easily when gizzard smooth muscle was homogenized in PBS. Immunoblot analysis of various tissues revealed that the amounts of Arp2/3 subunits were lower in striated muscle than in non-muscle and smooth muscle tissues. Amounts and ratio of the three subunits varied in tissues, as estimated by quantitative immunoblotting. With immunofluorescence microscopy, we also observed localization of Arp3 and p34-Arc in frozen sections of gizzard with different staining patterns around blood vessels. These results suggest that the Arp2/3 complex exists also in places where rapid actin polymerization does not occur, and that a part of the subunits may exist in different forms from the complex containing the seven subunits in some tissues.

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During the continuous culturing of neural PC12 cells, a drug hypersensitive PC12 mutant cell line (PC12m3) was obtained, which demonstrated high neurite outgrowth when stimulated by various drugs. When the immunosuppressant drug FK506 and nerve growth factor (NGF) were introduced to the PC12m3 cells, the frequency of neurite outgrowth increased approximately 40-fold for NGF alone. However, the effect of FK506 on neuritogenesis in PC12 parental and drug insensitive PC12m1 mutant cells was much lower than in PC12m3 cells. The sustained activation of mitogen-activated protein (MAP) kinase plays an important role in neurite outgrowth of PC12 cells. Interestingly, the drug hypersensitive PC12m3 cells exhibited the sustained activation of MAP kinase with FK506 in comparison to low or no activities in PC12 parental or drug insensitive PC12m1 cells. These results indicate that PC12m3 cells have a novel FK506-induced MAP kinase pathway for neuritogenesis.

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