Biomedical subjects

A Bradley

Publications and source records attributed to A Bradley.

At least 19 recordsLinked to original sources

PubMed1992

Alpha-inhibin is a tumour-suppressor gene with gonadal specificity in mice.

The inhibins are alpha:beta heterodimeric growth factors that are members of the transforming growth factor-beta family. To understand the physiological roles of the inhibins in mammalian development and reproduction, a targeted deletion of the alpha-inhibin gene was generated by homologous recombination in mouse embryonic stem cells. Mice homozygous for the null allele (inhibin-deficient) initially develop normally but every mouse ultimately develops mixed or incompletely differentiated gonadal stromal tumours either unilaterally or bilaterally. Inhibin is thus a critical negative regulator of gonadal stromal cell proliferation and the first secreted protein identified to have tumour-suppressor activity.

Animals↗

PubMed1992

Mice deficient for Rb are nonviable and show defects in neurogenesis and haematopoiesis.

The retinoblastoma gene, a prototypic tumour-suppressor gene, encodes a nuclear phosphoprotein (Rb). To understand better the role of Rb in development and in tumorigenesis, mice with an insertional mutation in exon 20 of the Rb-1 locus were generated. Homozygous mutants die before the 16th embryonic day with multiple defects. The haematopoietic system is abnormal; there is a significant increase in the number of immature nucleated erythrocytes. In the nervous system, ectopic mitoses and massive cell death are found, particularly in the hindbrain. All spinal ganglion cells die, but the neural retina is unaffected. Transfer of the human retinoblastoma (RB) mini-transgene into the mutant mice corrects the developmental defects. Thus, Rb is essential for normal mouse development.

Abnormalities, Multiple↗

PubMed1992

Structure of the mouse activin receptor type II gene.

Knowledge of the structures of the activin receptor genes is crucial to our understanding of the role of the activins, inhibins, and their receptors in developmental and physiological processes. The type II activin receptor (ActRc) has been shown to be a transmembrane protein with putative serine/threonine kinase activity. Using a human ActRc type II cDNA as a probe, 18 overlapping lambda clones containing portions of the mouse ActRc type II gene were isolated from a mouse 129SvE genomic library. Sequence analysis of the exons, exon-intron boundaries, and 5' and 3' non-translated regions as well as Southern blot analysis of mouse genomic DNA were used to establish the structure of the mouse ActRc type II gene. The mouse ActRc type II gene is encoded by 11 exons and spans greater than 66 kilobases. Two large introns (introns 1 and 4) contribute the majority of the gene size and are found to delineate exons which encode important domains of the activin receptor. Analysis of the 5' region of the gene reveals several putative transcription factor binding sites which may be important for the complex transcriptional regulation of this gene.

Activin Receptors↗

PubMed1992

The midbrain-hindbrain phenotype of Wnt-1-/Wnt-1- mice results from stepwise deletion of engrailed-expressing cells by 9.5 days postcoitum.

Mice homozygous for null alleles of the putative signaling molecule Wnt-1 have a reproducible phenotype: loss of the midbrain and adjacent cerebellar component of the metencephalon. By examining embryonic expression of the mouse engrailed (En) genes, from 8.0 to 9.5 days postcoitum, we demonstrate that Wnt-1 primarily regulates midbrain development. The midbrain itself is required for normal development of the metencephalon. Thus, the observed neonatal phenotype is explained by a series of early events, within 48 hr of neural plate induction, that leads to a complete loss of En domains in the anterior central nervous system. Wnt-1 and a related gene, Wnt-3a, are coexpressed from early somite stages in dorsal aspects of the myelencephalon and spinal cord. We suggest that functional redundancy between these two genes accounts for the lack of a caudal central nervous system phenotype.

Animals↗

PubMed1992

Mice deficient for p53 are developmentally normal but susceptible to spontaneous tumours.

Mutations in the p53 tumour-suppressor gene are the most frequently observed genetic lesions in human cancers. To investigate the role of the p53 gene in mammalian development and tumorigenesis, a null mutation was introduced into the gene by homologous recombination in murine embryonic stem cells. Mice homozygous for the null allele appear normal but are prone to the spontaneous development of a variety of neoplasms by 6 months of age. These observations indicate that a normal p53 gene is dispensable for embryonic development, that its absence predisposes the animal to neoplastic disease, and that an oncogenic mutant form of p53 is not obligatory for the genesis of many types of tumours.

Alleles↗

PubMed1992

Cloning of the human activin receptor cDNA reveals high evolutionary conservation.

The entire coding region of the human activin receptor was obtained from a human testis cDNA library. Analysis of the 1539 nucleotide (513 amino acid) sequence of the receptor reveals that there are only 83 nucleotide differences compared to the coding sequence of the mouse activin receptor. Similar to its ligands, the amino acid sequence of the activin receptor is highly conserved with only two conservative amino acid differences (Lys-39 and Val-92 in human versus Arg-39 and Ile-92 in the mouse). This high degree of conservation of the activin receptor illustrates a strong evolutionary selection and confirms that activin and its receptor play an important role in development.

Activin Receptors↗

PubMed1992

Clinical and pharmacological properties of ivermectin in rabbits and guinea pigs.

When 400 micrograms ivermectin/kg was administered subcutaneously to rabbits infected with the ear mite Psoroptes cuniculi it significantly reduced the clinical score, and when 500 micrograms ivermectin/kg was administered subcutaneously to guinea pigs with mange due to Trixacaurus caviae it resulted in a clinical cure. In rabbits a subcutaneous dose of 400 micrograms/kg produced high and sustained concentrations of ivermectin in the tissues and body fluids for at least 13 days and its rate of depletion from tissues was similar to that observed in sheep and rats. The mean (+/- sem) maximum concentration in plasma was 42.0 +/- 9.7 ng/ml 37.2 +/- 5.0 hours after administration and the area under the concentration-time curve was 3543 +/- 580 ng/ml hours. After the administration of 500 micrograms ivermectin/kg to guinea pigs orally, subcutaneously or topically the drug could be detected in the plasma only after subcutaneous administration. The mean concentration 72 hours after its administration to four guinea pigs was 0.7 +/- 0.3 ng/ml.

Animals↗

PubMed1992

The Wnt family of cell signalling molecules in postimplantation development of the mouse.

The mammalian Wnt gene family consists of at least ten members, all of which share a common structure. The N-terminus encodes a putative signal peptide sequence, suggesting that Wnt proteins are secreted. A number of absolutely conserved cysteine residues imply that inter- or intramolecular disulphide bonding is important to Wnt protein function. Wnt RNAs are localized to discrete regions of the postimplantation embryo and fetus, particularly within the developing central nervous system. Studies on Wnt gene expression strongly suggest that Wnt-mediated signalling is likely to be an important aspect of mouse development. One member of the family, Wnt-1, has been studied in some detail. By generating mutant alleles, we have demonstrated that Wnt-1 regulates regional development of the central nervous system at early somite stages. There is circumstantial evidence that some aspects of the pathway through which Wnt-1 action is mediated may be evolutionarily conserved. We propose that the Wnt family plays a major role in cell-cell interactions in the mouse.

Animals↗

PubMed1992

Genetic manipulation of the mouse via gene targeting in embryonic stem cells.

Gene targeting applied to totipotent embryonic stem (ES) cells is a very powerful means of creating highly specific mutations of genes in the mouse. The successful application of this technology is however constrained by both the types of mutations that can be generated at a target locus and the ability to reconstruct a germline chimera from the manipulated cells. We have developed two cell lines that can be routinely transmitted through the germline of chimeras after cloning and prolonged selection in tissue culture. We have also established a variety of methods for generating non-selected mutations at the X-linked hprt locus in ES cells. Our observations at this locus have enabled us to generate successfully a subtle mutation at the non-selectable Hox-2.6 locus.

Animals↗

PubMed1992

Genomic DNA microextraction: a method to screen numerous samples.

Many experimental designs require the analysis of genomic DNA from a large number of samples. Although the polymerase chain reaction (PCR) can be used, the Southern blot is preferred for many assays because of its inherent reliability. The rapid acceptance of PCR, despite a significant rate of false positive/negative results, is partly due to the disadvantages of the sample preparation process for Southern blot analysis. We have devised a rapid protocol to extract high-molecular-weight genomic DNA from a large number of samples. It involves the use of a single 96-well tissue culture dish to carry out all the steps of the sample preparation. This, coupled with the use of a multichannel pipette, facilitates the simultaneous analysis of multiple samples. The procedure may be automated since no centrifugation, mixing, or transferring of the samples is necessary. The method has been used to screen embryonic stem cell clones for the presence of targeted mutations at the Hox-2.6 locus and to obtain data from human blood.

Animals↗

PubMed1992

The effect of pupil size on chromostereopsis and chromatic diplopia: interaction between the Stiles-Crawford effect and chromatic aberrations.

Several studies have reported that the magnitude of chromostereopsis changes as the pupil size changes. Einthoven's theory, that chromostereopsis is determined by interocular differences of monocular transverse chromatic aberration, can not easily explain this change. Therefore, several alternative hypotheses have been introduced, most notably by Vos [(1960) Vision Research, 6, 105-107], who argues that shifts in chromostereopsis with pupil size are due to decentration of the peak of the Stiles-Crawford effect (SCE) with respect to the pupil. We tested this hypothesis by measuring chromostereopsis under both scotopic (no SCE) and photopic conditions with centered and decentered artificial pupils. The results show that the SCE plays an important role in the effect of pupil size on chromostereopsis. Similar changes were also measured in monocular chromatic diplopia which supports the hypothesis that the effect of pupil size on chromostereopsis is due to monocular mechanisms.

Adult↗

PubMed1992

Modifying the mouse: design and desire.

Genetic modification of endogenous genes in mice has become possible by applying gene targeting techniques to embryonic stem (ES) cells and using specific clones of cells to generate mice. Despite the experimental opportunities offered by the creation of organisms with specific genetic changes, there are considerable technical obstacles which can confound the routine implementation of this technology. This review addresses some recent advances in the ability to construct mice with a variety of genetic modifications. These include an increased understanding of the basic cell biology and in vitro growth characteristics of ES cells, which has facilitated germ line transmission of manipulated clones on a routine basis. The techniques that are used to isolate "targeted" clones of ES cells have been summarized, and the current status of strategies which have been successfully used to make very specific modifications of the genome are discussed.

Alleles↗

PubMed1992

Glenn A. Fry Award Lecture 1991: perceptual manifestations of imperfect optics in the human eye: attempts to correct for ocular chromatic aberration.

The profession of optometry has been very successful in providing optical corrections for spherocylindrical refractive errors. In this paper, I examine one attempt to improve retinal image quality beyond that afforded by a standard refractive correction. Ocular chromatic aberration is one of the factors that prevent retinal image quality from reaching the upper limit set by the wave nature of light. It can be subdivided into three primary aberrations (wavelength-dependent differences in imaging plane, image position, and image size). We have been able to measure all three of these using psychophysical techniques. Although attempts to provide an optical correction for wavelength-dependent refractive errors have been optically successful, they have failed to improve vision. Several possible explanations are given for this failure.

Awards and Prizes↗

PubMed1992

Chromatic aberration and optical power of a diffractive bifocal contact lens.

Although diffractive contact lenses have been well documented in theory, no definitive experimental data have been reported which confirm that the near image is in fact created by diffraction rather than by refraction. We have tested the diffraction hypothesis for one type of diffractive contact lens (the Hydron Echelon bifocal) experimentally by measuring its longitudinal chromatic aberration in isolation and when worn on the eye. The basis of this test is that, according to theory, diffractive lenses should have chromatic aberration which is opposite in sign to that measured for the eye. Objective measurements of chromatic aberration were made with a focimeter when the lens was in a wet cell. Subjective measurements were made with a Badal optometer when the lens was worn on the eye. Four control experiments were conducted to provide baseline measurements of the eye's chromatic aberration, against which we compared the results obtained for the diffractive contact lens. The data were also compared with conventional measurements of refractive error obtained by standard subjective techniques and by an automated infrared refractor. Our results showed that the longitudinal chromatic aberration of the diffraction image of the Echelon bifocal lens was about one-half that obtained under the four control conditions: for the naked eye, for the nondiffraction image of the Echelon lens, or for either image of a refractive bifocal contact lens (CIBA Bisoft). These results are consistent with the theoretical prediction that the negative chromatic aberration of a diffractive contact lens should partially cancel the positive chromatic aberration of the human eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Color Perception↗

PubMed1992

Imaging FWC.

Explore the source record for details and available documents.

Contrast Sensitivity↗

PubMed1992

Use of interferometric visual stimulators in optometry.

Professor Fergus Campbell and colleagues pioneered the use of the interferometric visual stimulator as a tool for separating optical from neural factors which limit spatial vision. The device is now commercially available as a clinical instrument which 'bypasses the optics of the eye' to measure the potential visual acuity of the neural portion of the visual system. However, the clinician needs to be aware of limitations of current instrument designs and recent advances in our understanding of peripheral vision, to correctly interpret patient data gathered with these devices.

Humans↗

PubMed1992

The role and fate of DNA ends for homologous recombination in embryonic stem cells.

We have analyzed the gene-targeting frequencies and recombination products generated by a series of vectors which target the hprt locus in embryonic stem cells and found the existence of alternative pathways that depend on the location of the double-strand break within the vector. A double-strand break in the targeting homology was found to increase the targeting frequency compared with a double-strand break at the edge of or outside the target homology; this finding agrees with the double-strand break repair model proposed for Saccharomyces cerevisiae. Although a double-strand break in the homology is important for efficient targeting, observations reported here suggest that the terminal ends are not always directly involved in the initial recombination event. Short terminal heterologous sequences which block the homologous ends of the vector may be incorporated into the target locus. A modification of the double-strand break repair model is described to account for this observation.

Animals↗

PubMed1992

Investigation of coelectroporation as a method for introducing small mutations into embryonic stem cells.

We have investigated coelectroporation as a method for introducing minor genetic changes into specific genes in embryonic stem cells. A selectable marker (neo) and a targeting replacement vector designed to insert a 4-bp insertion into exon 3 of the mouse hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene were coelectroporated into embryonic stem cells and selected in G418 and 6-thioguanine (6-TG). HPRT-negative clones were obtained at a frequency of approximately 1 per 520 G418r clones. Southern analysis and the polymerase chain reaction were used to demonstrate that 3 of 36 of the 6-TG-resistant clones had the desired 4-bp insertion without any other disruption of the HPRT locus. Initial studies indicated that the other 33 6-TG-resistant clones probably resulted from the targeted integration of a concatemer containing both the targeting construct and the selectable neo gene.

Blotting, Southern↗