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Biomedical subjects

D D Brown

Publications and source records attributed to D D Brown.

At least 37 records · Page 2Linked to original sources
PubMed1997

A set of novel tadpole specific genes expressed only in the epidermis are down-regulated by thyroid hormone during Xenopus laevis metamorphosis.

Four genes were identified in a screen for thyroid hormone-induced down-regulation of gene expression in Xenopus laevis tadpole tails. All four encode extracellular glycoproteins that are expressed exclusively in the apical cell layer of the entire tadpole epidermis, which is the equivalent of the mammalian fetal periderm. The onset of the four novel genes' expression late in embryogenesis, their activity throughout the life of the tadpole, their repression by exogenously added thyroid hormone, and the spontaneous cessation of their expression at the end of tadpole life are closely coordinated. These facts suggest that the protein products of these genes form a novel albeit temporary barrier or other structure in the tadpole epidermis that functions in lieu of the cornified, stratified epithelium of the adult epidermis. We have exploited the cloning of these genes for use as cell-specific markers to follow the appearance and loss of apical cells during development. We were able to demonstrate directly that the apical cells are derived from a stratification of the embryonic ectoderm at the onset of the formation of a true epidermis. The apical cells uniformly cover the surface of the tadpole until metamorphosis, when the expression of the four larval epidermis-specific genes is lost coordinately over the entire tadpole. In contrast, the adult epidermis develops with a distinct regional specificity: adult keratin is first expressed up to a line separating the body and tail epidermis and finally appears in the tail only at metamorphic climax. Finally, our analysis reveals that the TH-induced down-regulated gene expression program during metamorphosis is very different from the previously described up-regulated program which involves multiple cell types and several waves of gene expression changes. The down-regulated program only consists of the repression of a small number of genes which are expressed in larval cells preprogrammed to die during the larval to adult transition at metamorphosis.

Amino Acid Sequence↗
PubMed1996

Truncated exponential versus damped sinusoidal waveform shocks for transthoracic defibrillation.

Currently available transthoracic defibrillators use either a damped sinusoidal or truncated exponential (TE) waveform. Truncated exponential waveforms deliver a long pulse if the transthoracic impedance is high; it has been suggested that such a long pulse may be less effective for defibrillation. Our objective was to compare the ability of damped sinusoidal (DS) waveform shocks versus TE waveform shocks to terminate ventricular fibrillation (VF) and achieve survival from witnessed cardiac arrest. We retrospectively reviewed field-recorded electrocardiograms from 86 patients with witnessed VF, treated by prehospital personnel equipped with DS or TE waveform defibrillators. Forty-four patients received 130 shocks from TE defibrillators; 42 patients received 108 shocks from DS defibrillators. There were no significant differences in time from arrest to first shock (8.0 vs 8.1 minutes), nor were there any differences in the size of the communities involved. The shocks resulted in the following rhythms: organized rhythm: TE: 15 of 130 (12%), DS: 24 of 108 (22%), p = 0.10 (NS); persistent VF: TE: 85 of 130 (65%), DS: 45 of 108 (42%), p <0.01; asystole: TE: 30 of 130 (23%), DS: 39 of 108 (36%), p = NS; and survival to hospital discharge: TE: 5 of 44 (11%), DS: 8 of 42 (19%), p = NS. We conclude that DS waveforms terminated VF more frequently than TE, but there was no significant difference in resumption of an organized rhythm or survival. A prospective comparison of these 2 waveforms is needed.

Aged↗
PubMed1996

The thyroid hormone-induced tail resorption program during Xenopus laevis metamorphosis.

Genes that are up- and down-regulated by thyroid hormone in the tail resorption program of Xenopus laevis have been isolated by a gene expression screen, sequenced, and identified in the GenBank data base. The entire program is estimated to consist of fewer than 35 up-regulated and fewer than 10 down-regulated genes; 17 and 4 of them, respectively, have been isolated and characterized. Up-regulated genes whose function can be predicted on the basis of their sequence include four transcription factors (including one of the thyroid hormone receptors), an extracellular matrix component (fibronectin) and membrane receptor (integrin), four proteinases, a deiodinase that degrades thyroid hormone, and a protein that binds the hypothalamic corticotropin-releasing factor, which has been implicated in controlling thyroid hormone synthesis in Xenopus tadpoles. All four down-regulated genes encode extracellular proteins that are expressed in tadpole epidermis. This survey of the program provides insights into the biology of metamorphosis.

Animals↗
PubMed1996

The analysis of complex developmental programmes: amphibian metamorphosis.

Metamorphosis in frogs and toads is a set of complex developmental programmes controlled by thyroid hormone (TH). The pervasive and dramatic changes that occur as a tadpole turns into a frog have captured the attention of life scientists from many disciplines. For evolutionary biologists, the extent to which related organisms incorporate metamorphosis in their life cycle is of paramount interest. For specialists who study cell death, a burgeoning field of research interest, apoptosis is a major event in metamorphosis. It can be induced in many tissues, even in whole organs, by TH. For endocrinologists, TH-induced metamorphosis is a model for the general problem of the molecular basis of TH action and the interaction of the hypothalamus, the pituitary and the thyroid glands. However, the emphasis in this essay is the use of amphibian metamorphosis to study complex developmental programmes such as vertebrate organogenesis which can be initiated by the simple addition of TH to the tadpole's rearing water.

Animals↗
PubMed1996

A comparison of four aqueous-miscible liquid scintillation cocktails with an alpha/beta discriminating Wallac 1415 liquid scintillation counter.

This work was directed towards an assessment of alpha/beta separation, alpha detection efficiency, energy resolution, quench sensitivity, and background count rate for four commercially available liquid scintillation cocktails using a Wallac 1415 liquid scintillation counter. The four cocktails were Wallac Oy OptiPhase HiSafe 3 and HiSafe 2, and Packard Ultima Gold AB and Ultima Gold XR. Aqueous radioactive solutions were prepared in 0.1 N HCl and contained 241Am (5.49 MeV alpha) and 90Sr/90Y (0.46/2.28 MeV beta-). The radioactive solutions were loaded into the cocktails at a ratio of 1 mL of aqueous solution to 10 mL of cocktail in 20 mL high-density polyethylene (HDPE) liquid scintillation vials. The effect of three levels of chemical quench on the various properties of interest was assessed for each of the LS cocktails. Alpha/beta discrimination was quantified by the fraction of beta interactions that "spillover" into the alpha window and vice versa, at the working discriminator setting. The working discriminator setting was defined as the point where the spillover of beta interactions into the alpha window and alpha interactions into the beta window were equal. For the low-quench samples, Ultima Gold AB had the lowest spillover (0.25%) and Ultima Gold XR had the highest spillover (0.8%). For the high-quench samples, the spillover ranged from 4% (HiSafe 3) to 10.5% (Ultima Gold XR). The detection efficiency for 241Am was nearly 100% and decreased with increased quench because of loss from spillover. HiSafe 2 gave the best energy resolution, 500 keV (FWHM), for the low-quench sample. The lowest background count rate was achieved with Ultima Gold AB, 0.056 cpm in the 241Am window, for a low-quench blank.

Alpha Particles↗
PubMed1994

Quantitation of endogenous thyroid hormone receptors alpha and beta during embryogenesis and metamorphosis in Xenopus laevis.

Greater than 90% of the endogenous thyroid hormone receptor proteins TR alpha and TR beta in tissues of Xenopus laevis comigrate with their respective in vitro synthesized counterparts, and these major components are not phosphorylated detectably. Maternally inherited TR alpha protein is stable through early embryogenesis during a time in which there is no detectable TR alpha mRNA synthesis. At stage 35 when TR alpha mRNA is first detectable, the inherited TR alpha protein is present at about 100 molecules/cell. TR alpha protein subsequently increases to levels of about 1500 and 6000 molecules/cell in tail and head regions, respectively, in stage 52 tadpoles. Even though TR alpha mRNA gradually increases during metamorphosis (from stage 52 to 62), TR alpha protein remains constant, suggesting strongly that post-transcriptional events control the ultimate levels of TR alpha protein. In contrast, there is no detectable TR beta protein (less than 100 molecules/cell) throughout embryogenesis until stage 52. Both TR beta mRNA and protein rise along with the increase in endogenous thyroid hormone, reaching a maximum at the climax of metamorphosis, when TR beta protein exceeds TR alpha protein in concentration. As with TR alpha protein, TR beta protein in tail is consistently about one-fourth that of TR beta protein in the head region. The number of TR alpha protein molecules in extracts of premetamorphic tadpoles and cultured cells grown in the absence of thyroid hormone fully accounts for all of the sites to which 125I-T3 bind. We interpret this to mean that TR alpha protein must be a necessary, if not sufficient, component in the pathway toward metamorphosis triggered by thyroid hormone and required for the phenomenon of competence in tissues and cells.

Amino Acid Sequence↗
PubMed1994

A thyroid hormone-regulated gene in Xenopus laevis encodes a type III iodothyronine 5-deiodinase.

The type III iodothyronine 5-deiodinase metabolizes thyroxine and 3,5,3'-triiodothyronine to inactive metabolites by catalyzing the removal of iodine from the inner ring. The enzyme is expressed in a tissue-specific pattern during particular stages of development in amphibia, birds, and mammals. Recently, a PCR-based subtractive hybridization technique has been used to isolate cDNAs prepared from Xenopus laevis tadpole tail mRNA that represent genes upregulated by thyroid hormone during metamorphosis. Sequence analysis of one of these cDNAs (XL-15) revealed regions of homology to the mRNA encoding the rat type I (outer ring) 5'-deiodinase, including a conserved UGA codon that encodes selenocysteine in the mammalian enzyme. We report here that the protein encoded by the XL-15 cDNA efficiently catalyzes the (inner ring) 5-deiodination of 3,5,3'-triiodothyronine with a Km value of 2 nM and is resistant to inhibition by propylthiouracil and aurothioglucose. Our analysis confirms that the UGA codon encodes a selenocysteine that is critical for the catalytic activity of the enzyme. In addition, the direct induction of XL-15 mRNA levels by thyroid hormone in X. laevis tadpole tail tissue and cultured cell lines correlates closely with increases in 5- (but not 5'-) deiodinase activity. These findings indicate that the XL-15 cDNA encodes a type III 5-deiodinase and underscores the importance of the trace element selenium in thyroid hormone metabolism.

Amino Acid Sequence↗
PubMed1994

Enhanced auscultation with a new graphic display system.

BACKGROUND: To provide an objective method to support and teach auscultation, a new portable system (graphic display system) was evaluated for graphic display and printing of heart sounds. METHODS: Ninety-one patients from three institutions, with a variety of heart sound abnormalities, were studied by two examiners. A graphic recording was made in each and compared with the auscultatory findings. RESULTS: The findings of the graphic system confirmed the auscultatory impressions of both examiners in 77 (85%) of the 91 cases. Brief sound transients, such as split second sounds and ejection sounds, third heart sounds, and prosthetic opening and closing sounds, were all regularly recorded with the graphic system, often allowing resolution when examiners were in disagreement. Graphic recordings commonly were at variance with examiners in detecting fourth sounds, possibly because of examiners' difficulty in distinguishing these from split first sounds as well as limitations of the graphic system itself. High-pitched murmurs of low intensity (< grade 2), as exemplified by those of aortic and mitral regurgitation, were occasionally missed by the graphic system, probably because of baseline interference by background noise. CONCLUSIONS: The graphic display system can often provide more information than can be obtained by standard auscultation alone, especially in the detection of low-frequency and multiple sounds, and in the accurate timing of intervals. It is often unable to detect soft high-frequency murmurs. Permanent records allow for more objective comparison of the auscultatory findings of various examiners at different times. This system provides an excellent means by which auscultation skills may be taught or enhanced, especially since its speed and portability allow immediate feedback for comparison with auditory perceptions.

Computer Graphics↗
PubMed1993

The earliest changes in gene expression in tadpole intestine induced by thyroid hormone.

Genes induced by thyroid hormone (TH) to change their expression in the Xenopus laevis gastrointestinal (GI) tract have been isolated using a subtractive hybridization method. An exhaustive search for down-regulated genes identified a single gene. Thirty-two different cDNA fragments derived from the up-regulated mRNA of tadpole intestine 18 h after addition of TH were cloned. They map to no more than 22 distinct genes. The isolation of multiple cDNA fragments derived from a single mRNA indicates that the complexity of up-regulated genes is limited. Both ubiquitous and intestine-specific up-regulated genes were found in this screen. The majority of these genes respond directly to TH induction as judged by the resistance of up-regulation to inhibitors of protein synthesis. The biological significance of these genes is supported by their dramatic regulation in the GI tract during spontaneous metamorphosis.

Animals↗
PubMed1993

Thyroid hormone-induced gene expression program for amphibian tail resorption.

The changes in gene expression leading to tail resorption that are initiated by thyroid hormone (TH) were studied in Xenopus laevis. Four of the less than 10 genes that are down-regulated during this period have been isolated; their mRNAs decay with identical kinetics. Twenty of the approximately 35 genes that are up-regulated in the first 48 h have been isolated. The up-regulated genes fall into two kinetic patterns. After a lag of several hours, the direct response genes (including thyroid hormone receptor beta) increase their mRNA level steadily for 24-48 h. The delayed genes respond mainly in the second 24 h after TH addition. The importance of these genes for tail resorption is supported by the fact that they are all regulated developmentally during normal metamorphosis in tail and respond to hormone induction when the tail becomes competent to respond to TH. The relatively simple gene expression program leading to tail resorption is contrasted with the complex and multiple periods of gene expression during limb development. The gene expression screen defines the tail resorption program and has isolated the majority of TH-regulated genes.

Animals↗
PubMed1993

Cultured cells as a model for amphibian metamorphosis.

Gene expression screens have been applied to a cultured cell line of Xenopus laevis, XL-177, to isolate genes that are up- and down-regulated in the first 8 h after thyroid hormone (TH) induction. At least 14 up-regulated genes were isolated from TH-induced cells grown in the presence or absence of cycloheximide, an inhibitor of protein synthesis. These genes respond directly to TH as demonstrated by the resistance of up-regulation to protein synthesis inhibition in the cultured cells or in tadpoles. Kinetics of mRNA accumulation after TH induction is similar for these genes, including those that are superinduced by cycloheximide. Their mRNAs start to be up-regulated several hours after TH treatment and reach maximum levels between 8 and 16 h. These genes show up-regulation in one or more tadpole organs in response to exogenous TH. Only a few minimally down-regulated genes were identified. Fourteen of the 20 genes that were found to be up-regulated by TH in tadpole tail are also up-regulated in XL-177 cells. Their up-regulation falls into the same two kinetic patterns in the cultured cells as it does in tadpole tail. Another cell line of X. laevis, XLA, is greatly reduced in its ability to up-regulate the same genes isolated from XL-177 cells and tadpole tails in response to TH. Thus these cell lines make up a model system to examine the interactions of gene expression triggered by TH during amphibian metamorphosis.

Animals↗
PubMed1993

Expression of the Xenopus laevis prolactin and thyrotropin genes during metamorphosis.

The cDNAs encoding Xenopus laevis prolactin (PRL) and the alpha and beta subunits of thyroid-stimulating hormone (TSH alpha and TSH beta, respectively) have been cloned from a pituitary library. Results of developmental RNA blot analysis contradict the long-held biological role for PRL as a juvenilizing hormone in amphibia. The pituitary gland of a premetamorphic tadpole expresses PRL mRNA at very low levels. The abundance of PRL mRNA increases late in metamorphosis as a response to thyroid hormone (TH), suggesting that PRL is more likely to have a function in the frog than in the tadpole. TSH alpha and -beta mRNA levels increase through prometamorphosis; this rise does not appear to be regulated directly by TH. At climax, both TH and TSH mRNA levels drop. The sequential morphological changes that characterize prometamorphosis depend upon the gradual increase of endogenous TH, which peaks at climax. This increase in TH in turn depends upon the lack of a traditional thyroid-pituitary negative-feedback loop throughout prometamorphosis.

Animals↗