3D Ultrastructure of the Cochlear Outer Hair Cell Lateral Wall Revealed By Electron Tomography

Outer Hair Cells (OHCs) in the mammalian cochlea display a unique type of voltage-induced mechanical movement termed electromotility, which amplifies auditory signals and contributes to the sensitivity and frequency selectivity of mammalian hearing. Electromotility occurs in the OHC lateral wall, but it is not fully understood how the supramolecular architecture of the lateral wall enables this unique form of cellular motility. Employing electron tomography of high-pressure frozen and freeze-substituted OHCs, we visualized the 3D structure and organization of the membrane and cytoskeletal components of the OHC lateral wall. The subsurface cisterna (SSC) is a highly prominent feature, and we report that the SSC membranes and lumen possess hexagonally ordered arrays of particles. We also find the SSC is tightly connected to adjacent actin filaments by short filamentous protein connections. Pillar proteins that join the plasma membrane to the cytoskeleton appear as variable structures considerably thinner than actin filaments and significantly more flexible than actin-SSC links. The structurally rich organization and rigidity of the SSC coupled with apparently weaker mechanical connections between the plasma membrane (PM) and cytoskeleton reveal that the membrane-cytoskeletal architecture of the OHC lateral wall is more complex than previously appreciated. These observations are important for our understanding of OHC mechanics and need to be considered in computational models of OHC electromotility that incorporate subcellular features.

Meiotic cellular rejuvenation is coupled to nuclear remodeling in budding yeast

Production of healthy gametes in meiosis relies on the quality control and proper distribution of both nuclear and cytoplasmic contents. Meiotic differentiation naturally eliminates age-induced cellular damage by an unknown mechanism. Using time-lapse fluorescence microscopy in budding yeast, we found that nuclear senescence factors - including protein aggregates, extrachromosomal ribosomal DNA circles, and abnormal nucleolar material - are sequestered away from chromosomes during meiosis II and subsequently eliminated. A similar sequestration and elimination process occurs for the core subunits of the nuclear pore complex in both young and aged cells. Nuclear envelope remodeling drives the formation of a membranous compartment containing the sequestered material. Importantly, de novo generation of plasma membrane is required for the sequestration event, preventing the inheritance of long-lived nucleoporins and senescence factors into the newly formed gametes. Our study uncovers a new mechanism of nuclear quality control and provides insight into its function in meiotic cellular rejuvenation.

Genetic analysis of a novel tubulin mutation that redirects synaptic vesicle targeting and causes neurite degeneration in C. elegans

Neuronal cargos are differentially targeted to either axons or dendrites, and this polarized cargo targeting critically depends on the interaction between microtubules and molecular motors. From a forward mutagenesis screen, we identified a gain-of-function mutation in the C. elegans α-tubulin gene mec-12 that triggered synaptic vesicle mistargeting, neurite swelling and neurodegeneration in the touch receptor neurons. This missense mutation replaced an absolutely conserved glycine in the H12 helix with glutamic acid, resulting in increased negative charges at the C-terminus of α-tubulin. Synaptic vesicle mistargeting in the mutant neurons was suppressed by reducing dynein function, suggesting that aberrantly high dynein activity mistargeted synaptic vesicles. We demonstrated that dynein showed preference towards binding mutant microtubules over wild-type in microtubule sedimentation assay. By contrast, neurite swelling and neurodegeneration were independent of dynein and could be ameliorated by genetic paralysis of the animal. This suggests that mutant microtubules render the neurons susceptible to recurrent mechanical stress induced by muscle activity, which is consistent with the observation that microtubule network was disorganized under electron microscopy. Our work provides insights into how microtubule-dynein interaction instructs synaptic vesicle targeting and the importance of microtubule in the maintenance of neuronal structures against constant mechanical stress.

Axons and dendrites are two classes of neuronal process that differ in their functions and molecular compositions. Proteins important for synaptic functions are mostly synthesized in the cell body and sorted differentially into the axon or dendrites. Microtubules in the axon and dendrite maintain their structural integrity and regulate polarized protein transport into these compartments. We identified a novel α-tubulin mutation in C. elegans that caused mistargeting of synaptic vesicles and induced progressive neurite swelling, which resulted in late-onset neurodegeneration. We showed that this tubulin mutation weakened microtubule network and abnormally increased microtubule affinity for dynein, a motor protein responsible for cargo sorting to the dendrite. This enhanced microtubule-dynein affinity is due to augmented negative charge at the carboxyl terminus of α-tubulin. Neurite swelling and neurodegeneration could be ameliorated by reduced physical activity, suggesting that recurrent mechanical strain from muscle contraction jeopardized neurite integrity in the long run. Mutations in α- and β-tubulins are found in human neurological diseases; our findings therefore contribute to understanding the pathogenic mechanism of human neurological diseases associated with tubulin mutations.

The SUN protein UNC-84 is required only in force-bearing cells to maintain nuclear envelope architecture

SUN-KASH bridges that connect the nucleoskeleton to the cytoskeleton are only required to maintain nuclear envelope spacing in cells subjected to increased mechanical forces, such as muscle cells.

The nuclear envelope (NE) consists of two evenly spaced bilayers, the inner and outer nuclear membranes. The Sad1p and UNC-84 (SUN) proteins and Klarsicht, ANC-1, and Syne homology (KASH) proteins that interact to form LINC (linker of nucleoskeleton and cytoskeleton) complexes connecting the nucleoskeleton to the cytoskeleton have been implicated in maintaining NE spacing. Surprisingly, the NE morphology of most Caenorhabditis elegans nuclei was normal in the absence of functional SUN proteins. Distortions of the perinuclear space observed in unc-84 mutant muscle nuclei resembled those previously observed in HeLa cells, suggesting that SUN proteins are required to maintain NE architecture in cells under high mechanical strain. The UNC-84 protein with large deletions in its luminal domain was able to form functional NE bridges but had no observable effect on NE architecture. Therefore, SUN-KASH bridges are only required to maintain NE spacing in cells subjected to increased mechanical forces. Furthermore, SUN proteins do not dictate the width of the NE.

Out with the old and in with the new: rapid specimen preparation procedures for electron microscopy of sectioned biological material

This article presents the best current practices for preparation of biological samples for examination as thin sections in an electron microscope. The historical development of fixation, dehydration, and embedding procedures for biological materials are reviewed for both conventional and low temperature methods. Conventional procedures for processing cells and tissues are usually done over days and often produce distortions, extractions, and other artifacts that are not acceptable for today's structural biology standards. High-pressure freezing and freeze substitution can minimize some of these artifacts. New methods that reduce the times for freeze substitution and resin embedding to a few hours are discussed as well as a new rapid room temperature method for preparing cells for on-section immunolabeling without the use of aldehyde fixatives.

Rapid embedding methods into epoxy and LR White resins for morphological and immunological analysis of cryofixed biological specimens

A variety of specimens including bacteria, ciliates, choanoflagellates (Salpingoeca rosetta), zebrafish (Danio rerio) embryos, nematode worms (Caenorhabditis elegans), and leaves of white clover (Trifolium repens) plants were high pressure frozen, freeze-substituted, infiltrated with either Epon, Epon-Araldite, or LR White resins, and polymerized. Total processing time from freezing to blocks ready to section was about 6 h. For epoxy embedding the specimens were freeze-substituted in 1% osmium tetroxide plus 0.1% uranyl acetate in acetone. For embedding in LR White the freeze-substitution medium was 0.2% uranyl acetate in acetone. Rapid infiltration was achieved by centrifugation through increasing concentrations of resin followed by polymerization at 100°C for 1.5-2 h. The preservation of ultrastructure was comparable to standard freeze substitution and resin embedding methods that take days to complete. On-section immunolabeling results for actin and tubulin molecules were positive with very low background labeling. The LR White methods offer a safer, quicker, and less-expensive alternative to Lowicryl embedding of specimens processed for on-section immunolabeling without traditional aldehyde fixatives.

Interleukins in glioblastoma pathophysiology: implications for therapy

Despite considerable amount of research, the poor prognosis of patients diagnosed with glioblastoma multiforme (GBM) critically needs new drug development to improve clinical outcomes. The development of an inflammatory microenvironment has long been considered important in the initiation and progression of glioblastoma; however, the success of developing therapeutic approaches to target inflammation for GBM therapy has yet been limited. Here, we summarize the accumulating evidence supporting a role for inflammation in the pathogenesis of glioblastoma, discuss anti-inflammatory targets that could be relevant for GBM treatment and provide a perspective on the challenges faced in the development of drugs that target GBM inflammation. In particular, we will review the function of IL-1β, IL-6 and IL-8 as well as the potential of kinase inhibitors targeting key players in inflammatory cell signalling cascades such as JAK, JNK and p38 MAPK.

Transmission electron microscopy of thin sections of Drosophila: preparation of embryos using n-heptane and glutaraldehyde

This protocol describes the simultaneous permeabilization of Drosophila embryos with n-heptane and initial fixation with glutaraldehyde. Even though the vitelline membrane around the embryo is chemically permeabilized, it must be manually removed to achieve infiltration with embedding resins. Once the embryo is embedded, it can be sectioned for transmission electron microscopy (TEM). This procedure can produce excellent preservation for ultrastructural analysis, and is useful for situations where optimal preservation (e.g., by high-pressure freezing) is not required or is not feasible.

Preparation of Drosophila specimens for examination by transmission electron microscopy

There is no single, simple procedure for fixing and embedding all tissues for transmission electron microscopy (TEM). The chemistry of different cell types is to some extent unique, and this affects the way each cell type reacts to the wide array of fixatives, buffers, organic solvents, and resins used in TEM specimen preparation. A recurring theme in those organisms or cell types that are difficult to fix is the presence of a diffusion barrier that prevents the free diffusion of fixative and other chemicals in and out of the cell or tissue. This in turn means that fixation takes a relatively long time (measured in minutes or tens of minutes in some cases), during which the cells begin autolysis or are otherwise degraded from their original state. Drosophila requires specific preparation methods for TEM because most fly tissues are surrounded by significant diffusion barriers. In the embryo, it is the vitelline envelope, and in larvae and adults, it is the cuticle. In this article, we discuss methods that have evolved to cope with these barriers to achieve reasonable preservation of ultrastructure.

The T genotype of the MGMT C>T (rs16906252) enhancer single-nucleotide polymorphism (SNP) is associated with promoter methylation and longer survival in glioblastoma patients

Clinical studies in patients with newly diagnosed glioblastoma treated with temozolomide have shown that the methylation status of the O(6)-methylguanine-DNA methyltransferase (MGMT) gene is both predictive and prognostic of outcome. Methylation of the promoter region of MGMT is the most clinically relevant measure of MGMT expression and its assessment has become integral in current and planned clinical trials in glioblastoma. Our study confirmed that MGMT methylation, assessed by pyrosequencing, is associated with a significant survival benefit in glioblastoma patients treated with temozolomide (either concurrently with radiotherapy or sequential treatment). More interestingly, our study demonstrated that a promoter variant, the c.-56C>T (rs16906252) single nucleotide polymorphism (SNP) located within a cis-acting enhancer element at the proximal end of MGMT, is associated with the presence of MGMT promoter methylation in de novo glioblastoma. Furthermore, we show that the overall survival of patients carrying both the SNP and MGMT methylation showed a strong survival benefit when compared to either molecular event on their own. Promoter reporter experiments in MGMT methylated glioblastoma cell lines showed the T allele conferred a ∼30% reduction in normalised MGMT promoter activity compared to the wild-type haplotype. This might account for the propensity of the T allele to undergo promoter methylation, and in turn, the improved survival observed in carriers of the T allele. An independent validation on larger cohorts is required to confirm the prognostic and predictive value of individuals carrying the T allele.

Immunolabeling of thin sections of Drosophila tissues for transmission electron microscopy

The main advantage of electron microscopy (EM) for immunolabeling is resolution, but there is also another aspect that is often overlooked. For many investigators, the definitive image of an organelle is the one generated by EM. This is especially true for membranous organelles, with the possible exception of the nucleus and plant vacuoles. For example, references to the Golgi apparatus, smooth and rough endoplasmic reticulum, centriole, kinetochore, or mitochondrion typically bring to mind the images in an electron micrograph. The components of the cytoskeleton also have characteristic structural features that are associated with their EM image. Thus, it can be more effective for investigators to view gold particles superimposed over the image of a microtubule (MT) or mitochondrion using EM than to see bright dots or lines in the light microscope. This is especially true if the immunofluorescence image is of fixed cells. Here, we provide an overview of methods of EM immunolabeling used for localizing specific antigens on thin sections of Drosophila tissues.

Postembedding immunolabeling of thin sections of Drosophila tissues for transmission electron microscopy

Postembedding immunolabeling using resin sections is the recommended method for beginners carrying out electron microscopy (EM) immunolabeling. Postembedding labeling refers to labeling on sections, which is a method of gaining access to the interior of the cell without the harshness of detergent or ionic extraction as is performed with preembed labeling. Investigators already familiar with routine EM-sectioning techniques find EM immunolabeling using resin sections easiest to do, as procedures are similar to those used when performing light microscopy (LM) immunolabeling, but using a different resin. In addition, the overall preservation of structure is best in resin compared to use of cryosections or preembed labeling. The most critical component of immunoEM (iEM) is what primary antibody to use. This protocol descibes antibody labeling procedures for postembedding iEM using thin sections of Drosophila tissues.

Transmission electron microscopy of thin sections of Drosophila: conventional chemical fixation of embryos using trialdehyde

High-pressure freezing (HPF) followed by freeze-substitution is a valuable method for specimen preservation for transmission electron microscopy (TEM) in Drosophila. However, not all projects require this level of precision. In addition, some tissues are too large to fit into the HPF specimen carriers, and some fly tissues such as eyes and ovaries do not freeze well. This protocol describes a trialdehyde fixation procedure for embryos, to be used in situations where optimal preservation is not required or when HPF is not an option. Because the vitelline membrane is impermeable to aqueous solvents, it is necessary to either mechanically disrupt it or render it permeable by treatment with organic solvents. Good ultrastructural preservation has been achieved by puncturing embryos immersed in fixative with extremely sharp tungsten needles, as described here.

Transmission electron microscopy of thin sections of Drosophila: high-pressure freezing and freeze-substitution

The state of the art in fine-structure preservation for thin sectioning can be achieved by using fast-freezing technology followed by freeze substitution and embedding in resin. Samples prepared by high-pressure freezing are estimated to be "fixed" in 20-50 msec. Fast freezing also freezes every cell component regardless of its chemistry. Once frozen, tissues can be processed in a variety of ways before viewing in the electron microscope; here we describe only freeze substitution. In freeze substitution, cells are dehydrated at very low temperatures and cell water is replaced with organic solvent at -80°C to -90°C. At this temperature, large molecules such as proteins are immobilized, yet smaller molecules such as water (ice) can be dissolved and replaced with organic solvents, e.g., acetone. The ideal way to do freeze substitution is with a dedicated freeze-substitution device such as the Leica AFS2 system. These devices allow programming of the times and temperatures needed. Alternatively, if this equipment is not available, freeze substitution can still be performed using items commonly found around the laboratory, as is described here. This protocol is useful for preparing thin sections of Drosophila when the best possible preservation of ultrastructure and antigenicity is required.

Three-dimensional ultrastructure of the septin filament network in Saccharomyces cerevisiae

Septins are essential for membrane compartmentalization and remodeling. Electron tomography of yeast bud necks shows filaments perpendicular and parallel to the mother-bud axis that resemble in vitro septin arrays. Filaments are still present, although disordered, in mutants lacking a single septin, underscoring the importance of septin assembly.

Septins are conserved GTP-binding proteins involved in membrane compartmentalization and remodeling. In budding yeast, five mitotic septins localize at the bud neck, where the plasma membrane is enriched in phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P₂). We previously established the subunit organization within purified yeast septin complexes and how these hetero-octamers polymerize into filaments in solution and on PtdIns4,5P₂-containing lipid monolayers. How septin ultrastructure in vitro relates to the septin-containing filaments observed at the neck in fixed cells by thin-section electron microscopy was unclear. A morphological description of these filaments in the crowded space of the cell is challenging, given their small cross section. To examine septin organization in situ, sections of dividing yeast cells were analyzed by electron tomography of freeze-substituted cells, as well as by cryo–electron tomography. We found networks of filaments both perpendicular and parallel to the mother–bud axis that resemble septin arrays on lipid monolayers, displaying a repeat pattern that mirrors the molecular dimensions of the corresponding septin preparations in vitro. Thus these in situ structures most likely represent septin filaments. In viable mutants lacking a single septin, in situ filaments are still present, although more disordered, consistent with other evidence that the in vivo function of septins requires filament formation.

Freeze substitution in 3 hours or less

Freeze substitution is a process for low temperature dehydration and fixation of rapidly frozen cells that usually takes days to complete. New methods for freeze substitution have been developed that require only basic laboratory tools: a platform shaker, liquid nitrogen, a metal block with holes for cryotubes and an insulated container such as an ice bucket. With this equipment, excellent freeze substitution results can be obtained in as little as 90 min for cells of small volume such as bacteria and tissue culture cells. For cells of greater volume or that have significant diffusion barriers such as cuticles or thick cell walls, one can extend the time to 3 h or more with dry ice. The 3-h method works well for all manner of specimens, including plants and Caenorhabditis elegans as well as smaller samples. Here, we present the basics of the techniques and some results from Nicotiana leaves, C. elegans adult worms, Escherichia coli and baby hamster kidney tissue culture cells.

RAB-5- and RAB-11-dependent vesicle-trafficking pathways are required for plasma membrane repair after attack by bacterial pore-forming toxin

Pore-forming toxins (PFTs) secreted by pathogenic bacteria are the most common bacterial protein toxins and are important virulence factors for infection. PFTs punch holes in host cell plasma membranes, and although cells can counteract the resulting membrane damage, the underlying mechanisms at play remain unclear. Using Caenorhabditis elegans as a model, we demonstrate in vivo and in an intact epithelium that intestinal cells respond to PFTs by increasing levels of endocytosis, dependent upon RAB-5 and RAB-11, which are master regulators of endocytic and exocytic events. Furthermore, we find that RAB-5 and RAB-11 are required for protection against PFT and to restore integrity to the plasma membrane. One physical mechanism involved is the RAB-11-dependent expulsion of microvilli from the apical side of the intestinal epithelial cells. Specific vesicle-trafficking pathways thus protect cells against an attack by PFTs on plasma membrane integrity, via altered plasma membrane dynamics.

A review of high-pressure freezing preparation techniques for correlative light and electron microscopy of the same cells and tissues

In this paper, we review some published studies using correlative light and electron microscopy methods. We further refined our criteria to include only those studies using live cells for light microscope and where high-pressure freezing was the method of specimen preparation for electron microscopy. High-pressure freezing is especially important for some difficult-to-fix samples, and for optimal preservation of ultrastructure in samples larger than a few micrometres. How the light microscope observations are done is completely sample dependent, but the choice of high-pressure freezer depends on the speed required to capture (freeze) the biological event of interest. For events requiring high time resolution (in the 4-5 s range) the Leica EM PACT2 with rapid transfer system works well. For correlative work on structures of interest that are either non-motile or moving slowly (minutes rather than seconds), any make of high-pressure freezer will work. We also report on some efforts to improve the capabilities of the Leica EM PACT2 rapid transfer system.

Microarray gene expression and immunohistochemistry analyses of adrenocortical tumors identify IGF2 and Ki-67 as useful in differentiating carcinomas from adenomas

The management of adrenocortical tumors (ACTs) is complex. The Weiss score is the present most widely used system for ACT diagnosis. An ACT is scored from 0 to 9, with a higher score correlating with increased malignancy. However, ACTs with a score of 3 can be phenotypically benign or malignant. Our objective is to use microarray profiling of a cohort of adrenocortical carcinomas (ACCs) and adrenocortical adenomas (ACAs) to identify discriminatory genes that could be used as an adjunct to the Weiss score. A cohort of Weiss score defined ACCs and ACAs were profiled using Affymetrix HGU133plus2.0 genechips. Genes with high-discriminatory power were identified by univariate and multivariate analyses and confirmed by quantitative real-time reverse transcription PCR and immunohistochemistry (IHC). The expression of IGF2, MAD2L1, and CCNB1 were significantly higher in ACCs compared with ACAs while ABLIM1, NAV3, SEPT4, and RPRM were significantly lower. Several proteins, including IGF2, MAD2L1, CCNB1, and Ki-67 had high-diagnostic accuracy in differentiating ACCs from ACAs. The best results, however, were obtained with a combination of IGF2 and Ki-67, with 96% sensitivity and 100% specificity in diagnosing ACCs. Microarray gene expression profiling accurately differentiates ACCs from ACAs. The combination of IGF2 and Ki-67 IHC is also highly accurate in distinguishing between the two groups and is particularly helpful in ACTs with Weiss score of 3.

Controlled microaspiration for high-pressure freezing: a new method for ultrastructural preservation of fragile and sparse tissues for TEM and electron tomography

High-pressure freezing is the preferred method to prepare thick biological specimens for ultrastructural studies. However, the advantages obtained by this method often prove unattainable for samples that are difficult to handle during the freezing and substitution protocols. Delicate and sparse samples are difficult to manipulate and maintain intact throughout the sequence of freezing, infiltration, embedding and final orientation for sectioning and subsequent transmission electron microscopy. An established approach to surmount these difficulties is the use of cellulose microdialysis tubing to transport the sample. With an inner diameter of 200 microm, the tubing protects small and fragile samples within the thickness constraints of high-pressure freezing, and the tube ends can be sealed to avoid loss of sample. Importantly, the transparency of the tubing allows optical study of the specimen at different steps in the process. Here, we describe the use of a micromanipulator and microinjection apparatus to handle and position delicate specimens within the tubing. We report two biologically significant examples that benefit from this approach, 3D cultures of mammary epithelial cells and cochlear outer hair cells. We illustrate the potential for correlative light and electron microscopy as well as electron tomography.

Autophagy counterbalances endoplasmic reticulum expansion during the unfolded protein response

The protein folding capacity of the endoplasmic reticulum (ER) is regulated by the unfolded protein response (UPR). The UPR senses unfolded proteins in the ER lumen and transmits that information to the cell nucleus, where it drives a transcriptional program that is tailored to re-establish homeostasis. Using thin section electron microscopy, we found that yeast cells expand their ER volume at least 5-fold under UPR-inducing conditions. Surprisingly, we discovered that ER proliferation is accompanied by the formation of autophagosome-like structures that are densely and selectively packed with membrane stacks derived from the UPR-expanded ER. In analogy to pexophagy and mitophagy, which are autophagic processes that selectively sequester and degrade peroxisomes and mitochondria, the ER-specific autophagic process described utilizes several autophagy genes: they are induced by the UPR and are essential for the survival of cells subjected to severe ER stress. Intriguingly, cell survival does not require vacuolar proteases, indicating that ER sequestration into autophagosome-like structures, rather than their degradation, is the important step. Selective ER sequestration may help cells to maintain a new steady-state level of ER abundance even in the face of continuously accumulating unfolded proteins.

The authors describe an ER-specific autophagic process induced by the unfolded protein response (UPR), which sequesters ER into autophagosome-like bodies. This process may be involved in re-establishing homeostasis when unfolded proteins accumulate inside cells.

Recent advances in high-pressure freezing: equipment- and specimen-loading methods

This chapter is an update of material first published by McDonald in the first volume of this book. Here, we discuss the improvements in the technology and the methodology of high-pressure freezing (HPF) since that article was published. First, we cover the latest innovation in HPF, the Leica EM PACT2. This machine differs significantly from the BAL-TEC HPM 010 high-pressure freezer, which was the main subject of the former chapter. The EM PACT2 is a smaller, portable machine and has an optional attachment, the Rapid Transfer System (RTS). This RTS permits easy and reproducible loading of the sample and allows one to do correlative light and electron microscopy with high time resolution. We also place more emphasis in this article on the details of specimen loading for HPF, which is considered the most critical phase of the whole process. Detailed procedures are described for how to high-pressure freeze cells in suspension, cells attached to substrates, tissue samples, or whole organisms smaller than 300 microm, and tissues or organisms greater than 300 microm in size. We finish the article with a brief discussion of freeze substitution and recommend some sample protocols for this procedure.

Oocyte CD9 is enriched on the microvillar membrane and required for normal microvillar shape and distribution

Microvilli are found on the surface of many cell types, including the mammalian oocyte, where they are thought to act in initial contact of sperm and oocyte plasma membranes. CD9 is currently the only oocyte protein known to be required for sperm-oocyte fusion. We found CD9 is localized to the oocyte microvillar membrane using transmission electron microscopy (TEM). Scanning electron microscopy (SEM) showed that CD9 null oocytes, which are unable to fuse with sperm, have an altered length, thickness and density of their microvilli. One aspect of this change in morphology was quantified using TEM by measuring the radius of curvature at the microvillar tips. A small radius of curvature is thought to promote fusibility and the radius of curvature of microvillar tips on CD9 wild-type oocytes was found to be half that of the CD9 null oocytes. We found that oocyte CD9 co-immunoprecipitates with two Ig superfamily cis partners, EWI-2 and EWI-F, which could have a role in linking CD9 to the oocyte microvillar actin core. We also examined latrunculin B-treated oocytes, which are known to have reduced fusion ability, and found altered microvillar morphology by SEM and TEM. Our data suggest that microvilli may participate in sperm-oocyte fusion. Microvilli could act as a platform to concentrate adhesion/fusion proteins and/or provide a membrane protrusion with a low radius of curvature. They may also have a dynamic interaction with the sperm that serves to capture the sperm cell and bring it into close contact with the oocyte plasma membrane.

High-pressure freezing, cellular tomography, and structural cell biology

Structural cell biology, which we define as electron microscopic analysis of intact cells, suffered a loss of interest and activity following the advances in light microscopy beginning in the 1990s. Interestingly, it is the wealth of detailed observation in the light microscope that is one of the driving forces for the current renewed interest in electron microscopy (EM). A great many cellular details are simply beyond the resolving power of the light microscope. In this article, we describe how electron microscopists are responding to the demands for better preservation of cells and for ways to view cell ultrastructure in three dimensions at high resolution. We discuss how low temperature methods, especially high-pressure freezing and freeze substitution, reduce the artifacts of conventional EM specimen preparation. We also give a brief introduction to cellular electron tomography, a powerful analytical method that can give near-atomic resolution of cell ultrastructure in three-dimensional (3-D) models.

Functional modulation of IFT kinesins extends the sensory repertoire of ciliated neurons in Caenorhabditis elegans

The diversity of sensory cilia on Caenorhabditis elegans neurons allows the animal to detect a variety of sensory stimuli. Sensory cilia are assembled by intraflagellar transport (IFT) kinesins, which transport ciliary precursors, bound to IFT particles, along the ciliary axoneme for incorporation into ciliary structures. Using fluorescence microscopy of living animals and serial section electron microscopy of high pressure–frozen, freeze-substituted IFT motor mutants, we found that two IFT kinesins, homodimeric OSM-3 kinesin and heterotrimeric kinesin II, function in a partially redundant manner to build full-length amphid channel cilia but are completely redundant for building full-length amphid wing (AWC) cilia. This difference reflects cilia-specific differences in OSM-3 activity, which serves to extend distal singlets in channel cilia but not in AWC cilia, which lack such singlets. Moreover, AWC-specific chemotaxis assays reveal novel sensory functions for kinesin II in these wing cilia. We propose that kinesin II is a “canonical” IFT motor, whereas OSM-3 is an “accessory” IFT motor, and that subtle changes in the deployment or actions of these IFT kinesins can contribute to differences in cilia morphology, cilia function, and sensory perception.

Spindle pole body duplication in fission yeast occurs at the G1/S boundary but maturation is blocked until exit from S by an event downstream of cdc10+

The regulation and timing of spindle pole body (SPB) duplication and maturation in fission yeast was examined by transmission electron microscopy. When cells are arrested at G1 by nitrogen starvation, the SPB is unduplicated. On release from G1, the SPBs were duplicated after 1-2 h. In cells arrested at S by hydroxyurea, SPBs are duplicated but not mature. In G1 arrest/release experiments with cdc2.33 cells at the restrictive temperature, SPBs remained single, whereas in cells at the permissive temperature, SPBs were duplicated. In cdc10 mutant cells, the SPBs seem not only to be duplicated but also to undergo partial maturation, including invagination of the nuclear envelope underneath the SPB. There may be an S-phase-specific inhibitor of SPB maturation whose expression is under control of cdc10(+). This model was examined by induction of overreplication of the genome by overexpression of rum1p or cdc18p. In cdc18p-overexpressing cells, the SPBs are duplicated but not mature, suggesting that cdc18p is one component of this feedback mechanism. In contrast, cells overexpressing rum1p have large, deformed SPBs accompanied by other features of maturation and duplication. We propose a feedback mechanism for maturation of the SPB that is coupled with exit from S to trigger morphological changes.

Actin dynamics coupled to clathrin-coated vesicle formation at the trans-Golgi network

In diverse species, actin assembly facilitates clathrin-coated vesicle (CCV) formation during endocytosis. This role might be an adaptation specific to the unique environment at the cell cortex, or it might be fundamental, facilitating CCV formation on different membranes. Proteins of the Sla2p/Hip1R family bind to actin and clathrin at endocytic sites in yeast and mammals. We hypothesized that Hip1R might also coordinate actin assembly with clathrin budding at the trans-Golgi network (TGN). Using deconvolution and time-lapse microscopy, we showed that Hip1R is present on CCVs emerging from the TGN. These vesicles contain the mannose 6-phosphate receptor involved in targeting proteins to the lysosome, and the actin nucleating Arp2/3 complex. Silencing of Hip1R expression by RNAi resulted in disruption of Golgi organization and accumulation of F-actin structures associated with CCVs on the TGN. Hip1R silencing and actin poisons slowed cathepsin D exit from the TGN. These studies establish roles for Hip1R and actin in CCV budding from the TGN for lysosome biogenesis.

High-pressure freezing of isolated gastric glands provides new insight into the fine structure and subcellular localization of H+/K+-ATPase in gastric parietal cells

High-pressure freezing (HPF) is currently the most reliable method to obtain an adequately frozen sample for high-resolution morphological evaluation. Here we applied the HPF technique to isolated rabbit gastric glands to reveal structural evidence that may be correlated with functional activity of gastric parietal cells. This approach provided well-preserved fine structure and excellent antigenicity of several parietal cell proteins. Microtubules were abundant in the cytoplasm and frequently appeared to be associating with tubulovesicles. Interestingly, many electron-dense coated vesicles were apparent around the intracellular canaliculi (IC) of resting parietal cells, consistent with active membrane retrieval from the apical membranes. Immunolabeling of H+/K+-ATPase was evident on the endocytic components (e.g., multivesicular bodies) and tubulovesicles. After histamine stimulation, the parietal cells characteristically showed expanded IC membranes with varied features of their apical microvilli. The labeling density of H+/K+-ATPase was four-fold higher on the IC membrane of stimulated parietal cells than on that of resting parietal cells. Immunolabeling of ezrin was clearly identified on the IC and basolateral membranes of parietal cells, corresponding to their F-actin-rich sites. The present findings provide a new insight into the correlation of cell structure and function in gastric parietal cells.

Morphologically distinct microtubule ends in the mitotic centrosome of Caenorhabditis elegans

During mitosis, the connections of microtubules (MTs) to centrosomes and kinetochores are dynamic. From in vitro studies, it is known that the dynamic behavior of MTs is related to the structure of their ends, but we know little about the structure of MT ends in spindles. Here, we use high-voltage electron tomography to study the centrosome- and kinetochore-associated ends of spindle MTs in embryonic cells of the nematode, Caenorhabditis elegans. Centrosome-associated MT ends are either closed or open. Closed MT ends are more numerous and are uniformly distributed around the centrosome, but open ends are found preferentially on kinetochore-attached MTs. These results have structural implications for models of MT interactions with centrosomes.

Mycobacterium marinum escapes from phagosomes and is propelled by actin-based motility

Mycobacteria are responsible for a number of human and animal diseases and are classical intracellular pathogens, living inside macrophages rather than as free-living organisms during infection. Numerous intracellular pathogens, including Listeria monocytogenes, Shigella flexneri, and Rickettsia rickettsii, exploit the host cytoskeleton by using actin-based motility for cell to cell spread during infection. Here we show that Mycobacterium marinum, a natural pathogen of fish and frogs and an occasional pathogen of humans, is capable of actively inducing actin polymerization within macrophages. M. marinum that polymerized actin were free in the cytoplasm and propelled by actin-based motility into adjacent cells. Immunofluorescence demonstrated the presence of host cytoskeletal proteins, including the Arp2/3 complex and vasodilator-stimulated phosphoprotein, throughout the actin tails. In contrast, Wiskott-Aldrich syndrome protein localized exclusively at the actin-polymerizing pole of M. marinum. These findings show that M. marinum can escape into the cytoplasm of infected macrophages, where it can recruit host cell cytoskeletal factors to induce actin polymerization leading to direct cell to cell spread.

Ultrastructural localization of spicule matrix proteins in normal and metalloproteinase inhibitor-treated sea urchin primary mesenchyme cells

Studies of the sea urchin larval skeleton have contributed greatly to our understanding of the process of biomineralization. In this study we have undertaken an investigation of the morphology of skeleton formation and the localization of proteins involved in the process of spicule formation at the electron microscope level. Sea urchin primary mesenchyme cells undergo a number of morphological changes as they synthesize the larval skeleton. They form a large spicule compartment that surrounds the growing spicule and, as spicule formation comes to an end, the density of the cytoplasm decreases. Inhibition of spicule formation by specific matrix metalloproteinase inhibitors or serum deprivation has some subtle effects on the morphology of cells and causes the accumulation of specific classes of vesicles. We have localized proteins of the organic matrix of the spicule and found that one protein, SM30, is localized to the Golgi apparatus and transport vesicles in the cytoplasm as well as throughout the occluded protein matrix of the spicule itself. This localization suggests that SM30 is an important structural protein in the spicule. Another spicule matrix protein, SM50, has a similar cytoplasmic localization, but in the spicule much of it is localized at the periphery of the spicule compartment, and consequently it may play a role in the assembly of new material onto the growing spicule or in the maintenance of the integrity of the matrix surrounding the spicule.

Direct attachment of cell suspensions to high-pressure freezing specimen planchettes

We describe a procedure for high-pressure freezing (HPF) of cultured cells using the HPF aluminium planchettes as a substrate. Cells are either grown directly on planchettes covered with Matrigel or allowed to attach to poly-l-lysine-coated planchettes. This method allows for rapid transfer of the cells into the HPF and minimizes physical and physiological trauma to the cells. Furthermore, the yield of well-frozen cells approaches 100% for every cell type we have tried so far. In this report, we show well-preserved ultrastructure in mitotic and interphase HeLa cells, isolated gastric parietal cells and isolated gastric glands. Immunogold labelling of H+/K+-ATPase is shown in parietal cells of isolated gastric glands embedded in LR White resin. The aluminium planchettes appear to have little effect on cell physiology, as demonstrated by the fact that parietal cells cultured for 24-28 h on the planchettes retain their responsiveness to stimulation with histamine.

Dynamic phosphoregulation of the cortical actin cytoskeleton and endocytic machinery revealed by real-time chemical genetic analysis

We used chemical genetics to control the activity of budding yeast Prk1p, which is a protein kinase that is related to mammalian GAK and AAK1, and which targets several actin regulatory proteins implicated in endocytosis. In vivo Prk1p inhibition blocked pheromone receptor endocytosis, and caused cortical actin patches to rapidly aggregate into large clumps that contained Abp1p, Sla2p, Pan1p, Sla1p, and Ent1p. Clump formation depended on Arp2p, suggesting that this phenotype might result from unregulated Arp2/3-stimulated actin assembly. Electron microscopy/immunoelectron microscopy analysis and tracking of the endocytic membrane marker FM4-64 revealed vesicles of likely endocytic origin within the actin clumps. Upon inhibitor washout, the actin clumps rapidly disassembled, and properly polarized actin patches reappeared. Our results suggest that actin clumps result from blockage at a normally transient step during which actin assembly is stimulated by endocytic proteins. Thus, we revealed tight phosphoregulation of an intrinsically dynamic, actin patch-related process, and propose that Prk1p negatively regulates the actin assembly-stimulating activity of endocytic proteins.

Rapid translocation of polarized MDCK cell monolayers by Leptospira interrogans, an invasive but nonintracellular pathogen

Pathogenic spirochetes of the genus Leptospira are a major cause of human zoonotic infectious disease worldwide. After gaining entry through the skin, the organism causes disease by hematogenously disseminating to multiple organs. The mechanism by which it penetrates the mammalian cell barriers to disseminate is not well understood. In this study, we used a low-passage-number isolate of Leptospira interrogans to elucidate the invasive potential of this spirochete. Quantification of bacteria by dark-field microscopy revealed that pathogenic spirochetes were able to translocate through polarized MDCK cell monolayers at a rate significantly greater than that of nonpathogenic Leptospira or a recognized invasive bacterial pathogen, Salmonella: In contrast to Salmonella, L. interrogans did not alter transepithelial electrical resistance during cell translocation. Both transmission and scanning electron microscopy revealed tight association of the extracellular spirochetes with the host cell plasma membrane, without membrane perturbations suggestive of cytoskeletal rearrangement. Spirochetes were not observed within intercellular junctions or membrane-bound compartments inside cells. They were found within the cytoplasm of only 8% of the counted cells. These results indicate that Leptospira is an invasive but not a facultative intracellular organism. We propose that the rapid translocation of mammalian cells by pathogenic Leptospira is a mechanism designed to evade killing by host cells that permits the organism to quickly reach the bloodstream and disseminate to multiple organs.

A new approach for high-pressure freezing of primary culture cells: the fine structure and stimulation-associated transformation of cultured rabbit gastric parietal cells

A newly designated procedure for high-pressure freezing of primary culture cells provided excellent ultrastructure of rabbit gastric parietal cells. The isolated parietal cells were cultivated on Matrigel-coated aluminium plates for conventional subsequential cryoimmobilization by high-pressure freezing. The ultrastructure of different organelles (Golgi apparatus, mitochondria, multivesicular bodies, etc.) was well preserved compared to conventional chemical fixation. In detail, actin filaments were clearly shown within the microvilli and the subapical cytoplasm. Another striking finding on the cytoskeleton system is the abundance of microtubules among the tubulovesicles. Interestingly, some microtubules appeared to be associating with tubulovesicles. A large number of electron-dense coated pits and vesicles were observed around the apical membrane vacuoles in cimetidine-treated resting parietal cells, consistent with an active membrane uptake in the resting state. Immunogold labelling of H+/K+-ATPase was seen on the tubulovesicular membranes. When stimulated with histamine, the cultured parietal cells undergo morphological transformation, resulting in great expansion of apical membrane vacuoles. Immunogold labelling of H+/K+-ATPase was present not only on the microvilli of expanded apical plasma membrane vacuoles but also in the electron-dense coated pits. The present findings provide a clue to vesicular membrane trafficking in cultured gastric parietal cells, and assure the utility of the new procedure for high-pressure freezing of primary culture cells.

Three-dimensional reconstruction of cytoplasmic membrane networks in parietal cells

There is general agreement that stimulation and consequent secretion of gastric parietal cells result in a great expansion of the apical canalicular membrane at the expense of an extensive intracellular network of membranes rich in the gastric proton pump (H,K-ATPase). However, there is ongoing controversy as to the precise nature of the intracellular membrane network,conventionally called tubulovesicles. At the heart of this controversy lies the question of whether tubulovesicles are a distinct membrane compartment or whether they are continuous with the apical plasma membrane.

To address this controversy we used high-pressure, rapid freezing techniques to fix non-stimulated (resting) rabbit gastric glands for electron microscopy. Ultra-thin (60-70 nm) serial sections were used for conventional TEM; 400-500 nm sections were used for tomography. Images were digitized and models constructed using Midas and Imod software(http://bio3d.colorado.edu). Images were aligned and contours drawn on specific cellular structures. The contours from a stack of serial sections were arranged into objects and meshed into 3D structures. For resting parietal cells our findings are as follows:(1) The apical canaliculus is a microvilli-decorated, branching membrane network that extends into and throughout the parietal cell. This agrees well with a host of previous studies. (2) The plentiful mitochondria form an extensive reticular network throughout the cytoplasm. This has not previously been reported for the parietal cell, and the significance of this observation and the dynamics of the mitochondrial network remain unknown. (3)H,K-ATPase-rich membranes do include membrane tubules and vesicles; however,the tubulovesicular compartment is chiefly comprised of small stacks of cisternae. Thus a designation of tubulocisternae seems appropriate; however,in the resting cell there are no continuities between the apical canaliculus and the tubulocisternae or between tubulocisternae. These data support the recruitment-recycling model of parietal cell stimulation.

HIM-10 is required for kinetochore structure and function on Caenorhabditis elegans holocentric chromosomes

Macromolecular structures called kinetochores attach and move chromosomes within the spindle during chromosome segregation. Using electron microscopy, we identified a structure on the holocentric mitotic and meiotic chromosomes of Caenorhabditis elegans that resembles the mammalian kinetochore. This structure faces the poles on mitotic chromosomes but encircles meiotic chromosomes. Worm kinetochores require the evolutionarily conserved HIM-10 protein for their structure and function. HIM-10 localizes to the kinetochores and mediates attachment of chromosomes to the spindle. Depletion of HIM-10 disrupts kinetochore structure, causes a failure of bipolar spindle attachment, and results in chromosome nondisjunction. HIM-10 is related to the Nuf2 kinetochore proteins conserved from yeast to humans. Thus, the extended kinetochores characteristic of C. elegans holocentric chromosomes provide a guide to the structure, molecular architecture, and function of conventional kinetochores.

Dynamic localization of the Swe1 regulator Hsl7 during the Saccharomyces cerevisiae cell cycle

In Saccharomyces cerevisiae, entry into mitosis requires activation of the cyclin-dependent kinase Cdc28 in its cyclin B (Clb)-associated form. Clb-bound Cdc28 is susceptible to inhibitory tyrosine phosphorylation by Swe1 protein kinase. Swe1 is itself negatively regulated by Hsl1, a Nim1-related protein kinase, and by Hsl7, a presumptive protein-arginine methyltransferase. In vivo all three proteins localize to the bud neck in a septin-dependent manner, consistent with our previous proposal that formation of Hsl1-Hsl7-Swe1 complexes constitutes a checkpoint that monitors septin assembly. We show here that Hsl7 is phosphorylated by Hsl1 in immune-complex kinase assays and can physically associate in vitro with either Hsl1 or Swe1 in the absence of any other yeast proteins. With the use of both the two-hybrid method and in vitro binding assays, we found that Hsl7 contains distinct binding sites for Hsl1 and Swe1. A differential interaction trap approach was used to isolate four single-site substitution mutations in Hsl7, which cluster within a discrete region of its N-terminal domain, that are specifically defective in binding Hsl1. When expressed in hsl7Delta cells, each of these Hsl7 point mutants is unable to localize at the bud neck and cannot mediate down-regulation of Swe1, but retains other functions of Hsl7, including oligomerization and association with Swe1. GFP-fusions of these Hsl1-binding defective Hsl7 proteins localize as a bright perinuclear dot, but never localize to the bud neck; likewise, in hsl1Delta cells, a GFP-fusion to wild-type Hsl7 or native Hsl7 localizes to this dot. Cell synchronization studies showed that, normally, Hsl7 localizes to the dot, but only in cells in the G1 phase of the cell cycle. Immunofluorescence analysis and immunoelectron microscopy established that the dot corresponds to the outer plaque of the spindle pole body (SPB). These data demonstrate that association between Hsl1 and Hsl7 at the bud neck is required to alleviate Swe1-imposed G2-M delay. Hsl7 localization at the SPB during G1 may play some additional role in fine-tuning the coordination between nuclear and cortical events before mitosis.

Age-related changes in detecting a mistuned harmonic

The effects of age on discriminating simultaneous sounds were investigated by comparing the hearing threshold in detecting a mistuned harmonic in young, middle-aged, and older adults. The stimuli were complex sounds containing multiple harmonics, one of which could be "mistuned" so that it was no longer an integer multiple of the fundamental. Older adults had higher thresholds than middle-aged or young adults. The effect of age was greater for short than for long duration sounds and remained even after controlling for hearing sensitivity. The results are consistent with an age-related decline in parsing simultaneous auditory events, which may contribute to the speech perception difficulties in the elderly.

Clathrin in gastric acid secretory (parietal) cells: biochemical characterization and subcellular localization

Clathrin from H-K-ATPase-rich membranes derived from the tubulovesicular compartment of rabbit and hog gastric acid secretory (parietal) cells was characterized biochemically, and the subcellular localization of membrane-associated clathrin in parietal cells was characterized by immunofluorescence, electron microscopy, and immunoelectron microscopy. Clathrin from H-K- ATPase-rich membranes was determined to be comprised of conventional clathrin heavy chain and a predominance of clathrin light chain A. Clathrin and adaptors could be induced to polymerize quantitatively in vitro, forming 120-nm-diameter basketlike structures. In digitonin-permeabilized resting parietal cells, the intracellular distribution of immunofluorescently labeled clathrin was suggestive of labeling of the tubulovesicular compartment. Clathrin was also unexpectedly localized to canalicular (apical) membranes, as were alpha-adaptin and dynamin, suggesting that this membrane domain of resting parietal cells is endocytotically active. At the ultrastructural level, clathrin was immunolocalized to canalicular and tubulovesicular membranes. H-K-ATPase was immunolocalized to the same membrane domains as clathrin but did not appear to be enriched at the specific subdomains that were enriched in clathrin. Finally, in immunofluorescently labeled primary cultures of parietal cells, in contrast to the H-K-ATPase, intracellular clathrin was found not to translocate to the apical membrane on secretagogue stimulation. Taken together, these biochemical and morphological data provide a framework for characterizing the role of clathrin in the regulation of membrane trafficking from tubulovesicles and at the canalicular membrane in parietal cells.

The coronin-like protein POD-1 is required for anterior-posterior axis formation and cellular architecture in the nematode caenorhabditis elegans

Establishment of anterior-posterior (a-p) polarity in the Caenorhabditis elegans embryo depends on filamentous (F-) actin. Previously, we isolated an F-actin-binding protein that was enriched in the anterior cortex of the one-cell embryo and was hypothesized to link developmental polarity to the actin cytoskeleton. Here, we identify this protein, POD-1, as a new member of the coronin family of actin-binding proteins. We have generated a deletion within the pod-1 gene. Elimination of POD-1 from early embryos results in a loss of physical and molecular asymmetries along the a-p axis. For example, PAR-1 and PAR-3, which themselves are polarized and required for a-p polarity, are delocalized in pod-1 mutant embryos. However, unlike loss of PAR proteins, loss of POD-1 gives rise to the formation of abnormal cellular structures, namely large vesicles of endocytic origin, membrane protrusions, unstable cell divisions, a defective eggshell, and deposition of extracellular material. We conclude that, analogous to coronin, POD-1 plays an important role in intracellular trafficking and organizing specific aspects of the actin cytoskeleton. We propose models to explain how the role of POD-1 in basic cellular processes could be linked to the generation of polarity along the embryonic a-p axis.

The bipolar kinesin, KLP61F, cross-links microtubules within interpolar microtubule bundles of Drosophila embryonic mitotic spindles

Previous genetic and biochemical studies have led to the hypothesis that the essential mitotic bipolar kinesin, KLP61F, cross-links and slides microtubules (MTs) during spindle assembly and function. Here, we have tested this hypothesis by immunofluorescence and immunoelectron microscopy (immunoEM). We show that Drosophila embryonic spindles at metaphase and anaphase contain abundant bundles of MTs running between the spindle poles. These interpolar MT bundles are parallel near the poles and antiparallel in the midzone. We have observed that KLP61F motors, phosphorylated at a cdk1/cyclin B consensus domain within the BimC box (BCB), localize along the length of these interpolar MT bundles, being concentrated in the midzone region. Nonphosphorylated KLP61F motors, in contrast, are excluded from the spindle and display a cytoplasmic localization. Immunoelectron microscopy further suggested that phospho-KLP61F motors form cross-links between MTs within interpolar MT bundles. These bipolar KLP61F MT-MT cross-links should be capable of organizing parallel MTs into bundles within half spindles and sliding antiparallel MTs apart in the spindle midzone. Thus we propose that bipolar kinesin motors and MTs interact by a "sliding filament mechanism" during the formation and function of the mitotic spindle.

Saccharomyces cerevisiae Duo1p and Dam1p, novel proteins involved in mitotic spindle function

In this paper, we describe the identification and characterization of two novel and essential mitotic spindle proteins, Duo1p and Dam1p. Duo1p was isolated because its overexpression caused defects in mitosis and a mitotic arrest. Duo1p was localized by immunofluorescence, by immunoelectron microscopy, and by tagging with green fluorescent protein (GFP), to intranuclear spindle microtubules and spindle pole bodies. Temperature-sensitive duo1 mutants arrest with short spindles. This arrest is dependent on the mitotic checkpoint. Dam1p was identified by two-hybrid analysis as a protein that binds to Duo1p. By expressing a GFP-Dam1p fusion protein in yeast, Dam1p was also shown to be associated with intranuclear spindle microtubules and spindle pole bodies in vivo. As with Duo1p, overproduction of Dam1p caused mitotic defects. Biochemical experiments demonstrated that Dam1p binds directly to microtubules with micromolar affinity. We suggest that Dam1p might localize Duo1p to intranuclear microtubules and spindle pole bodies to provide a previously unrecognized function (or functions) required for mitosis.

The rim sign in hepatic abscess: case report and review of the literature

We studied a previously healthy patient who presented with a 3-wk history of fever, flu-like symptoms and abdominal pain.

METHODS

Blood cultures were positive for Escherichia coli. A computed tomography (CT) scan revealed a 2-cm low-density focus in the right hepatic lobe. A technetium-99m-mebrofenin scan showed a photopenic area in the right hepatic lobe surrounded by a rim of activity greater than the adjacent parenchymal activity.

RESULTS

Gallbladder visualization was normal and the diagnosis of hepatic abscess was made. CT-guided percutaneous drainage of the lesion yielded six cc of pus, the culture of which grew E. coli, Prevotella and Bacteroides fragilis. Drainage and a 6-wk course of intravenous antibiotics were followed by clinical improvement and resolution of the abscess by CT.

CONCLUSION

The rim sign and its possible mechanism of causation in hepatic abscess are discussed in this report, together with a review of the literature.

Three-dimensional ultrastructural analysis of the Saccharomyces cerevisiae mitotic spindle

The three dimensional organization of microtubules in mitotic spindles of the yeast Saccharomyces cerevisiae has been determined by computer-aided reconstruction from electron micrographs of serially cross-sectioned spindles. Fifteen spindles ranging in length from 0.6-9.4 microns have been analyzed. Ordered microtubule packing is absent in spindles up to 0.8 micron, but the total number of microtubules is sufficient to allow one microtubule per kinetochore with a few additional microtubules that may form an interpolar spindle. An obvious bundle of about eight interpolar microtubules was found in spindles 1.3-1.6 microns long, and we suggest that the approximately 32 remaining microtubules act as kinetochore fibers. The relative lengths of the microtubules in these spindles suggest that they may be in an early stage of anaphase, even though these spindles are all situated in the mother cell, not in the isthmus between mother and bud. None of the reconstructed spindles exhibited the uniform populations of kinetochore microtubules characteristic of metaphase. Long spindles (2.7-9.4 microns), presumably in anaphase B, contained short remnants of a few presumed kinetochore microtubules clustered near the poles and a few long microtubules extending from each pole toward the spindle midplane, where they interdigitated with their counterparts from the other pole. Interpretation of these reconstructed spindles offers some insights into the mechanisms of mitosis in this yeast.

Interpolar spindle microtubules in PTK cells

Spindle microtubules (MTs) in PtK1 cells, fixed at stages from metaphase to telophase, have been reconstructed using serial sections, electron microscopy, and computer image processing. We have studied the class of MTs that form an interdigitating system connecting the two spindle poles (interpolar MTs or ipMTs) and their relationship to the spindle MTs that attach to kinetochores (kMTs). Viewed in cross section, the ipMTs cluster with antiparallel near neighbors throughout mitosis; this bundling becomes much more pronounced as anaphase proceeds. While the minus ends of most kMTs are near the poles, those of the ipMTs are spread over half of the spindle length, with at least 50% lying > 1.5 microns from the poles. Longitudinal views of the ipMT bundles demonstrate a major rearrangement of their plus ends between mid- and late anaphase B. However, the minus ends of these MTs do not move appreciably farther from the spindle midplane, suggesting that sliding of these MTs contributes little to anaphase B. The minus ends of ipMTs are markedly clustered in the bundles of kMTs throughout anaphase A. These ends lie close to kMTs much more frequently than would be expected by chance, suggesting a specific interaction. As sister kinetochores separate and kMTs shorten, the minus ends of the kMTs remain associated with the spindle poles, but the minus ends of many ipMTs are released from the kMT bundles, allowing the spindle pole and the kMTs to move away from the ipMTs as the spindle elongates.