Multi-scale imaging of anticancer platinum(iv) compounds in murine tumor and kidney

Nano-scale secondary ion mass spectrometry (NanoSIMS) enables trace element and isotope analyses with high spatial resolution. This unique capability has recently been exploited in several studies analyzing the subcellular distribution of Au and Pt anticancer compounds. However, these studies were restricted to cell culture systems. To explore the applicability to the in vivo setting, we developed a combined imaging approach consisting of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), NanoSIMS and transmission electron microscopy (TEM) suitable for multi-scale detection of the platinum distribution in tissues. Applying this approach to kidney and tumor samples upon administration of selected platinum(iv) anticancer prodrugs revealed uneven platinum distributions on both the organ and subcellular scales. Spatial platinum accumulation patterns were quantitatively assessed by LA-ICP-MS in histologically heterogeneous organs (e.g., higher platinum accumulation in kidney cortex than in medulla) and used to select regions of interest for subcellular-scale imaging with NanoSIMS. These analyses revealed cytoplasmic sulfur-rich organelles accumulating platinum in both kidney and malignant cells. Those in the tumor were subsequently identified as organelles of lysosomal origin, demonstrating the potential of the combinatorial approach for investigating therapeutically relevant drug concentrations on a submicrometer scale.

Mdm38 protein depletion causes loss of mitochondrial K+/H+ exchange activity, osmotic swelling and mitophagy

Loss of the MDM38 gene product in yeast mitochondria results in a variety of phenotypic effects including reduced content of respiratory chain complexes, altered mitochondrial morphology and loss of mitochondrial K(+)/H(+) exchange activity resulting in osmotic swelling. By use of doxycycline-regulated shut-off of MDM38 gene expression, we show here that loss of K(+)/H(+) exchange activity and mitochondrial swelling are early events, associated with a reduction in membrane potential and fragmentation of the mitochondrial reticulum. Changes in the pattern of mitochondrially encoded proteins are likely to be secondary to the loss of K(+)/H(+) exchange activity. The use of a novel fluorescent biosensor directed to the mitochondrial matrix revealed that the loss of K(+)/H(+) exchange activity was immediately followed by morphological changes of mitochondria and vacuoles, the close association of these organelles and finally uptake of mitochondrial material by vacuoles. Nigericin, a K(+)/H(+) ionophore, fully prevented these effects of Mdm38p depletion. We conclude that osmotic swelling of mitochondria triggers selective mitochondrial autophagy or mitophagy.

High-pressure freezing of epithelial cells on sapphire coverslips

Rapid freezing of cell monolayers at ambient pressure is limited regarding the thickness of ice crystal damage-free freezing. The specific freezing conditions of the cells under investigation are decisive for the success of such methods. Improved reproducibility of results could be expected by cryoimmobilization at high pressure because this achieves a greater thickness of adequate freezing. In a novel approach, we tested the suitability of sapphire discs as cell substrata for high-pressure freezing. Frozen samples on sapphire were subjected to freeze-substitution while in the same flat sample holders as used for high-pressure freezing. We obtained cells that displayed an excellent preservation of fine structure. Because sapphire is a tissue culture substratum suitable for light microscopy, its use in combination with high-pressure freezing could become a powerful tool in correlative studies of cell dynamics at light and electron microscopic levels.

Cryofixation of epithelial cells grown on sapphire coverslips by impact freezing

Rapid cryofixation of cells cultured on coverslips without the use of chemical fixatives has proved advantageous for the immunolocalization of antigens by electron microscopy. Here, we demonstrate the application of sapphire-attached tissue culture cells (PtK2 epithelial cells and mouse myoblasts) to metal-mirror impact freezing. The potential of the Leica EM-CPC cryoworkstation for routine freezing and for safe transfer of the cryofrozen samples into a sapphire disc magazine for freeze-substitution (SD-FS unit) has been exploited. Subsequently, the SD-FS unit has been tested for its use in methanol freeze-substitution and low temperature embedding for immunoelectron microscopy. The structural preservation of Lowicryl HM20-embedded cells has been assessed as being free of damage by large ice crystals.

Salmonella-induced caspase-2 activation in macrophages: a novel mechanism in pathogen-mediated apoptosis

The enterobacterial pathogen Salmonella induces phagocyte apoptosis in vitro and in vivo. These bacteria use a specialized type III secretion system to export a virulence factor, SipB, which directly activates the host's apoptotic machinery by targeting caspase-1. Caspase-1 is not involved in most apoptotic processes but plays a major role in cytokine maturation. We show that caspase-1-deficient macrophages undergo apoptosis within 4-6 h of infection with invasive bacteria. This process requires SipB, implying that this protein can initiate the apoptotic machinery by regulating components distinct from caspase-1. Invasive Salmonella typhimurium targets caspase-2 simultaneously with, but independently of, caspase-1. Besides caspase-2, the caspase-1-independent pathway involves the activation of caspase-3, -6, and -8 and the release of cytochrome c from mitochondria, none of which occurs during caspase-1-dependent apoptosis. By using caspase-2 knockout macrophages and chemical inhibition, we establish a role for caspase-2 in both caspase-1-dependent and -independent apoptosis. Particularly, activation of caspase-1 during fast Salmonella-induced apoptosis partially relies on caspase-2. The ability of Salmonella to induce caspase-1-independent macrophage apoptosis may play a role in situations in which activation of this protease is either prevented or uncoupled from the induction of apoptosis.

Association of mitochondria with plectin and desmin intermediate filaments in striated muscle

Plectin (M(r) > 500,000) is a versatile and widely expressed cytolinker protein. In striated muscle it is predominantly found at the Z-disc level where it colocalizes with the intermediate filament protein desmin. Both proteins show altered labeling patterns in tissues of muscular dystrophy patients. Moreover, mutations in the plectin gene lead to the autosomal recessive human disorder epidermolysis bullosa simplex with muscular dystrophy, and defects in the desmin gene have been shown to cause familiar cardiac and skeletal myopathy. Since intermediate filaments (IFs) in striated muscle tissue have been found to be intimately associated with mitochondria, we investigated whether plectin is involved in this association. Using postembedding immunogold labeling of Lowicryl sections and immunogold labeling of ultrathin cryosections, we show that plectin is associated with desmin IFs linking myofibrils to mitochondria at the level of the Z-disc and along the entire length of the sarcomere. The localization of plectin label at the mitochondrial membrane itself was consistent with a putative linker function of plectin between desmin IFs and the mitochondrial surface. In mitochondrion-rich muscle fibers, both plectin and desmin were part of an ordered arrangement of mitochondrial side branches, which wound around myofibrils adjacent to the Z-discs and were anchored into a filamentous network transversing from one fibril to the other. The association of mitochondria with plectin and IFs was seen also in tissues without regular distribution patterns of mitochondria, such as heart muscle and neonatal skeletal muscle tissues. These data were supplemented with in vitro binding assays showing direct interaction of plectin with desmin via its carboxy-terminal IF-binding domain. As a cytolinker protein associated with mitochondria and desmin IFs, plectin could play an important role in the positioning and shape formation, in particular branching, of mitochondrial organelles in striated muscle tissues.

Nucleolar segregation during apoptosis of haemopoietic stem cell line FDCP-Mix

The programmed elimination of cells during apoptosis is distinct from necrosis both morphologically and biochemically. Currently, the morphological description of apoptosis discriminates between the segregation of the nucleolus and the so called 'chromatin condensation'. The latter originates from observations of electron dense material adjacent to the nuclear envelope of apoptotic nuclei. Although there is ample evidence for an involvement of DNA in electron dense marginations, their true nature is still unknown. By studying apoptosis in FDCP-Mix, a pluripotent murine haemopoietic stem cell line, we found morphological and histochemical evidence that electron dense material at the nuclear envelope (NE) has emerged as a result of the segregation of nucleoli in association with the nuclear membrane. The remaining electron dense and homogenous bulk of the nucleolus labels for RNAse-gold, but even more intensely for DNAse-gold, and therefore could possibly be mistaken as 'condensed chromatin' in the light microscope. The labelling of the electron dense material for DNase-gold in FDCP-Mix could be explained by a migration of DNA into the bulk of the nucleoli at an early stage of cell death.

Accessing nuclear structure for field emission, in lens, scanning electron microscopy (FEISEM)

Scanning electron microscopy (SEM) has had a shorter time course in biology than conventional transmission electron microscopy (TEM) but has nevertheless produced a wealth of images that have significantly complemented our perception of biological structure and function from TEM information. By its nature, SEM is a surface imaging technology, and its impact at the subcellular level has been restricted by the considerably reduced resolution in conventional SEM in comparison to TEM. This restriction has been removed by the recent advent of high-brightness sources used in lensfield emission instruments (FEISEM) which have produced resolution of around 1 nanometre, which is not usually a limiting figure for biological material. This communication reviews our findings in the use of FEISEM in the imaging of nuclear surfaces, then associated structures, such as nuclear pore complexes, and the relationships of these structures with cytoplasmic and nucleoplasmic elements. High resolution SEM allows the structurally orientated cell biologist to visualise, directly and in three dimensions, subcellular structure and its modulation with a view to understanding, its functional significance. Clearly, intracellular surfaces require separation from surrounding structural elements in vivo to allow surface imaging, and we review a combination of biochemical and mechanical isolation methods for nuclear surfaces.

Linking integrin alpha6beta4-based cell adhesion to the intermediate filament cytoskeleton: direct interaction between the beta4 subunit and plectin at multiple molecular sites

Recent studies with patients suffering from epidermolysis bullosa simplex associated with muscular dystrophy and the targeted gene disruption in mice suggested that plectin, a versatile cytoskeletal linker and intermediate filament-binding protein, may play an essential role in hemidesmosome integrity and stabilization. To define plectin's interactions with hemidesmosomal proteins on the molecular level, we studied its interaction with the uniquely long cytoplasmic tail domain of the beta4 subunit of the basement membrane laminin receptor integrin alpha6beta4 that has been implicated in connecting the transmembrane integrin complex with hemidesmosome-anchored cytokeratin filaments. In vitro binding and in vivo cotransfection assays, using recombinant mutant forms of both proteins, revealed their direct interaction via multiple molecular domains. Furthermore, we show in vitro self-interaction of integrin beta4 cytoplasmic domains, as well as disruption of intermediate filament network arrays and dislocation of hemidesmosome-associated endogenous plectin upon ectopic overexpression of this domain in PtK2 and/or 804G cells. The close association of plectin molecules with hemidesmosomal structures and their apparent random orientation was indicated by gold immunoelectron microscopy using domain-specific antibodies. Our data support a model in which plectin stabilizes hemidesmosomes, via directly interlinking integrin beta4 subunits and cytokeratin filaments.

Apoptosis in haemopoietic progenitor cells exposed to extremely low-frequency magnetic fields

Epidemiological studies have indicated a modestly increased risk for the development of acute myeloid leukaemia in children who live close to high-voltage power-lines. Recent evidence has suggested that a common property shared by a number of known and suspected tumour promoters is their ability to block the process of apoptosis. Therefore, one possible mechanistic explanation for the apparent leukaemogenic effect of weak, low-frequency magnetic fields, such as emitted by power-lines and electrical appliances, would be their expression of tumour-promoting activity by interfering with the regulation of apoptosis in multipotent haemopoietic progenitor cells. In order to test this hypothesis, we have employed the well-characterized multipotential haemopoietic progenitor cell line FDCP-mix(A4). These cells are non-leukaemic and undergo apoptosis when deprived of appropriate growth factors such as Interleukin-3. We have tested a series of different regimes of weak, low-frequency magnetic fields: nulled fields, Ca2+-ion cyclotron resonance conditions at 50 Hz, and vertical 50 Hz fields of 6 microT(RMS), 1 mT(RMS) and 2 mT(RMS), exposing the cells for 2 hours, 24 hours, 4 days or 7 days under various culture conditions. We have not seen any significant alteration in apoptosis induced by any of the exposure regimes tested. We therefore conclude that the regulation of viability and apoptosis in FDCP-mix(A4) cells is not disturbed by weak magnetic fields of the magnitude and type indicated.

Macromolecular substructure in nuclear pore complexes by in-lens field-emission scanning electron microscopy

Scanning electron microscopy (SEM) has produced a wealth of novel images that have significantly complemented our perception of biological structure and function, derived initially from transmission electron microscopy (TEM) information. SEM is a surface imaging technology, and its impact at the subcellular level has been restricted by reduced resolution in comparison with TEM. Recently, SEM resolution has been considerably improved by the advent of high-brightness sources used in field-emission instruments (FEISEM) which have produced resolution of around 1 nm, virtually equivalent to TEM "working resolution." Here we review our findings in the use of FEISEM in the imaging of nuclear envelopes and their associated structures, such as nuclear pore complexes, and the relationships of structure and function. FEISEM allows the structurally orientated cell biologist to visualise, directly and in three dimensions, subcellular structure and its modulation with a view to understanding its functional significance.

DNA inclusions within autolytic cytoplasmic vacuoles of hemopoietic stem cell line FDCP-Mix

FDCP-Mix, a pluripotent routine hemopoietic stem cell line undergoes internucleosomal cleavage of DNA when induced to apoptosis either by drugs or by withdrawal of growth factor (IL-3), and also displays a pattern of nuclear morphology that is typical for apoptosis. However, increased autolytic activity in the cytoplasm precedes the nuclear changes. For etoposide-treated FDCP-Mix cells, mitochondria were identified as a target for autolytic digestion in large autolytic vacuoles, but during this period an increase in the number of mitochondria was observed. The autolytic vacuoles displayed variations in their content. Large, electron-dense inclusions resembling "condensed chromatin" could regularly be found in FDCP-Mix cells treated with low concentrations of etoposide (<4 microM). Confocal fluorescence microscopy and DNAse-gold labeling were employed to demonstrate the presence of DNA in the formation of the electron-dense inclusions within autolytic vacuoles. The identification of mitochondrial macroautophagy, the evidence for an etoposide-induced proliferation of mitochondria, and the fact that electron-dense inclusions are formed at a stage when the morphology of the nucleus is still not effected, suggests that the DNA within the autolytic vacuoles may be of mitochondrial origin.

Nuclear pore clustering is a consistent feature of apoptosis in vitro

Two cell lines which show different patterns of DNA fragmentation have been examined for variations of their nuclear morphology during apoptosis. FDCP-Mix, a pluripotent murine haemopoietic stem cell line which undergoes typical internucleosomal cleavage of DNA when induced to apoptosis either by drugs or withdrawal of growth factor (IL-3) was compared with the human lymphoid leukemia cell line MOLT-4, a cell line which undergoes apoptosis without production of a typical DNA 'ladder'. The nuclear morphology of FDCP-Mix cells was consistent after apoptotic induction by drug or by growth factor withdrawal. Apoptotic nuclear morphology for MOLT-4 and FDCP-Mix showed variations in the distribution, density and texture of the electron dense nuclear marginations. Despite these differences, clustering of nuclear pore complexes (NPCs) after treatment with the topoisomerase II inhibitor etoposide was a common phenomenon for both cell lines. Moreover, pore clustering for FDCP-Mix nuclei occurred independently from the way in which apoptosis was induced, either by growth factor withdrawal or etoposide treatment. In a novel approach, we visualised the clustering of NPCs three-dimensionally by field emission in-lens scanning electron microscopy (FEISEM).

Changes of mitochondrial mass in the hemopoietic stem cell line FDCP-mix after treatment with etoposide: a correlative study by multiparameter flow cytometry and confocal and electron microscopy

FDCP-Mix, a pluripotent murine hemopoietic stem cell line which undergoes typical internucleosomal cleavage of DNA when induced to apoptosis by either drugs or withdrawal of growth factor (interleukin-3) was studied after treatment with the topoisomerase II inhibitor etoposide (0.5-4 microM). An increase in autolytic activity was the major early morphological change within the cytoplasm, with mitochondria as the main target for autolytic digestion. Despite this macroautophagy, thin sections showed a high number of mitochondria, suggesting mitochondrial proliferation as a result of drug treatment. This observation of an increase in the number of mitochondria was confirmed by flow cytometric studies of mitochondrial overall mass. Multiparameter flow cytometry of cells double stained with propidium iodide and nonyl-acridine orange gave an accurate assay for mitochondrial mass in relation to cell cycle stages. The increase in mitochondrial mass was found in all cell cycle stages. The results suggest a drug-induced proliferation of mitochondria separate from the processes involved in the doubling of mitochondrial mass during the cell cycle and a decline of mitochondria in the later stages of apoptosis.

Preparation of isolated nuclei from K 562 haemopoietic cell line for high resolution scanning electron microscopy

The aim of the work is to visualise nuclear pore complexes (NPCs) in mammalian cells by high resolution scanning electron microscopy. A detergent-free isolation protocol was employed to obtain clean nuclei from the haemopoietic cell line K 562. Nuclear isolation was performed by mechanical homogenisation under hypotonic conditions followed by purification of the nuclear fraction. The isolated nuclei were attached to silicon chips, fixed, critical point dried, and sputter coated with a thin film (3-4 nm) of tantalum. Analysis of the nuclear surface by scanning electron microscopy (SEM) revealed a strong sensitivity of the outer nuclear membrane (ONM) to disruption during the isolation procedure. A significant reduction of the characteristic pattern of damage to the ONM was achieved by means of an isopicnic centrifugation on an isoosmolar balanced Percoll gradient. Analysis of the population of isolated nuclei by flow cytometry showed no signs of cell cycle specific losses of nuclei during isolation. The SEM investigations of the morphology of the nuclear envelope (NE) and of substructural details of NPCs and polyribosomes were performed using an in-lens field emission scanning electron microscope.