Dynamics of organelles in the mitotic spindles of living cells: membrane and microtubule interactions

Sculpting nuclear envelope identity from the endoplasmic reticulum during the cell cycle

The nuclear envelope (NE) regulates nuclear functions, including transcription, nucleocytoplasmic transport, and protein quality control. While the outer membrane of the NE is directly continuous with the endoplasmic reticulum (ER), the NE has an overall distinct protein composition from the ER, which is crucial for its functions. During open mitosis in higher eukaryotes, the NE disassembles during mitotic entry and then reforms as a functional territory at the end of mitosis to reestablish nucleocytoplasmic compartmentalization. In this review, we examine the known mechanisms by which the functional NE reconstitutes from the mitotic ER in the continuous ER-NE endomembrane system during open mitosis. Furthermore, based on recent findings indicating that the NE possesses unique lipid metabolism and quality control mechanisms distinct from those of the ER, we explore the maintenance of NE identity and homeostasis during interphase. We also highlight the potential significance of membrane junctions between the ER and NE.

A detection method for neuronal death indicates abnormalities in intracellular membranous components in neuronal cells that underwent delayed death

Acute neuronal degeneration is always preceded under the light and electron microscopes by a stage called microvacuolation, which is characterized by a finely vacuolar alteration in the cytoplasm of the neurons destined to death. In this study, we reported a method for detecting neuronal death using two membrane-bound dyes, rhodamine R6 and DiOC₆(3), which may be associated with the so-called microvacuolation. This new method produced a spatiotemporally similar staining pattern to Fluoro-Jade B in kainic acid-damaged brains in mice. Further experiments showed that increased staining of rhodamine R6 and DiOC₆(3) was observed only in degenerated neurons, but not in glia, erythrocytes, or meninges. Different from Fluoro-Jade-related dyes, rhodamine R6 and DiOC₆(3) staining is highly sensitive to solvent extraction and detergent exposure. Staining with Nile red for phospholipids and filipin III for non-esterified cholesterol supports that the increased staining of rhodamine R6 and DiOC₆(3) might be associated with increased levels of phospholipids and free cholesterol in the perinuclear cytoplasm of damaged neurons. In addition to kainic acid-injected neuronal death, rhodamine R6 and DiOC₆(3) were similarly useful for detecting neuronal death in ischemic models either in vivo or in vitro. As far as we know, the staining with rhodamine R6 or DiOC₆(3) is one of a few histochemical methods for detecting neuronal death whose target molecules have been well defined and therefore may be useful for explaining experimental results as well as exploring the mechanisms of neuronal death. (250 words).

A membrane reticulum, the centriculum, affects centrosome size and function in Caenorhabditis elegans

Centrosomes are cellular structures that nucleate microtubules. At their core is a pair of centrioles that recruit pericentriolar material (PCM). Although centrosomes are considered membraneless organelles, in many cell types, including human cells, centrosomes are surrounded by endoplasmic reticulum (ER)-derived membranes of unknown structure and function. Using volume electron microscopy (vEM), we show that centrosomes in the Caenorhabditis elegans (C. elegans) early embryo are surrounded by a three-dimensional (3D), ER-derived membrane reticulum that we call the centriculum, for centrosome-associated membrane reticulum. The centriculum is adjacent to the nuclear envelope in interphase and early mitosis and fuses with the fenestrated nuclear membrane at metaphase. Centriculum formation is dependent on the presence of an underlying centrosome and on microtubules. Conversely, increasing centriculum size by genetic means led to the expansion of the PCM, increased microtubule nucleation capacity, and altered spindle width. The effect of the centriculum on centrosome function suggests that in the C. elegans early embryo, the centrosome is not membraneless. Rather, it is encased in a membrane meshwork that affects its properties. We provide evidence that the centriculum serves as a microtubule "filter," preventing the elongation of a subset of microtubules past the centriculum. Finally, we propose that the fusion between the centriculum and the nuclear membrane contributes to nuclear envelope breakdown by coupling spindle elongation to nuclear membrane fenestration.

Microtubule cytoskeleton: The centrosome gains a membrane

Identification of a membrane structure, termed the 'centriculum', in Caenorhabditis elegans embryos challenges the textbook view of the centrosome - a major microtubule organizing center in animal cells - as an organelle that lacks a surrounding membrane.

C. elegans pronuclei fuse after fertilization through a novel membrane structure

After fertilization, parental genomes are enclosed in two separate pronuclei. In Caenorhabditis elegans, and possibly other organisms, when the two pronuclei first meet, the parental genomes are separated by four pronuclear membranes. To understand how these membranes are breached to allow merging of parental genomes we used focused ion beam scanning electron microscopy (FIB-SEM) to study the architecture of the pronuclear membranes at nanometer-scale resolution. We find that at metaphase, the interface between the two pronuclei is composed of two membranes perforated by fenestrations ranging from tens of nanometers to several microns in diameter. The parental chromosomes come in contact through one of the large fenestrations. Surrounding this fenestrated, two-membrane region is a novel membrane structure, a three-way sheet junction, where the four membranes of the two pronuclei fuse and become two. In the plk-1 mutant, where parental genomes fail to merge, these junctions are absent, suggesting that three-way sheet junctions are needed for formation of a diploid genome.

Between life and death: strategies to reduce phototoxicity in super-resolution microscopy

Super-resolution microscopy (SRM) enables non-invasive, molecule-specific imaging of the internal structure and dynamics of cells with sub-diffraction limit spatial resolution. One of its major limitations is the requirement for high-intensity illumination, generating considerable cellular phototoxicity. This factor considerably limits the capacity for live-cell observations, particularly for extended periods of time. Here, we give an overview of new developments in hardware, software and probe chemistry aiming to reduce phototoxicity. Additionally, we discuss how the choice of biological model and sample environment impacts the capacity for live-cell observations.

A novel, dynein-independent mechanism focuses the endoplasmic reticulum around spindle poles in dividing Drosophila spermatocytes

In dividing animal cells the endoplasmic reticulum (ER) concentrates around the poles of the spindle apparatus by associating with astral microtubules (MTs), and this association is essential for proper ER partitioning to progeny cells. The mechanisms that associate the ER with astral MTs are unknown. Because astral MT minus-ends are anchored by centrosomes at spindle poles, we hypothesized that the MT minus-end motor dynein mediates ER concentration around spindle poles. Live in vivo imaging of Drosophila spermatocytes revealed that dynein is required for ER concentration around centrosomes during late interphase. In marked contrast, however, dynein suppression had no effect on ER association with astral MTs and concentration around spindle poles in early M-phase. In fact, there was a sudden onset of ER association with astral MTs in dynein RNAi cells, revealing activation of an M-phase specific mechanism of ER-MT association. ER redistribution to spindle poles also did not require non-claret disjunctional (ncd), the other known Drosophila MT minus-end motor, nor Klp61F, a MT plus-end motor that generates spindle poleward forces. Collectively, our results suggest that a novel, M-phase specific mechanism of ER-MT association that is independent of MT minus-end motors is required for proper ER partitioning in dividing cells.

Ultrastructural analysis of mitotic Drosophila S2 cells identifies distinctive microtubule and intracellular membrane behaviors

BACKGROUND

S2 cells are one of the most widely used Drosophila melanogaster cell lines. A series of studies has shown that they are particularly suitable for RNAi-based screens aimed at the dissection of cellular pathways, including those controlling cell shape and motility, cell metabolism, and host-pathogen interactions. In addition, RNAi in S2 cells has been successfully used to identify many new mitotic genes that are conserved in the higher eukaryotes, and for the analysis of several aspects of the mitotic process. However, no detailed and complete description of S2 cell mitosis at the ultrastructural level has been done. Here, we provide a detailed characterization of all phases of S2 cell mitosis visualized by transmission electron microscopy (TEM).

RESULTS

We analyzed by TEM a random sample of 144 cells undergoing mitosis, focusing on intracellular membrane and microtubule (MT) behaviors. This unbiased approach provided a comprehensive ultrastructural view of the dividing cells, and allowed us to discover that S2 cells exhibit a previously uncharacterized behavior of intracellular membranes, involving the formation of a quadruple nuclear membrane in early prometaphase and its disassembly during late prometaphase. After nuclear envelope disassembly, the mitotic apparatus becomes encased by a discontinuous network of endoplasmic reticulum membranes, which associate with mitochondria, presumably to prevent their diffusion into the spindle area. We also observed a peculiar metaphase spindle organization. We found that kinetochores with attached k-fibers are almost invariably associated with lateral MT bundles that can be either interpolar bundles or k-fibers connected to a different kinetochore. This spindle organization is likely to favor chromosome alignment at metaphase and subsequent segregation during anaphase.

CONCLUSIONS

We discovered several previously unknown features of membrane and MT organization during S2 cell mitosis. The genetic determinants of these mitotic features can now be investigated, for instance by using an RNAi-based approach, which is particularly easy and efficient in S2 cells.

Proper symmetric and asymmetric endoplasmic reticulum partitioning requires astral microtubules

Mechanisms that regulate partitioning of the endoplasmic reticulum (ER) during cell division are largely unknown. Previous studies have mostly addressed ER partitioning in cultured cells, which may not recapitulate physiological processes that are critical in developing, intact tissues. We have addressed this by analysing ER partitioning in asymmetrically dividing stem cells, in which precise segregation of cellular components is essential for proper development and tissue architecture. We show that in Drosophila neural stem cells, called neuroblasts, the ER asymmetrically partitioned to centrosomes early in mitosis. This correlated closely with the asymmetric nucleation of astral microtubules (MTs) by centrosomes, suggesting that astral MT association may be required for ER partitioning by centrosomes. Consistent with this, the ER also associated with astral MTs in meiotic Drosophila spermatocytes and during syncytial embryonic divisions. Disruption of centrosomes in each of these cell types led to improper ER partitioning, demonstrating the critical role for centrosomes and associated astral MTs in this process. Importantly, we show that the ER also associated with astral MTs in cultured human cells, suggesting that this centrosome/astral MT-based partitioning mechanism is conserved across animal species.

Organelle asymmetry for proper fitness, function, and fate

During cellular division, centrosomes are tasked with building the bipolar mitotic spindle, which partitions the cellular contents into two daughter cells. While every cell will receive an equal complement of chromosomes, not every organelle is symmetrically passaged to the two progeny in many cell types. In this review, we highlight the conservation of nonrandom centrosome segregation in asymmetrically dividing stem cells, and we discuss how the asymmetric function of centrosomes could mediate nonrandom segregation of organelles and mRNA. We propose that such a mechanism is critical for insuring proper cell fitness, function, and fate.

Protein adaptation: mitotic functions for membrane trafficking proteins

Membrane trafficking and mitosis are two essential processes in eukaryotic cells. Surprisingly, many proteins best known for their role in membrane trafficking have additional 'moonlighting' functions in mitosis. Despite having proteins in common, there is insufficient evidence for a specific connection between these two processes. Instead, these phenomena demonstrate the adaptability of the membrane trafficking machinery that allows its repurposing for different cellular functions.

The role of clathrin in mitotic spindle organisation

Clathrin, a protein best known for its role in membrane trafficking, has been recognised for many years as localising to the spindle apparatus during mitosis, but its function at the spindle remained unclear. Recent work has better defined the role of clathrin in the function of the mitotic spindle and proposed that clathrin crosslinks the microtubules (MTs) comprising the kinetochore fibres (K-fibres) in the mitotic spindle. This mitotic function is unrelated to the role of clathrin in membrane trafficking and occurs in partnership with two other spindle proteins: transforming acidic coiled-coil protein 3 (TACC3) and colonic hepatic tumour overexpressed gene (ch-TOG; also known as cytoskeleton-associated protein 5, CKAP5). This review summarises the role of clathrin in mitotic spindle organisation with an emphasis on the recent discovery of the TACC3-ch-TOG-clathrin complex.

A high-resolution multimode digital microscope system

This chapter describes the development of a high-resolution, multimode digital imaging system based on a wide-field epifluorescent and transmitted light microscope, and a cooled charge-coupled device (CCD) camera. The three main parts of this imaging system are Nikon FXA microscope, Hamamatsu C4880 cooled CCD camera, and MetaMorph digital imaging system. This chapter presents various design criteria for the instrument and describes the major features of the microscope components-the cooled CCD camera and the MetaMorph digital imaging system. The Nikon FXA upright microscope can produce high resolution images for both epifluorescent and transmitted light illumination without switching the objective or moving the specimen. The functional aspects of the microscope set-up can be considered in terms of the imaging optics, the epi-illumination optics, the transillumination optics, the focus control, and the vibration isolation table. This instrument is somewhat specialized for microtubule and mitosis studies, and it is also applicable to a variety of problems in cellular imaging, including tracking proteins fused to the green fluorescent protein in live cells. The instrument is also valuable for correlating the assembly dynamics of individual cytoplasmic microtubules (labeled by conjugating X-rhodamine to tubulin) with the dynamics of membranes of the endoplasmic reticulum (labeled with DiOC6) and the dynamics of the cell cortex (by differential interference contrast) in migrating vertebrate epithelial cells. This imaging system also plays an important role in the analysis of mitotic mutants in the powerful yeast genetic system Saccharomyces cerevisiae.

Division of the intermediate compartment at the onset of mitosis provides a mechanism for Golgi inheritance

As mammalian cells prepare for mitosis, the Golgi ribbon is first unlinked into its constituent stacks and then transformed into spindle-associated, pleiomorphic membrane clusters in a process that remains enigmatic. Also, it remains unclear whether Golgi inheritance involves the incorporation of Golgi enzymes into a pool of coat protein I (COPI) vesicles, or their COPI-independent transfer to the endoplasmic reticulum (ER). Based on the observation that the intermediate compartment (IC) at the ER-Golgi boundary is connected to the centrosome, we examined its mitotic fate and possible role in Golgi breakdown. The use of multiple imaging techniques and markers revealed that the IC elements persist during the M phase, maintain their compositional and structural properties and remain associated with the mitotic spindle, forming circular arrays at the spindle poles. At G2/M transition, the movement of the pericentrosomal domain of the IC (pcIC) to the cell centre and its expansion coincide with the unlinking of the Golgi ribbon. At prophase, coupled to centrosome separation, the pcIC divides together with recycling endosomes, providing novel landmarks for mitotic entry. We provide evidence that the permanent IC elements function as way stations during the COPI-dependent dispersal of Golgi components at prometa- and metaphase, indicating that they correspond to the previously described Golgi clusters. In addition, they continue to communicate with the vesicular 'Golgi haze' and thus are likely to provide templates for Golgi reassembly. These results implicate the IC in mitotic Golgi inheritance, resulting in a model that integrates key features of the two previously proposed pathways.

Phosphoregulation of STIM1 leads to exclusion of the endoplasmic reticulum from the mitotic spindle

The endoplasmic reticulum (ER) undergoes significant reorganization between interphase and mitosis, but the underlying mechanisms are unknown. Stromal interaction molecule 1 (STIM1) is an ER Ca(2+) sensor that activates store-operated Ca(2+) entry (SOCE) and also functions in ER morphogenesis through its interaction with the microtubule +TIP protein end binding 1 (EB1). We previously demonstrated that phosphorylation of STIM1 during mitosis suppresses SOCE. We now show that STIM1 phosphorylation is a major regulatory mechanism that excludes ER from the mitotic spindle. In mitotic HeLa cells, the ER forms concentric sheets largely excluded from the mitotic spindle. We show that STIM1 dissociates from EB1 in mitosis and localizes to the concentric ER sheets. However, a nonphosphorylatable STIM1 mutant (STIM1(10A)) colocalized extensively with EB1 and drove ER mislocalization by pulling ER tubules into the spindle. This effect was rescued by mutating the EB1 interaction site of STIM1(10A), demonstrating that aberrant association of STIM1(10A) with EB1 is responsible for the ER mislocalization. A STIM1 phosphomimetic exhibited significantly impaired +TIP tracking in interphase but was ineffective at inhibiting SOCE, suggesting different mechanisms of regulation of these two STIM1 functions by phosphorylation. Thus, ER spindle exclusion and ER-dependent Ca(2+) signaling during mitosis require multimodal STIM1 regulation by phosphorylation.

Altering membrane topology with Sar1 does not impair spindle assembly in Xenopus egg extracts

Intracellular membrane networks including the endoplasmic reticulum (ER) and the Golgi apparatus experience dramatic reorganization upon entry into mitosis. However, the mechanisms driving these rearrangements and their importance for cell division are poorly understood. The GTPase Sar1 is a component of the secretory pathway and a key activator of anterograde transport of cargo from the ER to the Golgi. Here we show that Sar1 mutant proteins added to metaphase-arrested Xenopus laevis egg extracts cause dramatic effects on membrane organization. Live analysis of membrane structures in egg extract cytoplasm revealed a distinct network of sheets and tubules reflective of the organization of the ER in other systems. Addition of a constitutively active Sar1 GTPase mutant (H79G) increased membrane tubulation, while a dominant negative version Sar1 (T39N) impaired tubule organization. Although microtubule pelleting assays revealed that Sar1 associates with microtubules in the egg extract, and addition of Sar1 (H79G) mutant slightly destabilized spindle poles, bipolar spindle assembly was largely unaffected. Thus, spindles are stable to dramatic changes in mitotic membrane organization at metaphase, suggesting that mitotic membrane is not an upstream regulator of the mitotic spindle apparatus in Xenopus egg extracts.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Basic confocal microscopy

This unit introduces the reader to the basic principles of confocal microscopy and the design and capabilities of current confocal microscopes. The advantages and disadvantages of confocal microscopy compared to other techniques for fluorescence imaging are described. There are also practical guidelines for sample preparation and optimization of imaging parameters, as well as examples of some of the applications of confocal microscopy.

Mitotic moonlighting functions for membrane trafficking proteins

In recent years, cell biologists have uncovered a number of new functions for proteins that were previously thought to operate solely in membrane trafficking. These alternative roles, termed moonlighting functions, can occur at distinct intracellular sites or at different stages of the cell cycle. Here, I evaluate the evidence for mitotic moonlighting functions of proteins that have membrane trafficking roles during interphase. The aim is to identify key issues facing the field and to outline important questions for future work.

Cell Cycle-Dependent Localization of Voltage-Dependent Calcium Channels and the Mitotic Apparatus in a Neuroendocrine Cell Line(AtT-20)

Changes in intracellular calcium are necessary for the successful progression of mitosis in many cells. Both elevation and reduction in intracellular calcium can disrupt mitosis by mechanisms that remain ill defined. In this study we explore the role of transmembrane voltage-gated calcium channels (CaV channels) as regulators of mitosis in the mouse corticotroph cell line (AtT-20). We report that the nifedipine-sensitive isoform CaV1.2 is localized to the "poleward side" of kinetechores during metaphase and at the midbody during cytokinesis. A second nifedipine-sensitive isoform, CaV1.3, is present at the mid-spindle zone in telophase, but is also seen at the midbody. Nifedipine reduces the rate of cell proliferation, and, utilizing time-lapse microscopy, we show that this is due to a block at the prometaphase stage of the cell cycle. Using Fluo-4 we detect calcium fluxes at sites corresponding to the mid-spindle zone and the midbody region. Another calcium dye, Fura PE3/AM, causes an inhibition of mitosis prior to anaphase that we attribute to a chelation of intracellular calcium. Our results demonstrate a novel, isoform-specific localization of CaV1 channels during cell division and suggest a possible role for these channels in the calcium-dependent events underlying mitotic progression in pituitary corticotrophs.

A requirement for epsin in mitotic membrane and spindle organization

Eukaryotic cells possess a sophisticated membrane system to facilitate diverse functions. Whereas much is known about the nature of membrane systems in interphase, the organization and function of the mitotic membrane system are less well understood. In this study, we show that epsin, an endocytic adapter protein, regulates mitotic membrane morphology and spindle integrity in HeLa cells. Using epsin that harbors point mutations in the epsin NH2-terminal homology domain and spindle assembly assays in Xenopus laevis egg extracts, we show that epsin-induced membrane curvature is required for proper spindle morphogenesis, independent of its function in endocytosis during interphase. Although several other membrane-interacting proteins, including clathrin, AP2, autosomal recessive hypercholesterolemia, and GRASP65, are implicated in the regulation of mitosis, whether they participate through regulation of membrane organization is unclear. Our study of epsin provides evidence that mitotic membrane organization influences spindle integrity.

Cisternal organization of the endoplasmic reticulum during mitosis

The endoplasmic reticulum (ER) of animal cells is a single, dynamic, and continuous membrane network of interconnected cisternae and tubules spread out throughout the cytosol in direct contact with the nuclear envelope. During mitosis, the nuclear envelope undergoes a major rearrangement, as it rapidly partitions its membrane-bound contents into the ER. It is therefore of great interest to determine whether any major transformation in the architecture of the ER also occurs during cell division. We present structural evidence, from rapid, live-cell, three-dimensional imaging with confirmation from high-resolution electron microscopy tomography of samples preserved by high-pressure freezing and freeze substitution, unambiguously showing that from prometaphase to telophase of mammalian cells, most of the ER is organized as extended cisternae, with a very small fraction remaining organized as tubules. In contrast, during interphase, the ER displays the familiar reticular network of convolved cisternae linked to tubules.

Myofibril assembly visualized by imaging N-RAP, alpha-actinin, and actin in living cardiomyocytes

N-RAP is a striated muscle-specific scaffolding protein that organizes alpha-actinin and actin into symmetrical I-Z-I structures in developing myofibrils. Here we determined the order of events during myofibril assembly through time-lapse confocal microscopy of cultured embryonic chick cardiomyocytes coexpressing fluorescently tagged N-RAP and either alpha-actinin or actin. During de novo myofibril assembly, N-RAP assembled in fibrillar structures within the cell, with dots of alpha-actinin subsequently organizing along these structures. The initial fibrillar structures were reminiscent of actin fibrils, and coassembly of N-RAP and actin into newly formed fibrils supported this. The alpha-actinin dots subsequently broadened to Z-lines that were wider than the underlying N-RAP fibril, and N-RAP fluorescence intensity decreased. FRAP experiments showed that most of the alpha-actinin dynamically exchanged during all stages of myofibril assembly. In contrast, less than 20% of the N-RAP in premyofibrils was exchanged during 10-20 min after photobleaching, but this value increased to 70% during myofibril maturation. The results show that N-RAP assembles into an actin containing scaffold before alpha-actinin recruitment; that the N-RAP scaffold is much more stable than the assembling structural components; that N-RAP dynamics increase as assembly progresses; and that N-RAP leaves the structure after assembly is complete.

Basic confocal microscopy

Confocal microscopy produces sharp images of structures within relatively thick specimens (up to several hundred microns). It is particularly useful for examining fluorescent specimens. This overview intended to provide background and practical tips needed to get started with confocal microscopy. It begins with a description of the basis of optical sectioning, then discusses various types of confocal microscopes, and concludes with practical guidelines for sample preparation and optimizing image acquisition parameters.

Fluorescent staining of subcellular organelles: ER, Golgi complex, and mitochondria

The ability to distinguish and identify specific subcellular compartments is essential to understanding organelle function, biogenesis, and maintenance within cells and to defining protein trafficking pathways. Fluorescent dyes and/or fluorescently labeled lipid derivatives can be used to identify ER, Golgi complex, and mitochondria. Specific conditions for labeling each of these compartments are described.

Fluorescent speckle microscopy (FSM) of microtubules and actin in living cells

Fluorescent speckle microscopy (FSM), a combination of conventional wide-field fluorescent light microscopy and digital imaging with a low-noise, charge-coupled device (CCD) camera, has been developed to allow visualization of assembly/disassembly dynamics, movement, and turnover of macromolecule assemblies in vivo and in vitro. FSM uses a low level of fluorescent subunits to avoid high background. This produces an image of speckled molecules that co-assemble with endogenous molecules and are followed to characterize dynamic events in living cells.

Basic confocal microscopy

This unit introduces the reader to the basic principles of confocal microscopy and the design and capabilities of current confocal microscopes. The advantages and disadvantages of confocal microscopy compared to other techniques for fluorescence imaging are described. There are also practical guidelines for sample preparation and optimization of imaging parameters, as well as examples of some of the applications of confocal microscopy.

Mounting live cells attached to coverslips for microscopy

INTRODUCTIONThis article describes the mounting of coverslips containing live cells onto microscope slides.

Basic confocal microscopy

This comprehensive overview unit introduces the reader to confocal microscopy from the basic principles of imaging and optical sectioning, to selection of laser, practical guidelines for fixation, choice of fluorophore, control samples, and mounting the sample. There are also suggestions for optimizing the imaging parameters.

Imaging real-time gene expression in Mammalian cells with single-transcript resolution

INTRODUCTIONThe MS2 system provides optimal sensitivity for single-molecule detection in cells. It requires two genetically encoded moieties: a reporter mRNA that contains MS2 binding site (MBS) stem loops and a fluorescent MS2 coat protein (MCP-xFP) that binds to the stem loops with high affinity, thus tagging the mRNA within the cell. This protocol describes transfection of COS-7 cells with reporter RNA (e.g., pRSV-Z-24 MBS-β-actin) and MCP-xFP (e.g., pPolII-MCP-GFP-NLS) plasmids using calcium phosphate precipitation. The reporter mRNA plasmid must be co-transfected with the MCP-xFP-NLS plasmid for simultaneous expression in a cell. The unbound MCP-xFP-NLS is sequestered in the nucleus, leaving only the MCP-xFP-NLS that is bound to the reporter mRNA in the cytoplasm. This provides a high signal-to-noise ratio (SNR) that permits detection of single mRNA molecules. The Delta T Imaging System is used for image acquisition of fluorescent particles in the cells.

Cell and Molecular Biology of the Spindle Matrix

The concept of a spindle matrix has long been proposed to account for incompletely understood features of microtubule spindle dynamics and force production during mitosis. In its simplest formulation, the spindle matrix is hypothesized to provide a stationary or elastic molecular matrix that can provide a substrate for motor molecules to interact with during microtubule sliding and which can stabilize the spindle during force production. Although this is an attractive concept with the potential to greatly simplify current models of microtubule spindle behavior, definitive evidence for the molecular nature of a spindle matrix or for its direct role in microtubule spindle function has been lagging. However, as reviewed here multiple studies spanning the evolutionary spectrum from lower eukaryotes to vertebrates have provided new and intriguing evidence that a spindle matrix may be a general feature of mitosis.

Quantitative fluorescent speckle microscopy of cytoskeleton dynamics

Fluorescent speckle microscopy (FSM) is a technology used to analyze the dynamics of macromolecular assemblies in vivo and in vitro. Speckle formation by random association of fluorophores with a macromolecular structure was originally discovered for microtubules. Since then FSM has been expanded to study other cytoskeleton and cytoskeleton-binding proteins. Specialized software has been developed to convert the stochastic speckle image signal into spatiotemporal maps of polymer transport and turnover in living cells. These maps serve as a unique quantitative readout of the dynamic steady state of the cytoskeleton and its responses to molecular and genetic interventions, allowing a systematic study of the mechanisms of cytoskeleton regulation and its effect on cell function. Here, we explain the principles of FSM imaging and signal analysis, outline the biological questions and corresponding methodological advances that have led to the current state of FSM, and give a glimpse of new FSM modalities under development.

Basic confocal microscopy

This unit introduces the reader to the basic principles of confocal microscopy and the design and capabilities of current confocal microscopes. The advantages and disadvantages of confocal microscopy as compared to other techniques for fluorescence imaging are described. There are also practical guidelines for sample preparation and optimizing imaging parameters and examples of some of the applications of confocal microscopy.

Organization, dynamics and transmission of mitochondrial DNA: focus on vertebrate nucleoids

Eukaryotic cells contain numerous copies of the mitochondrial genome (from 50 to 100 copies in the budding yeast to some thousands in humans) that localize to numerous intramitochondrial nucleoprotein complexes called nucleoids. The transmission of mitochondrial DNA differs significantly from that of nuclear genomes and depends on the number, molecular composition and dynamic properties of nucleoids and on the organization and dynamics of the mitochondrial compartment. While the localization, dynamics and protein composition of mitochondrial DNA nucleoids begin to be described, we are far from knowing all mechanisms and molecules mediating and/or regulating these processes. Here, we review our current knowledge on vertebrate nucleoids and discuss similarities and differences to nucleoids of other eukaryots.

Overcoming photodamage in second-harmonic generation microscopy: real-time optical recording of neuronal action potentials

Second-harmonic generation (SHG) has proven essential for the highest-resolution optical recording of membrane potential (Vm) in intact specimens. Here, we demonstrate single-trial SHG recordings of neuronal somatic action potentials and quantitative recordings of their decay with averaging at multiple sites during propagation along branched neurites at distances up to 350 mum from the soma. We realized these advances by quantifying, analyzing, and thereby minimizing the dynamics of photodamage (PD), a frequent limiting factor in the optical imaging of biological preparations. The optical signal and the PD during SHG imaging of stained cultured Aplysia neurons were examined with intracellular electrode recordings monitoring the resting Vm variations induced by laser-scanning illumination. We found that the PD increased linearly with the dye concentration but grew with the cube of illumination intensity, leading to unanticipated optimization procedures to minimize PD. The addition of appropriate antioxidants in conjunction with an observed Vm recovery after termination of laser scanning further refined the imaging criteria for minimization and control of PD during SHG recording of action potentials. With these advances, the potential of SHG as an effective optical tool for neuroscience investigations is being realized.

Endoplasmic reticulum targeted GFP reveals ER organization in tobacco NT-1 cells during cell division

The endoplasmic reticulum (ER) of plant cells undergoes a drastic reorganization during cell division. In tobacco NT-1 cells that stably express a GFP construct targeted to the ER, we have mapped the reorganization of ER that occurs during mitosis and cytokinesis with confocal laser scanning microscopy. During division, the ER and nuclear envelope do not vesiculate. Instead, tubules of ER accumulate around the chromosomes after the nuclear envelope breaks down, with these tubules aligning parallel to the microtubules of the mitotic spindle. In cytokinesis, the phragmoplast is particularly rich in ER, and the transnuclear channels and invaginations present in many interphase cells appear to develop from ER tubules trapped in the developing phragmoplast. Drug studies, using oryzalin and latrunculin to disrupt the microtubules and actin microfilaments, respectively, demonstrate that during division, the arrangement of ER is controlled by microtubules and not by actin, which is the reverse of the situation in interphase cells.

Dynamics and inheritance of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of a polygonal array of interconnected tubules and sheets that spreads throughout the eukaryotic cell and is contiguous with the nuclear envelope. This elaborate structure is created and maintained by a constant remodeling process that involves the formation of new tubules, their cytoskeletal transport and homotypic fusion. Since the ER is a large, single-copy organelle, it must be actively segregated into daughter cells during cell division. Recent analysis in budding yeast indicates that ER inheritance involves the polarized transport of cytoplasmic ER tubules into newly formed buds along actin cables by a type V myosin. The tubules then become anchored to a site at the bud tip and this requires the Sec3p subunit of the exocyst complex. The ER is then propagated along the cortex of the bud to yield a cortical ER structure similar to that of the mother cell. In animal cells, the ER moves predominantly along microtubules, whereas actin fibers serve a complementary role. It is not yet clear to what extent the other components controlling ER distribution in yeast might be conserved in animal cells.

Golgi inheritance in mammalian cells is mediated through endoplasmic reticulum export activities

Golgi inheritance during mammalian cell division occurs through the disassembly, partitioning, and reassembly of Golgi membranes. The mechanisms responsible for these processes are poorly understood. To address these mechanisms, we have examined the identity and dynamics of Golgi proteins within mitotic membranes using live cell imaging and electron microscopy techniques. Mitotic Golgi fragments, seen in prometaphase and telophase, were found to localize adjacent to endoplasmic reticulum (ER) export domains, and resident Golgi transmembrane proteins cycled rapidly into and out of these fragments. Golgi proteins within mitotic Golgi haze-seen during metaphase-were found to redistribute with ER markers into fragments when the ER was fragmented by ionomycin treatment. The temperature-sensitive misfolding mutant ts045VSVG protein, when localized to the Golgi at the start of mitosis, became trapped in the ER at the end of mitosis in cells shifted to 40 degrees C. Finally, reporters for Arf1 and Sar1 activity revealed that Arf1 and Sar1 undergo sequential inactivation during mitotic Golgi breakdown and sequential reactivation upon Golgi reassembly at the end of mitosis. Together, these findings support a model of mitotic Golgi inheritance that involves inhibition and subsequent reactivation of cellular activities controlling the cycling of Golgi components into and out of the ER.

Cytoskeletal reorganization during process of apoptosis induced by cytostatic drugs in K-562 and HL-60 leukemia cell lines

The aim of the present study was to investigate the reorganization of F-actin, vimentin and tubulin in K-562 and HL-60 cell lines during apoptosis induced by etoposide, doxorubicin and taxol. The distribution of cytoskeletal proteins was analyzed by fluorescence microscopy. Actin was also studied by confocal microscopy and at the ultrastructural level. Changes in the distribution of cytoskeletal proteins were found to be dose-dependent and appeared to be more intense in HL-60 cells. Etoposide- and doxorubicin-treated cells showed similar changes in the distribution of F-actin, vimentin and tubulin. The reorganization of cytoskeletal proteins seemed to be consistent with features of apoptosis. An increase in bright staining of F-actin, vimentin and tubulin at the site of apoptotic bodies formation was observed. Immunogold labeling of actin in HL-60 cells was associated with features typical for apoptosis, i.e. compaction and margination of nuclear chromatin. K-562 cells showed cytoplasmic actin-positivity in the cytoplasm. Significant changes in morphology of HL-60 cells were found in the following concentrations: etoposide 20, 200 microM; doxorubicin 5, 10 microM and taxol 2-10 microM. The investigated proteins seemed to be involved in the above-reported apoptotic changes. Bright staining of F-actin, vimentin and tubulin, concentrated at the site of apoptotic bodies formation might suggested importance of these proteins for this process. Moreover, the increase in actin labeling in areas of chromatin compaction and margination of nuclear chromatin especially in HL-60 cells, which are more susceptible to apoptosis might implicate that actin might be involved in the chromatin remodeling during apoptosis.

Dynamics of the apical plasma membrane recycling system during cell division

The members of the family of Rab11 small GTPases are critical regulators of the plasma membrane vesicle recycling system. While previous studies have determined that the Golgi apparatus disperses during mitosis and reorganizes after cytokinesis, the fate of the recycling system during the cell cycle is more obscure. We have now studied in MDCK cells the fate during mitosis of an apical recycling system cargo, the polymeric IgA receptor (pIgAR), and regulators of the recycling system, Rab11a and its interacting proteins myosin Vb, Rab11-FIP1, Rab11-FIP2 and pp75/Rip11. Rab11a, pIgAR and myosin Vb containing vesicles dispersed into diffuse puncta in the cytosol during prophase and then became clustered near the spindle poles after metaphase, increasing in intensity throughout telophase. A similar pattern was observed for Rab11-FIP1 and Rab11-FIP2. However, Rab11-FIP1 lost colocalization with other recycling system markers during late prophase, relocating to the pericentriolar material. During telophase, Rab11-FIP1 returned to recycling system vesicles. Western blot analysis indicated that both Rab11a and pIgAR remained associated with membrane vesicles throughout the cell cycle. This behavior of the Rab11a-containing apical recycling endosome system during division was distinct from that of the Golgi apparatus. These results indicate that critical components of the apical recycling system remain associated on vesicles throughout the cell cycle and may provide a means for rapid re-establishment of plasma membrane components after mitosis.

Quantitative fluorescent speckle microscopy: where it came from and where it is going

Fluorescent speckle microscopy (FSM) is a technology for analysing the dynamics of macromolecular assemblies. Originally, the effect of random speckle formation was discovered with microtubules. Since then, the method has been expanded to other proteins of the cytoskeleton such as f-actin and microtubule binding proteins. Newly developed, specialized software for analysing speckle movement and photometric fluctuation in the context of polymer transport and turnover has turned FSM into a powerful method for the study of cytoskeletal dynamics in cell migration, division, morphogenesis and neuronal path finding. In all these settings, FSM serves as the quantitative readout to link molecular and genetic interventions to complete maps of the cytoskeleton dynamics and thus can be used for the systematic deciphering of molecular regulation of the cytoskeleton. Fully automated FSM assays can also be applied to live-cell screens for toxins, chemicals, drugs and genes that affect cytoskeletal dynamics. We envision that FSM has the potential to become a core tool in automated, cell-based molecular diagnostics in cases where variations in cytoskeletal dynamics are a sensitive signal for the state of a disease, or the activity of a molecular perturbant. In this paper, we review the origins of FSM, discuss these most recent technical developments and give a glimpse to future directions and potentials of FSM. It is written as a complement to the recent review (Waterman-Storer & Danuser, 2002, Curr. Biol., 12, R633-R640), in which we emphasized the use of FSM in cell biological applications. Here, we focus on the technical aspects of making FSM a quantitative method.

Computational analysis of F-actin turnover in cortical actin meshworks using fluorescent speckle microscopy

Fluorescent speckle microscopy (FSM) is a new imaging technique with the potential for simultaneous visualization of translocation and dynamic turnover of polymer structures. However, the use of FSM has been limited by the lack of specialized software for analysis of the positional and photometric fluctuations of hundreds of thousand speckles in an FSM time-lapse series, and for translating this data into biologically relevant information. In this paper we present a first version of a software for automated analysis of FSM movies. We focus on mapping the assembly and disassembly kinetics of a polymer meshwork. As a model system we have employed cortical F-actin meshworks in live newt lung epithelial cells. We lay out the algorithm in detail and present results of our analysis. The high spatial and temporal resolution of our maps reveals a kinetic cycling of F-actin, where phases of polymerization alternate with depolymerization in a spatially coordinated fashion. The cycle rates change when treating cells with a low dose of the drug latrunculin A. This shows the potential of this technique for future quantitative screening of drugs affecting the actin cytoskeleton. Various control experiments demonstrate that the algorithm is robust with respect to intensity variations due to noise and photobleaching and that effects of focus plane drifts can be eliminated by manual refocusing during image acquisition.

A high-speed multispectral spinning-disk confocal microscope system for fluorescent speckle microscopy of living cells

Fluorescent speckle microscopy (FSM) uses a small fraction of fluorescently labeled subunits to give macromolecular assemblies such as the cytoskeleton fluorescence image properties that allow quantitative analysis of movement and subunit turnover. We describe a multispectral microscope system to analyze the dynamics of multiple cellular structures labeled with spectrally distinct fluorophores relative to one another over time in living cells. This required a high-resolution, highly sensitive, low-noise, and stable imaging system to visualize the small number of fluorophores making up each fluorescent speckle, a means by which to switch between excitation wavelengths rapidly, and a computer-based system to integrate image acquisition and illumination functions and to allow a convenient interface for viewing multispectral time-lapse data. To reduce out-of-focus fluorescence that degrades speckle contrast, we incorporated the optical sectioning capabilities of a dual-spinning-disk confocal scanner. The real-time, full-field scanning allows the use of a low-noise, fast, high-dynamic-range, and quantum-efficient cooled charge-coupled device (CCD) as a detector as opposed to the more noisy photomultiplier tubes used in laser-scanning confocal systems. For illumination, our system uses a 2.5-W Kr/Ar laser with 100-300mW of power at several convenient wavelengths for excitation of few fluorophores in dim FSM specimens and a four-channel polychromatic acousto-optical modulator fiberoptically coupled to the confocal to allow switching between illumination wavelengths and intensity control in a few microseconds. We present recent applications of this system for imaging the cytoskeleton in migrating tissue cells and neurons.

Cytoplasmic dynein as a facilitator of nuclear envelope breakdown

During prophase in higher cells, centrosomes localize to deep invaginations in the nuclear envelope in a microtubule-dependent process. Loss of nuclear membranes in prometaphase commences in regions of the nuclear envelope that lie outside of these invaginations. Dynein and dynactin complex components concentrate on the nuclear envelope prior to any changes in nuclear envelope organization. These observations suggest a model in which dynein facilitates nuclear envelope breakdown by pulling nuclear membranes and associated proteins poleward along astral microtubules leading to nuclear membrane detachment. Support for this model is provided by the finding that interference with dynein function drastically alters nuclear membrane dynamics in prophase and prometaphase.