Search PubMed⌕ Search

Biomedical subjects

A Klug

Publications and source records attributed to A Klug.

At least 37 records · Page 2Linked to original sources
PubMed1999

Multiple components of ipsilaterally evoked inhibition in the inferior colliculus.

The central nucleus of the inferior colliculus (ICc) receives a large number of convergent inputs that are both excitatory and inhibitory. Although excitatory inputs typically are evoked by stimulation of the contralateral ear, inhibitory inputs can be recruited by either ear. Here we evaluate ipsilaterally evoked inhibition in single ICc cells in awake Mexican free-tailed bats. The principal question we addressed concerns the degree to which ipsilateral inhibition at the ICc suppresses contralaterally evoked discharges and thus creates the excitatory-inhibitory (EI) properties of ICc neurons. To study ipsilaterally evoked inhibition, we iontophoretically applied excitatory neurotransmitters and visualized the ipsilateral inhibition as a gap in the carpet of background activity evoked by the transmitters. Ipsilateral inhibition was seen in 86% of ICc cells. The inhibition in most cells had both glycinergic and GABAergic components that could be blocked by the iontophoretic application of bicuculline and strychnine. In 80% of the cells that were inhibited, the ipsilateral inhibition and contralateral excitation were temporally coincident. In many of these cells, the ipsilateral inhibition suppressed contralateral discharges and thus generated the cell's EI property in the ICc. In other cells, the ipsilateral inhibition was coincident with the initial portion of the excitation, but the inhibition was only 2-4 ms in duration and suppressed only the first few contralaterally evoked discharges. The suppression was so slight that it often could not be detected as a decrease in the spike count generated by increasing ipsilateral intensities. Twenty percent of the cells that expressed inhibition, however, had inhibitory latencies that were longer than the excitatory latencies. In these neurons, the inhibition arrived too late to suppress most or any of the discharges. Finally, in the majority of cells, the ipsilateral inhibition persisted for tens of milliseconds beyond the duration of the signal that evoked it. Thus ipsilateral inhibition has multiple components and one or more of these components are typically evoked in ICc neurons by sound received at the ipsilateral ear.

Animals↗
PubMed1998

Comprehensive DNA recognition through concerted interactions from adjacent zinc fingers.

Zinc fingers are small DNA-binding modules noted for their occurrence in a large number of eukaryotic transcription factors, and their use in protein engineering. Although it was expected that zinc fingers can bind to a wide diversity of DNA sequences, previous studies using model zinc finger domains from Zif268 (and Sp1) have revealed a potential limitation to the DNA-binding specificity. For example, phage display selection of individual zinc fingers to recognize trinucleotide DNA subsites returned fingers that bound specifically only to triplets of the form GNN, i.e., triplets with guanine at the 5' end. Following our recently reported work [Isalan, M., Choo, Y., and Klug, A. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 5617-5621], we now show that this limitation can be overcome by the concerted randomization of certain amino acid positions in adjacent zinc fingers that specify overlapping DNA subsites. This illustrates an important mechanism underlying DNA recognition by arrays of zinc fingers, and points the way to improved strategies for the design of highly specific zinc finger proteins that bind any given nucleotide sequence.

Amino Acid Sequence↗
PubMed1998

Mutation in the tau gene in familial multiple system tauopathy with presenile dementia.

Familial multiple system tauopathy with presenile dementia (MSTD) is a neurodegenerative disease with an abundant filamentous tau protein pathology. It belongs to the group of familial frontotemporal dementias with Parkinsonism linked to chromosome 17 (FTDP-17), a major class of inherited dementing disorders whose genetic basis is unknown. We now report a G to A transition in the intron following exon 10 of the gene for microtubule-associated protein tau in familial MSTD. The mutation is located at the 3' neighboring nucleotide of the GT splice-donor site and disrupts a predicted stem-loop structure. We also report an abnormal preponderance of soluble tau protein isoforms with four microtubule-binding repeats over isoforms with three repeats in familial MSTD. This most likely accounts for our previous finding that sarkosyl-insoluble tau protein extracted from the filamentous deposits in familial MSTD consists only of tau isoforms with four repeats. These findings reveal that a departure from the normal ratio of four-repeat to three-repeat tau isoforms leads to the formation of abnormal tau filaments. The results show that dysregulation of tau protein production can cause neurodegeneration and imply that the FTDP-17 gene is the tau gene. This work has major implications for Alzheimer's disease and other tauopathies.

Adenine↗
PubMed1997

Promoter-specific activation of gene expression directed by bacteriophage-selected zinc fingers.

It has been shown that sequence-specific DNA-binding domains containing zinc fingers can be selected from libraries displayed on filamentous bacteriophage. The affinity and specificity of these peptides are well characterised in vitro, but few data are available to demonstrate specific DNA binding and discrimination between closely related DNA sequences in vivo. Transient transactivation assays were performed in mammalian cells, using expression plasmids which produce different amounts of a model transcription factor containing a phage-selected zinc finger DNA-binding domain, and reporter plasmids which carry systematic variations of the promoter sequence. When the intracellular concentration of the transcription factor was appropriate, activation of gene expression was absolutely dependent on a promoter having the same DNA sequence as that originally used to select the zinc finger domain by phage display. However, excessive intracellular concentrations of the transcription factor resulted in some less-specific DNA binding, leading to gene activation from similar promoters containing a maximum of two base changes. Thus, provided delivery is carefully controlled, highly specific control of gene expression in vivo can be achieved using artificial transcription factors containing phage-selected zinc finger DNA-binding domains.

Amino Acid Sequence↗
PubMed1997

Synergy between adjacent zinc fingers in sequence-specific DNA recognition.

Zif268-like zinc fingers are generally regarded as independent DNA-binding modules that each specify three base pairs in adjacent, but discrete, subsites. However, crystallographic evidence suggests that a contact also can occur from the second helical position of one finger to the subsite of the preceding finger. Here we show for the three-finger DNA-binding domain of the protein Zif268, and a panel of variants, that deleting the putative contact from finger 3 can affect the binding specificity for the 5' base in the adjoining triplet, which forms part of the binding site of finger 2. This finding demonstrates that Zif268-like zinc fingers can specify overlapping 4-bp subsites, and that sequence specificity at the boundary between subsites arises from synergy between adjacent fingers. This has important implications for the design and selection of zinc fingers with novel DNA binding specificities.

Amino Acid Sequence↗
PubMed1997

Physical basis of a protein-DNA recognition code.

Can a stereochemical recognition code explain sequence-specific protein-nucleic acid interactions? Whereas a code that is generally applicable to DNA-binding proteins of all known structural families is unattainable, the indications are that a code can describe at least some of the interactions of classical zinc fingers with DNA. The crystal structures of related zinc finger-DNA complexes reveal a remarkable mode of interaction that sets the framework for this code, and recent biochemical studies have elucidated the intermolecular contacts (contingent on this framework) that result in specificity.

DNA↗
PubMed1997

Ganciclovir induces reproductive hazards in male rats after short-term exposure.

1. The effect of short-term treatment of ganciclovir on male reproduction in adult rats was studied. The animals were treated subcutaneously with either a single dose of 60 mg/kg daily for 5 days (Gan5day) or with 100 mg/kg administered three times at 4 h-intervals (Gan1day). The effects were investigated every 2 weeks up to 8 weeks, followed by investigations 16 and 24 weeks after treatment to detect the potential of recovery. 2. Time to mating was significantly increased in Gan1day group. The pregnancy index and outcome were only decreased 8 weeks (Gan5day and Gan1day) or 16 weeks (Gan1day) after treatment. 3. The lowest values of sperm variables studied were registered 8 weeks after treatment: The number of spermatid was reduced up to 4% (Gan5day) or 2% (Gan1day) of control; the sperm number was 5% and 8% of control in Gan5day and Gan1day, respectively. Over 80% of sperm were abnormal in Gan5day group, and only few normal sperm was detected in Gan1day group. 4. Morphological investigation of testes revealed a clearcut time-dependency effect. Four weeks after treatment distinct alterations were located exclusively in the peripheral part of the tubuli which included fat inclusions, cell and pyknotic nuclear debris and swellings of Sertoli cells. The effect was reversible 24 weeks after treatment. 5. Ganciclovir induces testicular damage and affects sperm variables after short-term exposure. The intensity and degree of the hazards varied in between the time of investigation after treatment.

Animals↗
PubMed1996

Capturing the structure of a catalytic RNA intermediate: the hammerhead ribozyme.

The crystal structure of an unmodified hammerhead RNA in the absence of divalent metal ions has been solved, and it was shown that this ribozyme can cleave itself in the crystal when divalent metal ions are added. This biologically active RNA fold is the same as that found previously for two modified hammerhead ribozymes. Addition of divalent cations at low pH makes it possible to capture the uncleaved RNA in metal-bound form. A conformational intermediate, having an additional Mg(II) bound to the cleavage-site phosphate, was captured by freeze-trapping the RNA at an active pH prior to cleavage. The most significant conformational changes were limited to the active site of the ribozyme, and the changed conformation requires only small additional movements to reach a proposed transition-state.

Base Sequence↗
PubMed1996

Ribozymes: structure and mechanism in RNA catalysis.

The hammerhead RNA is a small catalytic RNA found in a number of RNA virus genomes and virus-like RNAs. The recently determined crystal structures of hammerhead ribozymes reveal how a small RNA motif can fold up into a conformation suitable for mediating RNA cleavage.

Base Sequence↗
PubMed1995

Rapid crystallization of chemically synthesized hammerhead RNAs using a double screening procedure.

To find conditions for obtaining diffraction-quality crystals of a hammerhead RNA rapidly and reproducibly, we employed a "double screening" procedure in which we screened six different RNA synthetic constructs against 48 crystallization conditions using a newly devised sparse matrix. We obtained crystals immediately and diffraction-quality crystals of the sixth RNA construct within six months of initiating the screening of additional RNA sequences. The best crystals diffract to 2.9 A resolution when flash-cooled at synchrotron X-ray sources. Solid-support chemical synthesis combined with sparse matrix screening should allow rapid production of diffraction-quality crystals of a variety of small RNAs, reducing the time commitment for initiating such crystallography projects from several years to several months. The synthetic approach also makes introduction of modified bases to prevent self-cleavage and to generate isomorphous heavy-atom derivative crystals a rapid and straightforward process.

Base Sequence↗
PubMed1995

The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage.

We have solved the crystal structure of an all-RNA hammerhead ribozyme having a single 2'-O-methyl cytosine incorporated at the active site to prevent cleavage. The conditions used differ from those in another recent solution in four significant ways: first, it is an all-RNA ribozyme rather than a DNA-RNA hybrid; second, the connectivity of the ribozyme backbone strands is different; third, the crystals were grown in the presence of a much lower concentration of salt; and fourth, the crystal packing scheme is very different. Nevertheless, the three-dimensional structure of the all-RNA hammerhead ribozyme is similar to the previous structure. Five potential Mg(II)-binding sites are identified, including one positioned near the ribozyme catalytic pocket. Upon this basis, as well as upon comparisons with the metal-binding sites in the structurally homologous uridine turn of tRNAPhe, we propose a mechanism for RNA catalytic cleavage.

Base Sequence↗
PubMed1995

Gene regulatory proteins and their interaction with DNA.

The selective expression of a gene is achieved through the interaction of protein transcription factors with characteristic DNA sequences located in the regulatory region of the gene, which is usually distinct from the coding region. These proteins contain domains that bind specifically to the DNA sites (or response elements). Some general principles in the design of these DNA-binding domains are described, followed by examples of the different structural classes discovered so far and how they recognize their binding sites.

Animals↗
PubMed1995

Designing DNA-binding proteins on the surface of filamentous phage.

The strategy of molecular evolution by phage display recently has been applied to the study of interactions between protein and DNA. This technology will imminently enable DNA-binding proteins to be made to measure. In the first instance, this will greatly advance our understanding of protein-DNA interactions, but in the long term, it is expected to yield powerful tools for use in medicine and research.

Animals↗
PubMed1995

Characterisation of an antibody relevant to the neuropathology of Alzheimer disease.

The specificity of BR88, a polyclonal antibody we raised against amino acids 1-12 of beta/A4 (beta-amyloid protein), has been reexamined in view of a claim made that it cross-reacts with tau-protein, probably the sole component of the paired helical filaments of Alzheimer disease. We have used enzyme-linked immunosorbent assays, immunoblots, and immunohistochemistry, together with appropriate controls, to show that BR88 does not cross-react with tau. This contradicts the claim in question and confirms our previous findings in which BR88 was used to show that some tangle-bearing cells displayed the N-terminus of beta/A4 on their surfaces. The conclusions formed in that earlier work therefore remain intact.

Adult↗
PubMed1995

Glycine and GABA influence binaural processing in the inferior colliculus of the mustache bat.

1. The mammalian inferior colliculus contains large populations of binaural cells that are excited by stimulation of the contralateral ear and are inhibited by stimulation of the ipsilateral ear, and are called excitatory/inhibitory (EI) cells. Neurons with EI properties are initially created in the lateral superior olive (LSO), which, in turn, sends strong bilateral projections to the inferior colliculus. The questions that we address in this report are 1) whether the inhibition evoked by stimulation of the ipsilateral ear occurs at the inferior colliculus or whether it occurs in a lower nucleus, presumably the LSO; and 2) if the ipsilaterally evoked inhibition occurs at the inferior colliculus, is the inhibition a consequence of glycinergic innervation or is it a consequence of GABAergic innervation. To study these questions, we recorded from 61 EI neurons in the inferior colliculus of the mustache bat before and during the iontophoretic application of the glycine receptor antagonist, strychnine. We also tested the effects of the gamma-aminobutyric acid-A (GABAA) receptor antagonist, bicuculline, on 38 of the 61 neurons that were tested with strychnine. The main finding is that glycinergic or GABAergic inhibition, or both, contribute to the ipsilaterally evoked inhibition in approximately 50% of the EI neurons in the inferior colliculus. 2. Strychnine and bicuculline had different effects on the magnitude of the spike counts evoked by stimulation of the contralateral (excitatory) ear. On average, strychnine caused the maximum spike count evoked by contralateral stimulation to increase by only 23%. The relatively small effects of strychnine on response magnitude are in marked contrast to the effects of bicuculline, which usually caused much larger increases in spike counts. For example, although strychnine caused spike counts to more than double in approximately 25% of the collicular neurons, bicuculline caused a doubling of the spike count in approximately 60% of the cells. 3. The inhibitory influences of ipsilateral stimulation were evaluated by driving the neurons with a fixed intensity at the contralateral ear and then documenting the reductions in spike counts due to the presentation of progressively higher intensities at the ipsilateral ear. In 64% of the neurons sampled, blocking glycinergic inhibition with strychnine had little or no effect on the ipsilaterally evoked inhibition. These cells remained as strongly inhibited during the application of strychnine as they did before its application. In addition, the ipsilateral intensity that produced complete or nearly complete spike suppression in the predrug condition was also unchanged by strychnine. 4. In 36% of the neurons, strychnine markedly reduced the degree of ipsilaterally evoked spike suppression. In five of these neurons, there was a complete elimination of the ipsilateral inhibition: these neurons were transformed from strongly inhibited EI neurons into monaural neurons. 5. The influence of both strychnine and bicuculline was tested sequentially in 38 neurons. In about one-half of these cells, (53%, 20/38) the ipsilaterally evoked inhibition was unaffected by either drug. In 10 other units (26%), both drugs substantially reduced or eliminated the ipsilaterally evoked inhibition. In most of these cells, both bicuculline and strychnine reduced the ipsilaterally evoked inhibition to a similar degree. In the remaining eight cells studied with both drugs (21%), the ipsilaterally evoked inhibition was reduced or eliminated by one of the drugs, but not by both. 6. These results show that both glycinergic and GABAergic projections influence the ipsilaterally evoked inhibition in about one-half of the EI neurons in the inferior colliculus. The glycinergic inhibition elicited by ipsilateral stimulation is most likely due to projections from the ipsilateral lateral superior olive, whereas the GABAergic inhibition evoked by ipsilateral stimulation is most likely caused b

Acoustic Stimulation↗
PubMed1995

The crystal structure of an all-RNA hammerhead ribozyme.

We have solved the crystal structure of an all-RNA hammerhead ribozyme by isomorphous replacement which contains a single 2'-O-methyl-cytosine at the active site to prevent cleavage. We describe the hammerhead RNA structure from a point of view which shows how the structural elements are disposed to bring the active site nucleotide into the catalytic pocket of the ribozyme. Five potential Mg(II) binding sites can be identified in the hammerhead RNA electron density maps. Of these, one is a newly identified metal site positioned near the ribozyme catalytic pocket, and another site corresponds to a Mn(II) site identified in the previous hammerhead RNA structure. We propose a mechanism for RNA catalytic cleavage on the basis of this new metal-binding site, as well as upon comparisons between the catalytic pocket of the hammerhead RNA and the metal-binding sites in the structurally homologous uridine turn of the anticodon loop in tRNAPhe.

Base Sequence↗