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PubMed1997

Silicate core-organic refractory mantle particles as interstellar dust and as aggregated in comets and stellar disks.

The principal observational properties of silicate core-organic refractory mantle interstellar dust grains in the infrared at 3.4 microns and at 10 microns and 20 microns are discussed in terms of the cyclic evolution of particles forming in stellar atmospheres and undergoing subsequent accretion, photoprocessing and destruction (erosion). Laboratory plus space emulation of the photoprocessing of laboratory analog ices and refractories are discussed. The aggregated interstellar dust model of comets is summarized. The same properties required to explain the temperature and infrared properties of comet coma dust are shown to be needed to account for the infrared silicate and continuum emission of the beta Pictoris disk as produced by a cloud of comets orbiting the star.

Cosmic Dust↗
PubMed1995

Comets and life in the Universe.

The notion that comets supplied the primitive Earth with the requisite chemical species for the process of chemical evolution, which is widely held to have led to the origin of life on Earth, has now gained considerable intellectual momentum since its first formulation in 1961. In fact, in the fall of 1991, the University of Wisconsin-Eau Claire hosted a well attended scientific meeting devoted solely to the discussion of this topic, entitled Comets and the Origin and Evolution of Life [see Special Issue of Origins of Life, P.J. Thomas (ed), vol. 21(5-6)]. As a result of the above meeting, the recently completed COSPAR/World Space Congress Symposium on Extraterrestrial Organic Chemistry and the Origins of Life, and numerous independent reports, the role of comets in the Earth's biogenesis has been thoroughly addressed in the literature. At this time, in light of a few recent findings, we present here a concise review of this topic together with a brief discussion of the possible role of cometary material in the origin of life elsewhere in the Universe.

Carbon↗
PubMed1997

The contribution of cytotoxicity to DNA-effects in the single cell gel test (comet assay).

We evaluated genotoxic and cytotoxic effects of the three non-mutagenic and non-carcinogenic compounds p-nitrophenol, D-menthol and sodium N-lauroyl sarcosine which have previously been shown to induce DNA double strand breaks (DNA dsb) secondary to induced cytotoxicity. We tested whether genotoxic effects in the alkaline single cell gel test (comet assay) may be confounded by cytotoxicity-induced DNA dsb. Cell viability was determined at the end of the treatment using the fluorescein diacetate/ethidium bromide-assay and plating efficiency was used as an indicator of long-term survivability. Experiments with V79 Chinese hamster cells and human white blood cells revealed negative results in the comet assay despite strong cytotoxic effects. However, cells with extremely fragmented DNA ('clouds') occurred but were excluded from the evaluation under the principle that they represent dead cells. We also noticed a significant loss of cells at cytotoxic concentrations that might be attributed to the induction of highly fragmented DNA which is lost during electrophoresis. Since the comet assay allows the determination of DNA effects on the single cell level, a confounding effect of cytotoxicity on test results can be avoided.

Animals↗
PubMed1997

Detection of MNNG-induced DNA lesions in mammalian cells; validation of comet assay against DNA unwinding technique, alkaline elution of DNA and chromosomal aberrations.

Human cells (VH10 or Hep G2) and hamster cells V79 were exposed to different concentrations of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and the level of DNA lesions was evaluated by the DNA unwinding technique, alkaline elution of DNA and the comet assay. All three methods were able to detect the effects of MNNG but with a clear difference in sensitivity. At low concentrations of MNNG the most sensitive method appeared to be the comet assay. After the short-term treatment the comet assay was able to detect the lesions induced by MNNG at approx. 0.1 microgram/ml, alkaline elution of DNA at 1 microgram/ml and DNA unwinding at 1-2 micrograms/ml. MNNG treated VH10 cells, human lymphocytes and V79 cells were also tested cytogenetically, confirming that MNNG induced chromosomal aberrations at concentrations > 1 microgram/ml in VH10 cells (short-term treatment): > 0.2 microgram/ml in V79 cells (long-term treatment) and > 8 micrograms/ml in human lymphocytes (long-term treatment). In some experiments we tried to increase the level of MNNG-induced DNA breaks with help of DNA repair inhibitors cytosine arabinoside (Ara C) and hydroxyurea (HU) which were applied either after or during MNNG treatment. Our results showed that the level of MNNG-induced lesions was increased by simultaneous treatment of cells with MNNG and Ara C and HU. 2 x 10(-5) M Ara C and 2 x 10(-3) MHU were as effective as 10-times higher concentrations of inhibitors. Ara C and HU increased the level of MNNG-induced DNA breaks mainly in combination with lower concentrations of MNNG (< 2 micrograms/ml). Rejoining of DNA breaks was observed in human cells VH10 and Hep G2 as well as in Chinese hamster cells V79 damaged by both lower and higher MNNG-concentrations. All methods showed that MNNG-induced DNA breaks had been gradually rejoined.

Animals↗
PubMed1998

Evaluation of a tissue homogenization technique that isolates nuclei for the in vivo single cell gel electrophoresis (comet) assay: a collaborative study by five laboratories.

We evaluated a tissue homogenization technique that isolates nuclei for use in the in vivo comet assay. Five laboratories independently tested the technique using the liver, kidney, lung, spleen, and bone marrow of untreated and mutagen-treated male CD-1 mice. The direct mutagen methylmethanesulfonate (MMS) or the promutagen diethylnitrosamine (DEN) were injected intraperitoneally at maximum tolerated doses. Three and twenty-four hours later, the organs were removed and, except for bone marrow, were minced and homogenized and a nuclear suspension was prepared. The nuclear suspensions and bone marrow cells were used in the comet assay. None of the nuclear suspensions from the non-treated mice induced a positive response. All nuclear suspensions derived from the MMS-treated mice and those of the liver, kidney, and lung from DEN-treated mice induced positive responses in all the laboratories similarly. Reproducibility was demonstrated by five replicate studies in one laboratory. Furthermore, the organ-specific responses to MMS and DEN reflected the characteristic genotoxicity of the chemicals. We concluded from these results that the homogenization technique is a valid one to be used for mouse organs in the in vivo comet assay.

Animals↗
PubMed1999

Identification by microscopically controlled comet assay of peritoneal macrophages in a mixture of peritoneal exudate for DNA strand break analysis.

A simple modification of the alkaline comet assay allows the study of DNA damage in a specific cell type in a mixture of primary cells. Peritoneal macrophages from mice are selected from other peritoneal exudate cells without complex preparation and separation steps by their size and shape of the nuclei and their comets. The DNA damage can be well characterised by the manually monitored parameter 'tail length'. Complex measurement of the 'tail moment', often used for characterising DNA damage is not required, a fact which further simplifies the protocol. The distribution of tail length within one sample is symmetric and can be described by a Gaussian distribution and the mean tail length. As a first application, UV-A sensitivity of resident and stimulated macrophages was studied. The resident macrophages were more sensitive to UV-A than the stimulated ones. DNA damage repair follows the same simple monoexponential time course for both cell types. The simplicity of results, i.e., applicability of tail lengths and Gaussian statistics as well as monoexponential kinetics, suggest that microscopically controlled comet assay is well suited to study elementary processes of DNA damage induction and repair.

Animals↗
PubMed1999

Genotoxicity of inhalation anesthetics halothane and isoflurane in human lymphocytes studied in vitro using the comet assay.

The alkaline single cell gel electrophoresis (comet) assay was applied to study genotoxic properties of two inhalation anesthetics-halothane and isoflurane-in human peripheral blood lymphocytes (PBL). The cells were exposed in vitro to either halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) or isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) at concentrations 0.1-10 mM in DMSO. The anesthetics-induced DNA strand breaks as well as alkali-labile sites were measured as total comet length (i.e., increase of a DNA migration). Both analysed drugs were capable of increasing DNA migration in a dose-dependent manner. In experiments conducted at two different electrophoretic conditions (0. 56 and 0.78 V/cm), halothane was able to increase DNA migration to a higher extent than isoflurane. The comet assay detects DNA strand breaks induced directly by genotoxic agents as well as DNA degradation due to cell death. For this reason a contribution of toxicity in the observed effects was examined. We tested whether the exposed PBL were able to repair halothane- and isoflurane-induced DNA damage. The treated cells were incubated in a drug-free medium at 37 degrees C for 120 min to allow processing of the induced DNA damage. PBL exposed to isoflurane at 1 mM were able to complete repair within 60 min whereas for halothane a similar result was obtained at a concentration lower by one order of magnitude: the cells exposed to halothane at 1 mM removed the damage within 120 min only partly. We conclude that the increase of DNA migration induced in PBL by isoflurane at 1 mM and by halothane at 0.1 mM was not a result of cell death-associated DNA degradation but was caused by genotoxic action of the drugs. The DNA damage detected after the exposure to halothane at 1 mM was in part a result of DNA fragmentation due to cell death.

Adult↗
PubMed1999

Comparative in vitro and in vivo assessment of genotoxic effects of etoposide and chlorothalonil by the comet assay.

The alkaline single cell gel electrophoresis (comet) assay was used to assess in vitro and in vivo genotoxicity of etoposide, a topoisomerase II inhibitor known to induce DNA strand breaks, and chlorothalonil, a fungicide widely used in agriculture. For in vivo studies, rats were sacrificed at various times after treatment and the induction of DNA strand breaks was assessed in whole blood, bone marrow, thymus, liver, kidney cortex and in the distal part of the intestine. One hour after injection, etoposide induced DNA damage in all organs studied except kidney, especially in bone marrow, thymus (presence of HDC) and whole blood. As observed during in vitro comet assay on Chinese hamster ovary (CHO) cells, dose- and time-dependent DNA effects occurred in vivo with a complete disappearance of damage 24 h after administration. Even though apoptotic cells were detected in vitro 48 h after cell exposure to etoposide, such a result was not found in vivo. After chlorothalonil treatment, no DNA strand breaks were observed in rat organs whereas a clear dose-related DNA damage was observed in vitro. The discrepancy between in vivo and in vitro models could be explained by metabolic and mechanistic reasons. Our results show that the in vivo comet assay is able to detect the target organs of etoposide and suggest that chlorothalonil is devoid of appreciable in vivo genotoxic activity under the protocol used.

Animals↗
PubMed2000

The comet assay in eight mouse organs: results with 24 azo compounds.

The genotoxicity of 24 azo compounds selected from IARC (International Agency for Research on Cancer) groups 2A, 2B, and 3 were determined by the comet (alkaline single cell gel electrophoresis, SCG) assay in eight mouse organs. We treated groups of four mice once orally at the maximum tolerated dose (MTD) and sampled stomach, colon, liver, kidney, bladder, lung, brain, and bone marrow 3, 8, and 24 h after treatment. For the 17 azo compounds, the assay was positive in at least one organ; (1) 14 and 12 azo compounds induced DNA damage in the colon and liver, respectively, (2) the genotoxic effect of most of them was greatest in the colon, and (3) there were high positive responses in the gastrointestinal organs, but those organs are not targets for carcinogenesis. One possible explanation for this discrepancy is that the assay detects DNA damage induced shortly after administration of a relatively high dose, while carcinogenicity is detected after long treatment with relatively low doses. The metabolic enzymes may become saturated following high doses and the rates and pathways of metabolic activation and detoxification may differ following high single doses vs. low long-term doses. Furthermore, considering that spontaneous colon tumors are very rare in rats and mice, the ability to detect tumorigenic effects in the colon of those animals might be lower than the ability to detect genotoxic events in the comet assay. The in vivo comet assay, which has advantage of reflecting test chemical absorption, distribution, and excretion as well as metabolism, should be effective for estimating the risk posed by azo dyes to humans in spite of the difference in dosage regimen.

Animals↗
PubMed2002

Rationale and design of the carvedilol or metoprolol European trial in patients with chronic heart failure: COMET.

In large clinical trials both carvedilol and metoprolol have been shown to reduce mortality and morbidity in patients with chronic heart failure. Carvedilol is an adrenoceptor antagonist, which inhibits beta(1)-, beta(2)-, and alpha(1)-adrenergic receptors. Carvedilol has additional metabolic and antioxidant properties. Metoprolol is a selective antagonist of beta(1)-adrenergic receptors. The carvedilol or metoprolol European trial (COMET) is the first study to investigate whether beta-blocking agents with differing pharmacological profiles exert different effects on morbidity and mortality in patients with chronic heart failure. 3029 patients from 15 different European countries were enrolled into COMET and will be followed until 1020 fatal events have been observed, unless the data and safety monitoring committee (DSMC) recommends early termination. The target dose for carvedilol is 25 mg bid and for metoprolol tartrate 50 mg bid. This manuscript outlines the rationale, design and possible outcomes of COMET.

Adrenergic beta-Antagonists↗
PubMed1997

The HNC/HCN ratio in comets.

The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4-3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced in significant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus.

Astronomical Phenomena↗
PubMed2005

Extraterrestrial meteors: a martian meteor and its parent comet.

Regular meteor showers occur when a planet approaches the orbit of a periodic comet--for example, the Leonid shower is evident around 17 November every year as Earth skims past the dusty trail of comet Tempel-Tuttle. Such showers are expected to occur on Mars as well, and on 7 March last year, the panoramic camera of Spirit, the Mars Exploration Rover, revealed a curious streak across the martian sky. Here we show that the timing and orientation of this streak, and the shape of its light curve, are consistent with the existence of a regular meteor shower associated with the comet Wiseman-Skiff, which could be characterized as martian Cepheids.

Journal Article↗
PubMed1998

Comet-tailed drusen of the retinal pigment epithelium in angioid streaks.

PURPOSE: To describe a fundus finding in angioid streaks, namely comet-tailed drusen of the retinal pigment epithelium. METHODS: A review was carried out of fundus photographs in 110 cases of angioid streaks. RESULTS: Five patients had comet-tailed drusen and the halo tail was radial and parallel to adjacent angioid streaks. CONCLUSIONS: Comet-tailed drusen may represent zones of minor stress of the retinal pigment epithelium--Bruch's membrane complex compared with zones of major stress as in angioid streaks.

Adolescent↗
PubMed2003

The evolution of comets in the Oort cloud and Kuiper belt.

Comets are remnants from the time when the outer planets formed, approximately 4-4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.

Evolution, Chemical↗
PubMed2003

Compression forces generated by actin comet tails on lipid vesicles.

Polymerizing networks of actin filaments generate force for a variety of movements in living cells, including protrusion of filopodia and lamellipodia, intra- and intercellular motility of certain bacterial and viral pathogens, and motility of endocytic vesicles and other membrane-bound organelles. During actin-based motility, coexisting populations of actin filaments exert both pushing and retarding forces on the moving cargo. To examine the distribution and magnitude of forces generated by actin, we have developed a model system where large artificial lipid vesicles coated with the protein ActA from the bacterial pathogen Listeria monocytogenes are propelled by actin polymerization in cytoplasmic extract. We find that motile vesicles associated with actin comet tails are significantly deformed due to an inward compression force exerted by actin polymerization orthogonal to the direction of motion, which is >10-fold greater in magnitude than the component of the force exerted in the direction of motion. Furthermore, there is a spatial segregation of the pushing and retarding forces, such that pushing predominates along the sides of the vesicle, although retarding forces predominate at the rear. We estimate that the total net (pushing minus retarding) force generated by the actin comet tail is approximately 0.4-4 nN. In addition, actin comet tail formation is associated with polarization of the ActA protein on the fluid vesicle surface, which may reinforce the persistence of unidirectional motion by helping to maintain a persistent asymmetry of actin filament density.

Actins↗
PubMed1994

Comet assay studies of radiation-induced DNA damage and repair in various tumour cell lines.

We used the 'comet assay' to compare the amount of radiation-induced DNA damage in three tumour cell lines (MeWo, PECA 4451 and PECA 4197) and the extent of DNA repair in two of these lines (MeWo and PECA 4197). Tumour cells were irradiated with X-rays (0.1-10 Gy), embedded in agarose on slides, lysed with sodium dodecyl sulphate and exposed to an electric field. DNA migrated within the agarose and formed comets whose length depended on the amount of DNA damage. When the cells were incubated at 37 degrees C for various time intervals before electrophoresis started, the comets shrank in the course of time, indicating repair of DNA damage. All three cell lines showed the same extent of DNA damage after radiation exposure, despite the fact that in the colony-forming assay MeWo and PECA 4451 were much more sensitive to radiation exposure than PECA 4197. The repair characteristics, however, were markedly different for MeWo and PECA 4197 cells. PECA 4197 cells showed a much faster restoration of the original shape of the cell nucleus than MeWo cells.

DNA Damage↗
PubMed1996

Use of the alkaline comet assay to detect DNA repair deficiencies in human fibroblasts exposed to UVC, UVB, UVA and gamma-rays.

The alkaline single cell gel electrophoresis (comet) assay applied to human fibroblasts allowed us to analyze the response to components of the solar spectrum (UVB and UVA) in comparison with the well-established response to UVC and gamma-rays. DNA strand breaks related to nucleotide excision repair of DNA photoproducts were produced 1 h after exposure to UVB or UVC in the normal cell line but not in the repair deficient XPD and TTD-2 cell lines. In contrast, the immediate production of DNA strand breaks observed in all cell lines after exposure to UVA or gamma-rays was followed by restitution of high molecular weight DNA upon post-exposure incubation. These results imply that (1) fibroblasts as well as lymphocytes can be analysed by the comet assay and (2) the comet assay clearly distinguishes cellular nucleotide excision repair capacity without the use of inhibitors of DNA synthesis.

Cell Line↗
PubMed1999

Measurement of DNA base damage in cells exposed to low doses of gamma-radiation: comparison between the HPLC-EC and comet assays.

PURPOSE: Two different methods aimed at measuring DNA damage were compared. Monocytes were exposed to 60Co gamma-rays and the level of DNA damage was determined using either the HPLC-EC or comet assay. MATERIALS AND METHODS: The alkaline comet assay was used in association with the Fpg and Endo III DNA glycosylases to estimate the amount of modified bases together with strand breaks and alkali-labile sites. The HPLC-EC analysis was performed to measure 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) levels in cells. RESULTS: A correlation between these assays allowed the determination of the steady-state level and the yield of Fpg sensitive sites (assimilated to 8-oxodGuo), which were estimated to be 0.18 per 10(6) bp and 0.044 per 10(6) bp per Gy, respectively. Similar levels of Endo III sensitive sites were found. For the strand breaks and alkali-labile sites, the background level was 0.26 per 10(6) bp and the yield 0.123 per 10(6) bp per Gy. DISCUSSION: The modified comet assay appears to be an appropriate tool to estimate DNA base damage in cells exposed to low doses of gamma-radiation.

Chromatography, High Pressure Liquid↗