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M Meister

Publications and source records attributed to M Meister.

At least 19 recordsLinked to original sources
PubMed1997

The structure and precision of retinal spike trains.

Assessing the reliability of neuronal spike trains is fundamental to an understanding of the neural code. We measured the reproducibility of retinal responses to repeated visual stimuli. In both tiger salamander and rabbit, the retinal ganglion cells responded to random flicker with discrete, brief periods of firing. For any given cell, these firing events covered only a small fraction of the total stimulus time, often less than 5%. Firing events were very reproducible from trial to trial: the timing jitter of individual spikes was as low as 1 msec, and the standard deviation in spike count was often less than 0.5 spikes. Comparing the precision of spike timing to that of the spike count showed that the timing of a firing event conveyed several times more visual information than its spike count. This sparseness and precision were general characteristics of ganglion cell responses, maintained over the broad ensemble of stimulus waveforms produced by random flicker, and over a range of contrasts. Thus, the responses of retinal ganglion cells are not properly described by a firing probability that varies continuously with the stimulus. Instead, these neurons elicit discrete firing events that may be the fundamental coding symbols in retinal spike trains.

Action Potentials↗
PubMed1997

Adaptation of retinal processing to image contrast and spatial scale.

Owing to the limited dynamic range of a neuron's output, neural circuits are faced with a trade-off between encoding the full range of their inputs and resolving gradations among those inputs. For example, the ambient light level varies daily over more than nine orders of magnitude, whereas the firing rate of optic nerve fibres spans less than two. This discrepancy is alleviated by light adaptation: as the mean intensity increases, the retina becomes proportionately less sensitive. However, image statistics other than the mean intensity also vary drastically during routine visual processing. Theory predicts that an efficient visual encoder should adapt its strategy not only to the mean, but to the full shape of the intensity distribution. Here we report that retinal ganglion cells, the output neurons of the retina, adapt to both image contrast-the range of light intensities-and to spatial correlations within the scene, even at constant mean intensity. The adaptation occurs on a scale of seconds, one hundred times more slowly than the immediate light response, and involves 2-5-fold changes in the firing rate. It is mediated within the retinal network: two independent sites of modulation after the photoreceptor cells appear to be involved. Our results demonstrate a remarkable plasticity in retinal processing that may contribute to the contrast adaptation of human vision.

Action Potentials↗
PubMed1997

The light response of retinal ganglion cells is truncated by a displaced amacrine circuit.

The vertebrate retina contains ganglion cells that appear to be specialized for detecting temporal changes. The characteristic response of these cells is a transient burst of action potentials when a stationary image is presented or removed, and often a strong discharge to moving images. These transient and motion-sensitive responses are thought to result from processing in the inner retina that involves amacrine cells, but the critical interactions have been difficult to reveal. Here, we used a cell-ablation technique to remove a subpopulation of amacrine cells from the mouse retina. Their ablation changed transient ganglion cell responses into prolonged discharges. This suggests that transient responses are generated, at least in part, by a truncation of sustained excitatory input to the ganglion cells and that the ablated amacrine cells are critical for this process.

Animals↗
PubMed1996

Isoprenylation of the G protein gamma subunit is both necessary and sufficient for beta gamma dimer-mediated stimulation of phospholipase C.

We have previously shown that isoprenylation and/or additional post-translational processing of the G protein gamma 1 subunit carboxyl terminus is required for beta 1 gamma 1 subunit stimulation of phospholipase C-beta 2 (PLC beta 2) [Dietrich, A., Meister, M., Brazil, D., Camps, M., & Gierschik, P. (1994) Eur. J. Biochem. 219, 171-178]. To examine whether isoprenylation of the gamma 1 subunit alone is sufficient for beta 1 gamma 1-mediated PLC beta 2 stimulation or whether any of the two subsequent modifications, proteolytic removal of the carboxyl-terminal tripeptide and/or carboxylmethylation, is required for this effect, nonisoprenylated recombinant beta 1 gamma 1 dimers were produced in baculovirus-infected insect cells, purified to near homogeneity, and then isoprenylated in vitro using purified recombinant protein farnesyltransferase. Analysis of the beta 1 gamma 1 dimer after in vitro farnesylation by reversed phase high-performance liquid chromatography followed by delayed extraction matrix-assisted laser desorption/ionization mass spectrometry confirmed that the gamma 1 subunit was carboxyl-terminally farnesylated but not proteolyzed and carboxylmethylated. Functional reconstitution of in vitro-farnesylated beta 1 gamma 1 dimers with a recombinant PLC beta 2 isozyme revealed that farnesylation rendered recombinant nonisoprenylated beta 1 gamma 1 dimers capable of stimulating PLC beta 2 and that the degree of this stimulation was only approximately 45% lower for in vitro-farnesylated beta 1 gamma 1 dimers than for fully modified native beta 1 gamma 1 purified from bovine retinal rod outer segments. Taken together, these results suggest that isoprenylation of the gamma subunit is both necessary and sufficient for beta gamma dimer-mediated stimulation of phospholipase C.

Alkyl and Aryl Transferases↗
PubMed1996

Cloning of the gene encoding the antibacterial peptide drosocin involved in Drosophila immunity. Expression studies during the immune response.

A potent inducible antibacterial peptide carrying an O-glycosylated substitution has recently been isolated from Drosophila [Bulet, P., Dimarcq, J. L., Hetru, C., Lagueux, M., Charlet, M., Hegy, G., Van Dorsselaer, A. and Hoffmann, J. A. (1993) J. Biol. Chem. 268, 14893-14897]. Here we report cloning studies that show that Drosophila contains a single, intronless gene, located at position 51C1-6, which encodes the precursor protein from which drosocin is processed. The upstream and the downstream sequences of the drosocin gene contain putative cis-regulatory elements similar to mammalian regulatory motifs, namely three kappa B-related decameric sequences. The drosocin gene is silent in naive animals, and is strongly induced with acute phase kinetics after immune challenge in larvae and in adults. We have established several transgenic fly lines in which reporter genes were placed under the control of various drosocin promoter sequences. Our results indicate that 2.5 kb of upstream sequences confer inducibility and tissue specificity to the transgene, but that the level of its expression in the fat body after immune challenge is low. Addition of genomic regions downstream of the drosocin transcribed sequences results in increased transcription levels, which are similar for the fusion and the resident drosocin genes upon infection. Analysis of transgenic fly lines showed that the drosocin reporter gene is constitutively expressed in the oviducts of egg-laying females.

Amino Acid Sequence↗
PubMed1996

Multineuronal codes in retinal signaling.

The visual world is presented to the brain through patterns of action potentials in the population of optic nerve fibers. Single-neuron recordings show that each retinal ganglion cell has a spatially restricted receptive field, a limited integration time, and a characteristic spectral sensitivity. Collectively, these response properties define the visual message conveyed by that neuron's action potentials. Since the size of the optic nerve is strictly constrained, one expects the retina to generate a highly efficient representation of the visual scene. By contrast, the receptive fields of nearby ganglion cells often overlap, suggesting great redundancy among the retinal output signals. Recent multineuron recordings may help resolve this paradox. They reveal concerted firing patterns among ganglion cells, in which small groups of nearby neurons fire synchronously with delays of only a few milliseconds. As there are many more such firing patterns than ganglion cells, such a distributed code might allow the retina to compress a large number of distinct visual messages into a small number of optic nerve fibers. This paper will review the evidence for a distributed coding scheme in the retinal output. The performance limits of such codes are analyzed with simple examples, illustrating that they allow a powerful trade-off between spatial and temporal resolution.

Action Potentials↗
PubMed1996

Ecdysone and insect immunity: the maturation of the inducibility of the diptericin gene in Drosophila larvae.

The developmental analysis of the inducibility of the Drosophila diptericin gene promoter as a response to septic injury shows an important increase in the response during the third larval instar leading to a maximum in late larvae and early prepupae. This increase, or maturation, is temporally correlated with known ecdysone induced events of the salivary gland and we now present evidence, using wild type and mutant larvae, that it does indeed depend upon ecdysone. The response remains minimal in larvae carrying either the temperature sensitive ecdysone deficient late larval lethal allele ecd1, or l(1)t187, a deep orange allele known to be deficient in the ecdysone response. However, experiments with the late larval lethal Broad-Complex mutant l(1)t435 show that the regulation of this response is distinct from the developmental ecdysone regulated hierarchies.

Animals↗
PubMed1995

Concerted signaling by retinal ganglion cells.

To analyze the rules that govern communication between eye and brain, visual responses were recorded from an intact salamander retina. Parallel observation of many retinal ganglion cells with a microelectrode array showed that nearby neurons often fired synchronously, with spike delays of less than 10 milliseconds. The frequency of such synchronous spikes exceeded the correlation expected from a shared visual stimulus up to 20-fold. Synchronous firing persisted under a variety of visual stimuli and accounted for the majority of action potentials recorded. Analysis of receptive fields showed that concerted spikes encoded information not carried by individual cells; they may represent symbols in a multineuronal code for vision.

Action Potentials↗
PubMed1995

Identification of a three-amino-acid region in G protein gamma 1 as a determinant of selective beta gamma heterodimerization.

Guanine-nucleotide-binding protein beta and gamma subunits belong to large protein families encompassing at least five and ten members, respectively, from mammalian cells. The formation of stable beta gamma heterodimers is a selective process determined by the primary sequences of both the beta and gamma subunit. For example, gamma 2 dimerizes with both beta 1 and beta 2, gamma 1 with beta 1, but not with beta 2. To identify the structural elements of gamma subunits relevant to the selectivity of beta gamma dimerization, we have used the baculovirus-insect cell-expression system to produce chimeric beta and gamma subunits and have studied their dimerization using an assay based on the ability of isoprenylation-resistant gamma subunit mutants to draw beta subunits into the cytosol and including sucrose density gradient analysis of soluble recombinant beta gamma dimers. The results show that replacement of three consecutive residues of gamma 1, Cys36-Cys37-Glu38, by the corresponding residues of gamma 2, Ala33-Ala34-Ala35, suffices to render the mutant gamma 1 subunit capable of forming heterodimers with beta 2. The ability of mutant gamma 1 subunits to dimerize with beta 2 does not correlate with the probability of the mutated region to participate in coiled-coil structures. The tripeptide region identified here as a critical determinant of the selectivity of beta gamma dimer formation is distinct from, but partially overlaps with, the region reported by Lee et al. [Lee, C., Murakami, T. & Simonds, W. F. (1995) J. Biol. Chem. 270, 8779-8784]. The results of this study, therefore, not only extend the region of gamma 1 selecting between beta 1 and beta 2 to the five-residue sequence between Cys36 and Phe40, but also argue against the notion that the hydrophobic terminal residue of this motif represents the key determinant of selective beta gamma interaction.

Amino Acid Sequence↗
PubMed1995

A recessive mutation, immune deficiency (imd), defines two distinct control pathways in the Drosophila host defense.

In this paper we report a recessive mutation, immune deficiency (imd), that impairs the inducibility of all genes encoding antibacterial peptides during the immune response of Drosophila. When challenged with bacteria, flies carrying this mutation show a lower survival rate than wild-type flies. We also report that, in contrast to the antibacterial peptides, the antifungal peptide drosomycin remains inducible in a homozygous imd mutant background. These results point to the existence of two different pathways leading to the expression of two types of target genes, encoding either the antibacterial peptides or the antifungal peptide drosomycin.

Animals↗
PubMed1995

Redox equilibria between the regulatory thiols of light/dark-modulated chloroplast enzymes and dithiothreitol: fine-tuning by metabolites.

Three light/dark-modulated chloroplast enzymes, namely NADP-dependent malate dehydrogenase (EC 1.1.1.82), D-fructose 1,6-bisphosphatase (EC 3.1.3.11), and phosphoribulokinase (EC 2.7.1.19) were purified to apparent homogeneity from spinach leaves. Equilibrium constants for the covalent modification of the regulatory disulfide bonds of these enzymes in dithiothreitol (DTT)-redox buffer were determined according to a previously published method in the literature (Clancey and Gilbert (1987) J. Biol. Chem. 262, 13545-13549). The thiol/disulfide-redox potential (Kox) was defined as the ratio of reduced to oxidized dithiothreitol at which 50% of the maximal enzyme activity was observed after equilibrium had been established. All Kox values were very high, comparable to those of extracellular disulfide containing proteins: 0.23 +/- 0.02 for NADP-malate dehydrogenase, 0.59 +/- 0.17 for phosphoribulokinase, and 0.70 +/- 0.16 for D-fructose 1,6-bisphosphatase. The equilibrium constants for the reactions between these enzymes and the redox buffers were also determined in the presence of various concentrations of specific metabolites known to influence the rates of reduction and oxidation. Increasing concentrations of D-fructose 1,6-bisphosphate in the presence of Ca2+ shift the equilibrium constant between D-fructose 1,6-bisphosphatase and the DTT-redox buffer to much lower values. A decreasing NADPH/(NADP + NADPH) ratio increases the Kox of NADP-malate dehydrogenase in the redox buffer to very high values. For PRK, low concentrations of ATP result in a slight decrease of the Kox that is not further affected by higher ATP concentrations. The differences of the equilibrium constants of NADP-malate dehydrogenase and D-fructose 1,6-bisphosphatase as dependent upon the NADPH/(NADP + NADPH) ratio and the concentration of D-fructose 1,6-bisphosphate, respectively, reflect a mechanism of feed-back and feed-forward regulation by the product NADP and the substrate D-fructose 1,6-bisphosphate, respectively. Thus the actual activation state of these two key enzymes of chloroplast metabolism are determined in an independent manner. The relatively small effect of the ATP concentration upon the redox potential of phosphoribulokinase indicates that fine-regulation at this step might be achieved on another level (e.g., catalysis or aggregation state).

Chloroplasts↗
PubMed1995

Functional analysis and regulation of nuclear import of dorsal during the immune response in Drosophila.

In addition to its function in embryonic development, the NF-kappa B/rel-related gene dorsal (dl) of Drosophila is expressed in larval and adult fat body where its RNA expression is enhanced upon injury. Injury also leads to a rapid nuclear translocation of dl from the cytoplasm in fat body cells. Here we present data which strongly suggest that the nuclear localization of dl during the immune response is controlled by the Toll signaling pathway, comprising gene products that participate in the intracellular part of the embryonic dorsoventral pathway. We also report that in mutants such as Toll or cactus, which exhibit melanotic tumor phenotypes, dl is constitutively nuclear. Together, these results point to a potential link between the Toll signaling pathway and melanotic tumor induction. Although dl has been shown previously to bind to kappa B-related motifs within the promoter of the antibacterial peptide coding gene diptericin, we find that injury-induced expression of diptericin can occur in the absence of dl. Furthermore, the melanotic tumor phenotype of Toll and cactus is not dl dependent. These data underline the complexity of the Drosophila immune response. Finally, we observed that like other rel proteins, dl can control the level of its own transcription.

Animals↗
PubMed1995

Individual neurons dissociated from rat suprachiasmatic nucleus express independently phased circadian firing rhythms.

Within the mammalian hypothalamus, the suprachiasmatic nucleus (SCN) contains a circadian clock for timing of diverse neuronal, endocrine, and behavioral rhythms. By culturing cells from neonatal rat SCN on fixed microelectrode arrays, we have been able to record spontaneous action potentials from individual SCN neurons for days or weeks, revealing prominent circadian rhythms in firing rate. Despite abundant functional synapses, circadian rhythms expressed by neurons in the same culture are not synchronized. After reversible blockade of neuronal firing lasting 2.5 days, circadian firing rhythms re-emerge with unaltered phases. These data suggest that the SCN contains a large population of autonomous, single-cell circadian oscillators, and that synapses formed in vitro are neither necessary for operation of these oscillators nor sufficient for synchronizing them.

Action Potentials↗
PubMed1994

Insect immunity. A transgenic analysis in Drosophila defines several functional domains in the diptericin promoter.

Diptericins are antibacterial polypeptides which are strongly induced in the fat body and blood cells of dipteran insects in response to septic injury. The promoter of the single-copy, intronless diptericin gene of Drosophila contains several nucleotide sequences homologous to mammalian cis-regulatory motifs involved in the control of acute phase response genes. Extending our previous studies on the expression of the diptericin gene, we now report a quantitative analysis of the contribution of various putative regulatory elements to the bacterial inducibility of this gene, based on the generation of 60 transgenic fly lines carrying different elements fused to a reporter gene. Our data definitively identify two Kappa B-related motifs in the proximal promoter as the sites conferring inducibility and tissue-specific expression to the diptericin gene. These motifs alone, however, mediate only minimal levels of expression. Additional proximal regulatory elements are necessary to attain some 20% of the full response and we suspect a role for sequences homologous to mammalian IL6 response elements and interferon-gamma responsive sites in this up-regulation. The transgenic experiments also reveal the existence of a distal regulatory element located upstream of -0.6 kb which increases the level of expression by a factor of five.

Animals↗
PubMed1994

Characterization and transcriptional profiles of a Drosophila gene encoding an insect defensin. A study in insect immunity.

Insect defensins are a family of 4-kDa, cationic, inducible antibacterial peptides which bear six cysteine residues engaged in three intramolecular disulfide bridges. They owe their name to certain sequence similarities with defensins from mammalian neutrophiles and macrophages. We report the characterization of a novel defensin isoform from Drosophila and the cloning of the gene encoding a preprodefensin. The gene, which is intronless and present in a single copy/haploid genome, maps at position 46CD on the right arm of the second chromosome. The analysis of the upstream region of the gene reveals the presence of multiple putative cis-regulatory sequences similar to mammalian regulatory motifs of acute-phase-response genes. Transcriptional profiles indicate that the Drosophila defensin gene is induced by bacterial challenge with acute-phase kinetics. It is also expressed in the absence of immune challenge during metamorphosis. These and other data on the Drosophila defensin gene lead us to suggest that insect and mammalian defensins have evolved independently.

Amino Acid Sequence↗
PubMed1994

Stimulation of phospholipase C-beta 2 by recombinant guanine-nucleotide-binding protein beta gamma dimers produced in a baculovirus/insect cell expression system. Requirement of gamma-subunit isoprenylation for stimulation of phospholipase C.

Recombinant wild-type beta 1 gamma 1 dimers of signal-transducing guanine nucleotide-binding proteins (G proteins) and beta 1 gamma 1 dimers carrying a mutation known to block gamma-subunit isoprenylation (beta 1 gamma 1 C71S) were expressed in baculovirus-infected insect cells. Both wild-type and mutant beta 1 gamma 1 dimers were found in soluble fractions of infected cells upon subcellular fractionation. Anion exchange chromatographic and metabolic-radiolabeling studies revealed that the soluble beta 1 gamma 1 preparation contained approximately equal amounts of non-isoprenylated and isoprenylated beta 1 gamma 1 dimers. Soluble wild-type and mutant beta 1 gamma 1 dimers and native beta 1 gamma 1 dimers purified from bovine retina were reconstituted with recombinant phospholipase C-beta 2. Only isoprenylated beta 1 gamma 1 dimers were capable of stimulating phospholipase C-beta 2. The results show that gamma-subunit isoprenylation and/or additional post-translational processing of the protein are required for beta gamma subunit stimulation of phospholipase C.

Animals↗
PubMed1994

Multi-neuronal signals from the retina: acquisition and analysis.

Throughout the central nervous system, information about the outside world is represented collectively by large groups of cells, often arranged in a series of 2-dimensional maps connected by tracts with many fibers. To understand how such a circuit encodes and processes information, one must simultaneously observe the signals carried by many of its cells. This article describes a new method for monitoring the simultaneous electrical activity of many neurons in a functioning piece of retina. Extracellular action potentials are recorded with a planar array of 61 microelectrodes, which provides a natural match to the flat mosaic of retinal ganglion cells. The voltage signals are processed in real time to extract the spike trains from up to 100 neurons. We also present a method of visual stimulation and data analysis that allows a rapid characterization of each neuron's visual response properties. A randomly flickering display is used to elicit spike trains from the ganglion cell population. Analysis of the correlations between each spike train and the flicker stimulus results in a simple description of each ganglion cell's functional properties. The combination of these tools will allow detailed study of how the population of optic nerve fibers encodes a visual scene.

Action Potentials↗