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Biomedical subjects

A Sherman

Publications and source records attributed to A Sherman.

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

The Arabidopsis thaliana proton transporters, AtNhx1 and Avp1, can function in cation detoxification in yeast.

Overexpression of the Arabidopsis thaliana vacuolar H+-pyrophosphatase (AVP1) confers salt tolerance to the salt-sensitive ena1 mutant of Saccharomyces cerevisiae. Suppression of salt sensitivity requires two ion transporters, the Gef1 Cl- channel and the Nhx1 Na+/H+ exchanger. These two proteins colocalize to the prevacuolar compartment of yeast and are thought to be required for optimal acidification of this compartment. Overexpression of AtNHX1, the plant homologue of the yeast Na+/H+ exchanger, suppresses some of the mutant phenotypes of the yeast nhx1 mutant. Moreover, the level of AtNHX1 mRNA in Arabidopsis is increased in the presence of NaCl. The regulation of AtNHX1 by NaCl and the ability of the plant gene to suppress the yeast nhx1 mutant suggest that the mechanism by which cations are detoxified in yeast and plants may be similar.

Amino Acid Sequence↗
PubMed1999

Modeling study of the effects of overlapping Ca2+ microdomains on neurotransmitter release.

Although single-channel Ca2+ microdomains are capable of gating neurotransmitter release in some instances, it is likely that in many cases the microdomains from several open channels overlap to activate vesicle fusion. We describe a mathematical model in which transmitter release is gated by single or overlapping Ca2+ microdomains produced by the opening of nearby Ca2+ channels. This model accounts for the presence of a mobile Ca2+ buffer, provided either that the buffer is unsaturable or that it is saturated near an open channel with Ca2+ binding kinetics that are rapid relative to Ca2+ diffusion. We show that the release time course is unaffected by the location of the channels (at least for distances up to 50 nm), but paired-pulse facilitation is greater when the channels are farther from the release sites. We then develop formulas relating the fractional release following selective or random channel blockage to the cooperative relationship between release and the presynaptic Ca2+ current. These formulas are used with the transmitter release model to study the dependence of this form of cooperativity, which we call Ca2+ current cooperativity, on mobile buffers and on the local geometry of Ca2+ channels. We find that Ca2+ current cooperativity increases with the number of channels per release site, but is considerably less than the number of channels, the theoretical upper bound. In the presence of a saturating mobile buffer the Ca2+ current cooperativity is greater, and it increases more rapidly with the number of channels. Finally, Ca2+ current cooperativity is an increasing function of channel distance, particularly in the presence of saturating mobile buffer.

Calcium↗
PubMed1999

Modulation of the bursting properties of single mouse pancreatic beta-cells by artificial conductances.

Glucose triggers bursting activity in pancreatic islets, which mediates the Ca2+ uptake that triggers insulin secretion. Aside from the channel mechanism responsible for bursting, which remains unsettled, it is not clear whether bursting is an endogenous property of individual beta-cells or requires an electrically coupled islet. While many workers report stochastic firing or quasibursting in single cells, a few reports describe single-cell bursts much longer (minutes) than those of islets (15-60 s). We studied the behavior of single cells systematically to help resolve this issue. Perforated patch recordings were made from single mouse beta-cells or hamster insulinoma tumor cells in current clamp at 30-35 degrees C, using standard K+-rich pipette solution and external solutions containing 11.1 mM glucose. Dynamic clamp was used to apply artificial KATP and Ca2+ channel conductances to cells in current clamp to assess the role of Ca2+ and KATP channels in single cell firing. The electrical activity we observed in mouse beta-cells was heterogeneous, with three basic patterns encountered: 1) repetitive fast spiking; 2) fast spikes superimposed on brief (<5 s) plateaus; or 3) periodic plateaus of longer duration (10-20 s) with small spikes. Pattern 2 was most similar to islet bursting but was significantly faster. Burst plateaus lasting on the order of minutes were only observed when recordings were made from cell clusters. Adding gCa to cells increased the depolarizing drive of bursting and lengthened the plateaus, whereas adding gKATP hyperpolarized the cells and lengthened the silent phases. Adding gCa and gKATP together did not cancel out their individual effects but could induce robust bursts that resembled those of islets, and with increased period. These added currents had no slow components, indicating that the mechanisms of physiological bursting are likely to be endogenous to single beta-cells. It is unlikely that the fast bursting (class 2) was due to oscillations in gKATP because it persisted in 100 microM tolbutamide. The ability of small exogenous currents to modify beta-cell firing patterns supports the hypothesis that single cells contain the necessary mechanisms for bursting but often fail to exhibit this behavior because of heterogeneity of cell parameters.

Action Potentials↗
PubMed1999

Pill-induced gastric injury.

Pill-induced esophageal injury is a well described clinical entity. In contrast, pill-induced gastric injury has not been well characterized. In this report, we describe two patients with acute gastric ulcers due to pill ingestion and review the available literature on pill-induced gastric injury. The first patient presented with upper gastrointestinal hemorrhage and was found to have a large gastric ulcer with multiple potassium chloride pills in the ulcer crater. The second patient presented with odynophagia and endoscopy revealed doxycycline-induced esophageal and gastric injury. To our knowledge, this is the first case report of doxycycline-induced gastric ulcer.

Acute Disease↗
PubMed1998

Transposition of the Drosophila element mariner into the chicken germ line.

The ability of the Drosophila transposable element mariner to transpose in the chicken was tested using a plasmid carrying an active mariner element injected into chick zygotes. Surviving embryos and chicks were analyzed for presence of mariner. Analysis of embryos that survived for at least 12 days of development indicated that mariner had transposed at high frequency into the chicken genome. Germline transmission of mariner from one of three surviving birds confirmed transposition. Analysis of the first-generation (G1) chicks showed that they each contained between one and three copies of mariner. Six different transposition events were represented in the G1 birds, and the transposition was catalyzed by expression of the mariner element's transposase gene. Transmission from G1 to G2 occurred at a 1:1 ratio. Mariner therefore has potential for development as a vector for transgenesis in avian species.

Animals↗
PubMed1998

Factors associated with contractures in acute spinal cord injury.

The purpose of this study was to examine the occurrence of contractures in acute SCI and clarify possible contributing factors such as early versus late admission, level of injury, completeness of the lesion, pressure ulcers, spasticity, co-existent head injury, extremity fractures, heterotopic ossification (HO), peripheral nerve injury, and shoulder pain. The records of all patients (n = 482) admitted between 1990 and 1995 with acute SCI to a model SCI system were reviewed. Of 482 patients 44 (9%) (33 male, 11 female) developed contractures during their initial hospitalization. There were 30 tetraplegic and 14 paraplegic patients. Thirty-four patients had complete and 10 patients had incomplete lesions. The percentage of occurrence of contractures among patients admitted to the model system on the day of injury was 7.6% and among those admitted between 2 and 60 days of the injury was 15% (P = 0.05). Patients with a pressure ulcer (14.1%) were significantly more likely to have a contracture than patients without a pressure ulcer (7.1%) (P = 0.05). Contractures were also more common in patients with spasticity requiring medication; 12.7% of patients with spasticity compared to 7.8% without spasticity, had contractures (P < 0.05). Patients with a co-existent or suspected head injury (15%) were more likely to have contractures than patients without it (7.4%)(P < 0.05). Eighteen patients with a contracture had HO, extremity fracture, peripheral nerve injury or reflex sympathetic dystrophy. In conclusion, this study is one of the first to demonstrate a significant association of contractures in acute SCI with pressure ulcers and co-existent head injury and reaffirms the importance of early admission to a coordinated SCI center in the prevention of contractures.

Acute Disease↗
PubMed1998

Multiple and distinct activation and repression sequences mediate the regulated transcription of IME1, a transcriptional activator of meiosis-specific genes in Saccharomyces cerevisiae.

IME1 encodes a transcriptional activator required for the transcription of meiosis-specific genes and initiation of meiosis in Saccharomyces cerevisiae. The transcription of IME1 is repressed in the presence of glucose, and a low basal level of IME1 RNA is observed in vegetative cultures with acetate as the sole carbon source. Upon nitrogen depletion a transient induction in the transcription of IME1 is observed in MATa/MATalpha diploids but not in MAT-insufficient strains. In this study we demonstrate that the transcription of IME1 is controlled by an extremely unusual large 5' region, over 2,100 bp long. This area is divided into four different upstream controlling sequences (UCS). UCS2 promotes the transcription of IME1 in the presence of a nonfermentable carbon source. UCS2 is flanked by three negative regions: UCS1, which exhibits URS activity in the presence of nitrogen, and UCS3 and UCS4, which repress the activity of UCS2 in MAT-insufficient cells. UCS2 consists of alternate positive and negative elements: three distinct constitutive URS elements that prevent the function of any upstream activating sequence (UAS) under all growth conditions, a constitutive UAS element that promotes expression under all growth conditions, a UAS element that is active only in vegetative media, and two discrete elements that function as UASs in the presence of acetate. Sequence analysis of IME1 revealed the presence of two almost identical 30- to 32-bp repeats. Surprisingly, one repeat, IREd, exhibits constitutive URS activity, whereas the other repeat, IREu, serves as a carbon-source-regulated UAS element. The RAS-cyclic AMP-dependent protein kinase cAPK pathway prevents the UAS activity of IREu in the presence of glucose as the sole carbon source, while the transcriptional activators Msn2p and Msn4p promote the UAS activity of this repeat in the presence of acetate. We suggest that the use of multiple negative and positive elements is essential to restrict transcription to the appropriate conditions and that the combinatorial effect of the entire region leads to the regulated transcription of IME1.

Binding Sites↗
PubMed1997

Evidence that calcium release-activated current mediates the biphasic electrical activity of mouse pancreatic beta-cells.

The electrical response of pancreatic beta-cells to step increases in glucose concentration is biphasic, consisting of a prolonged depolarization with action potentials (Phase 1) followed by membrane potential oscillations known as bursts. We have proposed that the Phase 1 response results from the combined depolarizing influences of potassium channel closure and an inward, nonselective cation current (ICRAN) that activates as intracellular calcium stores empty during exposure to basal glucose (Bertram et al., 1995). The stores refill during Phase 1, deactivating ICRAN and allowing steady-state bursting to commence. We support this hypothesis with additional simulations and experimental results indicating that Phase 1 duration is sensitive to the filling state of intracellular calcium stores. First, the duration of the Phase 1 transient increases with duration of prior exposure to basal (2.8 mM) glucose, reflecting the increased time required to fill calcium stores that have been emptying for longer periods. Second, Phase 1 duration is reduced when islets are exposed to elevated K+ to refill calcium stores in the presence of basal glucose. Third, when extracellular calcium is removed during the basal glucose exposure to reduce calcium influx into the stores, Phase 1 duration increases. Finally, no Phase 1 is observed following hyperpolarization of the beta-cell membrane with diazoxide in the continued presence of 11 mm glucose, a condition in which intracellular calcium stores remain full. Application of carbachol to empty calcium stores during basal glucose exposure did not increase Phase 1 duration as the model predicts. Despite this discrepancy, the good agreement between most of the experimental results and the model predictions provides evidence that a calcium release-activated current mediates the Phase 1 electrical response of the pancreatic beta-cell.

Animals↗
PubMed1996

Double-strand breaks on YACs during yeast meiosis may reflect meiotic recombination in the human genome.

Meiotic recombination in the yeast Saccharomyces cerevisiae is initiated at double-strand breaks (DSBs), which occur preferentially at specific locations. Genetically mapped regions of elevated meiotic recombination ('hotspots') coincide with meiotic DSB sites, which can be identified on chromosome blots of meiotic DNA (refs 4,5; S.K. et al., manuscript submitted). The morphology of yeast artificial chromosomes (YACs) containing human DNA during the pachytene stage of meiosis resembles that of native yeast chromosomes. Homologous YAC pairs segregate faithfully and recombine at the high rates characteristic of S. cerevisiae (vs. approximately 0.4 cM/kb in S. cerevisiae versus approximately 10-3 cM/kb in humans). We have examined a variety of YACs carrying human DNA inserts for double-strand breakage during yeast meiosis. Each YAC has a characteristic set of meiotic DSB sites, as do yeast chromosomes (S.K. et al., manuscript submitted). We show that the positions of the DSB sites in the YACs depend on the human-derived DNA in the clones. The degree of double-strand breakage in yeast meiosis of the YACs in our study appears to reflect the degree of meiotic recombination in humans.

Chromosome Mapping↗
PubMed1996

Contributions of modeling to understanding stimulus-secretion coupling in pancreatic beta-cells.

Mechanisms of ionic control of insulin secretion in beta-cells of the pancreatic islets of Langerhans are reviewed. The focus is on aspects that have been treated by mathematical models, especially those related to bursting electrical activity. The study of these mechanisms is difficult because of the need to consider ionic fluxes, calcium handling, metabolism, and electrical coupling with other cells in the islet. The data come either from islets, where experimental maneuvers tend to have multiple effects, or from isolated cells, which have degraded electrical activity and secretory sensitivity. Modeling aids in the process by integrating data on individual components such as channels and calcium handling and testing hypotheses for coherence and quantitative plausibility. The study of a variety of models has led to some general mathematical results that have yielded qualitative model-independent insights.

Animals↗
PubMed1996

Single-domain/bound calcium hypothesis of transmitter release and facilitation.

1. We describe a model of transmitter release that is based on the finding that release can be gated during the opening of individual Ca2+ channels, suggesting that the release site can be activated by the Ca2+ domain under a single channel. In this model each release site contains four independent Ca2+ binding sites or gates with unbinding kinetics graded from slow to fast and affinities ranging from high to low. All four gates must be bound for release to occur. Thus synaptic dynamics are governed by the kinetics of Ca2+ binding and unbinding from release sites, not Ca2+ diffusion. 2. Fast facilitation occurs when an action potential invades a terminal with one or more ions remaining bound to the release sites. Residual free Ca2+ is not necessary for facilitation with this mechanism, but if present it would enhance facilitation by binding to high-affinity gates between pulses. 3. This model can account for key features of release. These include fourth-power cooperativity with regard to external Ca2+; a release time course that is virtually independent of an increase in quantal content; an inverse relation between external Ca2+ and the degree of facilitation; and a steplike increase in facilitation with increasing stimulus frequency, with each step corresponding to a unitary decline in the Ca2+ cooperativity. 4. Facilitation of single-channel-based secretion is shown to be robust even if channel opening is stochastic. Spontaneous release of transmitter, assumed to be due in part to spontaneous Ca2+ channel openings, is shown to be elevated during and after a train of impulses. 5. An extension of the model to include multiple Ca2+ channels per release site demonstrates that one role of overlapping Ca2+ domains may be to accentuate depolarization-evoked release relative to spontaneous release.

Action Potentials↗
PubMed1995

The Nir1 locus in barley is tightly linked to the nitrite reductase apoprotein gene Nii.

pBNiR1, a cDNA clone encoding part of the barley nitrite reductase apoprotein, was isolated from a barley (cv. Maris Mink) leaf cDNA library using the 1.85 kb insert of the maize nitrite reductase cDNA clone pCIB808 as a heterologous probe. The cDNA insert of pBNiR1 is 503 bp in length. The nucleotide coding sequence could be aligned with the 3' end of other higher plant nitrite reductase apoprotein cDNA sequences but diverges in the 3' untranslated region. The whole-plant barley mutant STA3999, previously isolated from the cultivar Tweed, accumulates nitrite after nitrate treatment in the light, has very much lowered levels of nitrite reductase activity and lacks detectable nitrite reductase cross-reacting material due to a recessive mutation in a single nuclear gene which we have designated Nir1. STA3999 has the characteristics expected of a nitrite reductase apoprotein gene mutant. Here we have used pB-NiR1 in RFLP analysis to determine whether the mutation carried by STA3999 is linked to the nitrite reductase apoprotein gene locus Nii. An RFLP was identified between the wild-type barley cultivars Tweed (major hybridising band of 11.5 kb) and Golden Promise (major hybridising band of 7.5 kb) when DraI-digested DNA was probed with the insert from the partial barley nitrite reductase cDNA clone, pBNiR1. DraI-digested DNA from the mutant STA3999 also exhibited a major hybridising band of 11.5 kb after hybridisation with the insert from pBNiR1.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗
PubMed1995

Positive and negative feedback loops affect the transcription of IME1, a positive regulator of meiosis in Saccharomyces cerevisiae.

The IME1 gene of Saccharomyces cerevisiae encodes a transcription factor that is required for the expression of meiosis-specific genes. Like many of the genes it regulates, IME1 itself is expressed according to the following complex pattern: barely detectable levels during vegetative growth, and high induced levels under starvation conditions, followed by a subsequent decline in the course of meiosis. This report examines the influence of Ime1 protein on its own expression, demonstrating feedback regulation. Disruption of either IME1 or IME2 leads to constantly increasing levels of Ime1-lacZ expression, under meiotic conditions. This apparent negative regulation is due to cis elements in the IME1 upstream region, which confer transient meiotic expression to heterologous promoter-less genes. A specific DNA/protein complex, whose level is transiently increased under meiotic conditions, is detected on this element. In ime1- diploids, the level of this DNA/protein complex increases, without any decline. These results indicate that the transient expression of IME1 is apparently due to transcriptional regulation. This report also presents evidence suggesting that Ime1p is directly responsible for regulating its own transcription. Positive feedback regulation in mitotic conditions is suggested by the observation that overexpression of Ime1p leads to increased levels of IME1-lacZ. Negative autoregulation in meiotic cultures is demonstrated by the observation that a specific point mutation in IME1, ime1-3, permits expression of meiosis-specific genes, as well as induction of meiosis, but is defective in negative-feedback regulation of IME1.

DNA, Fungal↗
PubMed1995

Estimating and eliminating junctional current in coupled cell populations by leak subtraction. A computational study.

The quantitative characterization of ion channel properties in pancreatic beta-cells under typical patch clamp conditions can be questioned because of the unreconciled differences in experimental conditions and observed behavior between microelectrode recordings of membrane potential in intact islets of Langerhans and patch recordings of single cells. Complex bursting is reliably observed in islets but not in isolated cells under patch clamp conditions. E. Rojas et al. (J. Membrane Biol. 143:65-77, 1995) have attempted to circumvent these incompatibilities by measuring currents in beta-cells in intact islets by voltage-clamping with intracellular microelectrodes (150-250 M omega tip resistance). The major potential pitfall is that beta-cells within the islet are electrically coupled, and contaminating coupling currents must be subtracted from current measurements, just as linear leak currents are typically subtracted. To characterize the conditions under which such coupling current subtraction is valid, we have conducted a computational study of a model islet. Assuming that the impaled cell is well clamped, we calculate the native and coupling components of the observed current. Our simulations illustrate that coupling can be reliably subtracted when neighbor cells' potentials are constant or vary only slowly (e.g., during their silent phases) but not when they vary rapidly (e.g., during their active phases). We also show how to estimate coupling conductances in the intact islet from measurements of coupling currents.

Animals↗
PubMed1995

Topological and phenomenological classification of bursting oscillations.

We describe a classification scheme for bursting oscillations which encompasses many of those found in the literature on bursting in excitable media. This is an extension of the scheme of Rinzel (in Mathematical Topics in Population Biology, Springer, Berlin, 1987), put in the context of a sequence of horizontal cuts through a two-parameter bifurcation diagram. We use this to describe the phenomenological character of different types of bursting, addressing the issue of how well the bursting can be characterized given the limited amount of information often available in experimental settings.

Animals↗