Geneviève Dupont - Academia.edu (original) (raw)
Papers by Geneviève Dupont
Cold Spring Harbor Perspectives in Biology, 2011
Calcium signaling results from a complex interplay between activation and inactivation of intrace... more Calcium signaling results from a complex interplay between activation and inactivation of intracellular and extracellular calcium permeable channels. This complexity is obvious from the pattern of calcium signals observed with modest, physiological concentrations of calcium-mobilizing agonists, which typically present as sequential regenerative discharges of stored calcium, a process referred to as calcium oscillations. In this review, we discuss recent advances in understanding the underlying mechanism of calcium oscillations through the power of mathematical modeling. We also summarize recent findings on the role of calcium entry through store-operated channels in sustaining calcium oscillations and in the mechanism by which calcium oscillations couple to downstream effectors.
The FASEB Journal, 2007
Benzene polyphosphates containing phosphate groups on one ring are Ins(1,4,5)P3 5-phosphatase inh... more Benzene polyphosphates containing phosphate groups on one ring are Ins(1,4,5)P3 5-phosphatase inhibitors when evaluated against type-I Ins(1,4,5)P3 5-phosphatase. A novel biphenyl derivative, biphenyl 2,3',4,5',6-pentakisphosphate, with five phosphate groups on two rings was synthesized: It inhibited the activity of two inositol 5-phosphatases: type I and SHIP2 with Ins(1,3,4,5)P4 as substrate. The inhibition was competitive with respect to the substrate. IC50 value measured in rat hepatocytes, which contains the native Ins(1,4,5)P3 5-phosphatase, was in the micromolar range at 1.0 microM Ins(1,4,5)P3 as substrate. Biphenyl 2,3',4,5',6-pentakisphosphate did not affect the activity of Ins(1,4,5)P3 3-kinase A in the 5-100 microM range. Surprisingly, experimental evidence supports an effect of biphenyl 2,3',4,5',6-pentakisphosphate at the level of the Ins(1,4,5)P3 receptor. Finally, when injected into rat hepatocytes, the analog affected the frequency of Ca2+ oscillations in a positive or negative way depending on its concentration. At very high concentrations of the analog, Ca2+ oscillations were even suppressed. These data were interpreted as a dual effect of the biphenyl 2,3',4,5',6-pentakisphosphate on cytosolic [Ca2+] increases: an activation effect through an increase in Ins(1,4,5)P3 level via Ins(1,4,5)P3 5-phosphatase inhibition and an inhibitory effect, which was exerted directly on the Ins(1,4,5)P3 receptor. Thus, our data show for the first time that the frequency of Ca2+ oscillations in response to a Ca2+-mobilizing agonist can be controlled by inhibitors of type-I Ins(1,4,5)P3 5-phosphatase.
médecine/sciences, 2011
Jusqu'à très récemment, les biologistes mesuraient la concentration d'une protéine ou la vitesse ... more Jusqu'à très récemment, les biologistes mesuraient la concentration d'une protéine ou la vitesse de transformation d'un métabolite par exemple à partir de populations cellulaires. L'extraordinaire développement de nouvelles techniques d'imagerie (microscopie confocale, fluorescence resonance energy transfer [FRET], fluorescence recovery after photobleaching [FRAP], etc.), combiné à l'utilisation de sondes fluorescentes performantes, de protéines chimères (green fluorescent protein [GFP], DsRed, etc.) ou de nanocristaux (quantum dots) permet actuellement une approche quantitative et dynamique de la biologie d'une seule cellule, et rend possible l'analyse par exemple de l'expression spécifique d'un gène ou de la variation d'un flux métabolique. Ces approches révèlent dans bien des cas une assez grande hétérogénéité spatiale, mais aussi temporelle, rappelant, si besoin est, que les cellules sont des entités extrêmement dynamiques . De manière plus inattendue, ces observations à l'échelle cellulaire mettent aussi en évidence le caractère aléatoire (stochastique) des événements moléculaires individuels. Le biologiste est dès lors confronté à de nouvelles questions : comment des événements qui se produisent sur > Les signaux calciques sont organisés dans le temps et dans l'espace, ce qui leur permet d'assurer une signalisation cellulaire spécifique et robuste. En réponse à une stimulation, la concentration nanomolaire du signal Ca 2+ peut augmenter de plusieurs dizaines de fois à proximité de quelques récepteurs de l'inositol (1, 4, 5) trisphosphate (InsP 3 ). Cela se fait sous la forme de vagues se propageant périodiquement dans un tissu ou un organe. Les études de la relation entre ces phénomènes, caractérisés par des échelles temporelles et spatiales très différentes, et les mécanismes qui en sont responsables, sont décrits dans cette revue par une approche fondée sur une interaction étroite entre expériences et modélisation, appliquée principalement à la signalisation calcique dans les hépatocytes. < des échelles de temps et d'espace très différentes sont-ils coordonnés ? Comment les cellules contrôlent-elles, ou même utilisent-elles, des fluctuations moléculaires pour réaliser des tâches dites robustes ? Pour répondre à ces questions, il est nécessaire d'associer démarches expérimentales et théoriques. Ce type d'approche pluridisciplinaire, utilisé avec succès depuis de nombreuses années en neurophysiologie, s'étend aujourd'hui à de nombreux aspects de la physiologie cellulaire. La signalisation calcique, objet de cette revue, en est un exemple frappant. Depuis les travaux de Ringer il y a près de 130 ans, l'importance du calcium dans la physiologie cellulaire n'est plus à démontrer, mais l'extraordinaire complexité de l'organisation dans le temps et dans l'espace de cette signalisation n'a été révelée que grâce aux progrès techniques réalisés depuis une petite vingtaine d'années. En combinant expériences et modélisation, nous avons pu disséquer certains aspects de cette signalisation, notamment dans les hépatocytes. En réponse à une stimulation extracellulaire, les hépatocytes augmentent leur concentration en Ca 2+ intracellulaire, celle-ci étant responsable de réponses physiologiques comme la production de glucose, la sécrétion biliaire, l'expression génétique ou la régénération du foie. Dans ces cellules, comme dans la plupart des cellules non excitables [2], des agonistes induisent la synthèse d'inositol (1, 4, 5) trisphosphate (InsP 3 ). L'InsP 3 formé se fixe sur des récepteurs (InsP 3 R)/canaux calciques présents dans la membrane du réticulum endoplasmique (RE). La libération de Ca 2+ via les récepteurs de l'InsP 3 est régulée par la concentration en Ca 2+ dans le cytoplasme ; c'est aussi le cas pour les
Biophysical Journal, 2008
Signal-induced Ca 21 oscillations have been observed in many cell types and play a primary role i... more Signal-induced Ca 21 oscillations have been observed in many cell types and play a primary role in cell physiology. Although it is the regular character of these oscillations that first catches the attention, a closer look at time series of Ca 21 increases reveals that the fluctuations on the period during individual spike trains are far from negligible. Here, we perform a statistical analysis of the regularity of Ca 21 oscillations in norepinephrine-stimulated hepatocytes and find that the coefficient of variation lies between 10% and 15%. Stochastic simulations based on Gillespie's algorithm and considering realistic numbers of Ca 21 ions and inositol trisphosphate (InsP 3 ) receptors account for this variability if the receptors are assumed to be grouped in clusters of a few tens of channels. Given the relatively small number of clusters (;200), the model predicts the existence of repetitive spikes induced by fluctuations (stochastic resonance). Oscillations of this type are found in hepatocytes at subthreshold concentrations of norepinephrine. We next predict with the model that the isoforms of the InsP 3 receptor can affect the variability of the oscillations. In contrast, possible accompanying InsP 3 oscillations have no impact on the robustness of signal-induced repetitive Ca 21 spikes.
Cell Calcium, 2011
In hepatocytes, as in other cell types, Ca(2+) signaling is subject to complex regulations, which... more In hepatocytes, as in other cell types, Ca(2+) signaling is subject to complex regulations, which result largely from the intrinsic characteristics of the different inositol 1,4,5-trisphosphate receptor (InsP(3)R) isoforms and from their interactions with other proteins. Although sigma1 receptors (Sig-1Rs) are widely expressed in the liver, their involvement in hepatic Ca(2+) signaling remains unknown. We here report that in this cell type Sig-1R interact with type 1 isoforms of the InsP(3) receptors (InsP(3)R-1). These results obtained by immunoprecipitation experiments are confirmed by the observation that Sig-1R proteins and InsP(3)R-1 colocalize in hepatocytes. However, Sig-1R ligands have no effect on InsP(3)-induced Ca(2+) release in hepatocytes. This can be explained by the rather low expression level expression of InsP(3)R-1. In contrast, we find that Sig-1R ligands can inhibit agonist-induced Ca(2+) signaling via an inhibitory effect on InsP(3) synthesis. We show that this inhibition is due to the stimulation of PKC activity by Sig-1R, resulting in the well-known down-regulation of the signaling pathway responsible for the transduction of the extracellular stimulus into InsP(3) synthesis. The PKC sensitive to Sig-1R activity belongs to the family of conventional PKC, but the precise molecular mechanism of this regulation remains to be elucidated.
Biology of the Cell, 2006
Background information. Oscillations of cytosolic Ca 2+ are well-known to rely on the regulatory ... more Background information. Oscillations of cytosolic Ca 2+ are well-known to rely on the regulatory properties of the InsP 3 R (inositol 1,4,5-trisphosphate receptor). Three isoforms of this channel have been identified. They differ in their regulatory properties by Ca 2+ and InsP 3 . Experiments in different cell types clearly indicate that the relative amounts of each isoform affect the time course of Ca 2+ changes after agonist stimulation. In the present study, we investigate whether different steady-state curves for the open probability of the InsP 3 Rs as a function of Ca 2+ imply different dynamical behaviours when these receptors are present in a cellular environment. We therefore describe by a specific phenomenological model the three main types of curves that have been reported: (i) the classical bell-shaped curve, (ii) the bell-shaped curve that is shifted towards higher Ca 2+ concentrations when InsP 3 is increased, and (iii) a monotonous increasing function of cytosolic Ca 2+ .
![Research paper thumbnail of What can we learn from the irregularity of Ca[sup 2+] oscillations?](https://attachments.academia-assets.com/42784750/thumbnails/1.jpg)
](Chaos: An Interdisciplinary Journal of Nonlinear Science, 2009
In most cells, Ca 2+ increases in response to external stimulation are organized in the form of o... more In most cells, Ca 2+ increases in response to external stimulation are organized in the form of oscillations and waves that sometimes propagate from one cell to another. Numerous experimental and theoretical studies reveal that this spatiotemporal organization contains a non-negligible level of stochasticity. In this study, we extend the previous work based on a statistical analysis of experimental Ca 2+ traces in isolated, hormone-stimulated hepatocytes and on stochastic simulations of Ca 2+ oscillations based on the Gillespie's algorithm. Comparison of the coefficients of variation in the periods of experimental and simulated Ca 2+ spikes provides information about the clustering and the specific subtypes of the Ca 2+ channels. In hepatocytes coupled by gap junctions, the global perfusion with a hormone leads to successive Ca 2+ responses, giving the appearance of an intercellular wave. Statistical analysis of experimental Ca 2+ oscillations in coupled hepatocytes confirms that this coordinated Ca 2+ spiking corresponds to a phase wave but suggests the existence of an additional coupling mechanism.
FEBS Letters, 2003
Receptor-mediated production of inositol 1,4,5-trisphosphate (InsP 3 ) initiates Ca 2+ release an... more Receptor-mediated production of inositol 1,4,5-trisphosphate (InsP 3 ) initiates Ca 2+ release and is responsible for cytosolic Ca 2+ oscillations. InsP 3 oscillations have also been observed in some cells. One of the enzymes controlling InsP 3 catabolism, the InsP 3 3-kinase, is stimulated by Ca 2+ ; this regulation is presumably part of the reason for InsP 3 oscillations that have been observed in some cells. Here, we investigate the possible role of Ca 2+ -activated InsP 3 catabolism on the characteristics of the InsP 3 -induced Ca 2+ oscillations. Numerical simulations show that if it is assumed that the Ca 2+ -independent InsP 3 catabolism is predominant, Ca 2+ oscillations remain qualitatively unchanged although the relative amplitude of the oscillations in InsP 3 concentrations becomes minimal. We tested this prediction in hepatocytes by masking the Ca 2+ -dependent InsP 3 catabolism by 3-kinase through the injection of massive amounts of InsP 3 5-phosphatase, which is not stimulated by Ca 2+ . We ¢nd that in such injected hepatocytes, Ca 2+ oscillations generated by modest agonist levels are suppressed, presumably because of the decreased dose in InsP 3 , but that at higher doses of agonist, oscillations reappear, with characteristics similar to those of untreated cells at low agonist doses. Altogether, these results suggest that oscillations in InsP 3 concentration due to Ca 2+ -stimulated InsP 3 catabolism do not play a major role for the oscillations in Ca 2+ concentration. ß 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.
The FASEB Journal, 2000
Intercellular calcium signals are propagated in multicellular hepatocyte systems as well as in th... more Intercellular calcium signals are propagated in multicellular hepatocyte systems as well as in the intact liver. The stimulation of connected hepatocytes by glycogenolytic agonists induces reproducible sequences of intracellular calcium concentration increases, resulting in unidirectional intercellular calcium waves. Hepatocytes are characterized by a gradient of vasopressin binding sites from the periportal to perivenous areas of the cell plate in hepatic lobules. Also, coordination of calcium signals between neighboring cells requires the presence of the agonist at each cell surface as well as gap junction permeability. We present a model based on the junctional coupling of several hepatocytes differing in sensitivity to the agonist and thus in the intrinsic period of calcium oscillations. In this model, each hepatocyte displays repetitive calcium spikes with a slight phase shift with respect to neighboring cells, giving rise to a phase wave. The orientation of the apparent calcium wave is imposed by the direction of the gradient of hormonal sensitivity. Calcium spikes are coordinated by the diffusion across junctions of small amounts of inositol 1,4,5-trisphosphate (InsP 3 ). Theoretical predictions from this model are confirmed experimentally. Thus, major physiological insights may be gained from this model for coordination and spatial orientation of intercellular signals.-Dupont, G., Tordjmann, T., Clair, C., Swillens, S., Claret, M., Combettes, L. Mechanism of receptororiented intercellular calcium wave propagation in hepatocytes. FASEB J. 14, 279 -289 (2000)
Biophysical Journal, 2013
Hint2, one of the five members of the superfamily of the histidine triad AMP-lysine hydrolase pro... more Hint2, one of the five members of the superfamily of the histidine triad AMP-lysine hydrolase proteins, is expressed in mitochondria of various cell types. In human adrenocarcinoma cells, Hint2 modulates Ca(2+) handling by mitochondria. As Hint2 is highly expressed in hepatocytes, we investigated if this protein affects Ca(2+) dynamics in this cell type. We found that in hepatocytes isolated from Hint2(-/-) mice, the frequency of Ca(2+) oscillations induced by 1 μM noradrenaline was 150% higher than in the wild-type. Using spectrophotometry, we analyzed the rates of Ca(2+) pumping in suspensions of mitochondria prepared from hepatocytes of either wild-type or Hint2(-/-) mice; we found that Hint2 accelerates Ca(2+) pumping into mitochondria. We then resorted to computational modeling to elucidate the possible molecular target of Hint2 that could explain both observations. On the basis of a detailed model for mitochondrial metabolism proposed in another study, we identified the respiratory chain as the most probable target of Hint2. We then used the model to predict that the absence of Hint2 leads to a premature opening of the mitochondrial permeability transition pore in response to repetitive additions of Ca(2+) in suspensions of mitochondria. This prediction was then confirmed experimentally.
Cold Spring Harbor Perspectives in Biology, 2011
Calcium signaling results from a complex interplay between activation and inactivation of intrace... more Calcium signaling results from a complex interplay between activation and inactivation of intracellular and extracellular calcium permeable channels. This complexity is obvious from the pattern of calcium signals observed with modest, physiological concentrations of calcium-mobilizing agonists, which typically present as sequential regenerative discharges of stored calcium, a process referred to as calcium oscillations. In this review, we discuss recent advances in understanding the underlying mechanism of calcium oscillations through the power of mathematical modeling. We also summarize recent findings on the role of calcium entry through store-operated channels in sustaining calcium oscillations and in the mechanism by which calcium oscillations couple to downstream effectors.
The FASEB Journal, 2007
Benzene polyphosphates containing phosphate groups on one ring are Ins(1,4,5)P3 5-phosphatase inh... more Benzene polyphosphates containing phosphate groups on one ring are Ins(1,4,5)P3 5-phosphatase inhibitors when evaluated against type-I Ins(1,4,5)P3 5-phosphatase. A novel biphenyl derivative, biphenyl 2,3&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,4,5&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,6-pentakisphosphate, with five phosphate groups on two rings was synthesized: It inhibited the activity of two inositol 5-phosphatases: type I and SHIP2 with Ins(1,3,4,5)P4 as substrate. The inhibition was competitive with respect to the substrate. IC50 value measured in rat hepatocytes, which contains the native Ins(1,4,5)P3 5-phosphatase, was in the micromolar range at 1.0 microM Ins(1,4,5)P3 as substrate. Biphenyl 2,3&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,4,5&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,6-pentakisphosphate did not affect the activity of Ins(1,4,5)P3 3-kinase A in the 5-100 microM range. Surprisingly, experimental evidence supports an effect of biphenyl 2,3&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,4,5&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,6-pentakisphosphate at the level of the Ins(1,4,5)P3 receptor. Finally, when injected into rat hepatocytes, the analog affected the frequency of Ca2+ oscillations in a positive or negative way depending on its concentration. At very high concentrations of the analog, Ca2+ oscillations were even suppressed. These data were interpreted as a dual effect of the biphenyl 2,3&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,4,5&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;,6-pentakisphosphate on cytosolic [Ca2+] increases: an activation effect through an increase in Ins(1,4,5)P3 level via Ins(1,4,5)P3 5-phosphatase inhibition and an inhibitory effect, which was exerted directly on the Ins(1,4,5)P3 receptor. Thus, our data show for the first time that the frequency of Ca2+ oscillations in response to a Ca2+-mobilizing agonist can be controlled by inhibitors of type-I Ins(1,4,5)P3 5-phosphatase.
médecine/sciences, 2011
Jusqu'à très récemment, les biologistes mesuraient la concentration d'une protéine ou la vitesse ... more Jusqu'à très récemment, les biologistes mesuraient la concentration d'une protéine ou la vitesse de transformation d'un métabolite par exemple à partir de populations cellulaires. L'extraordinaire développement de nouvelles techniques d'imagerie (microscopie confocale, fluorescence resonance energy transfer [FRET], fluorescence recovery after photobleaching [FRAP], etc.), combiné à l'utilisation de sondes fluorescentes performantes, de protéines chimères (green fluorescent protein [GFP], DsRed, etc.) ou de nanocristaux (quantum dots) permet actuellement une approche quantitative et dynamique de la biologie d'une seule cellule, et rend possible l'analyse par exemple de l'expression spécifique d'un gène ou de la variation d'un flux métabolique. Ces approches révèlent dans bien des cas une assez grande hétérogénéité spatiale, mais aussi temporelle, rappelant, si besoin est, que les cellules sont des entités extrêmement dynamiques . De manière plus inattendue, ces observations à l'échelle cellulaire mettent aussi en évidence le caractère aléatoire (stochastique) des événements moléculaires individuels. Le biologiste est dès lors confronté à de nouvelles questions : comment des événements qui se produisent sur > Les signaux calciques sont organisés dans le temps et dans l'espace, ce qui leur permet d'assurer une signalisation cellulaire spécifique et robuste. En réponse à une stimulation, la concentration nanomolaire du signal Ca 2+ peut augmenter de plusieurs dizaines de fois à proximité de quelques récepteurs de l'inositol (1, 4, 5) trisphosphate (InsP 3 ). Cela se fait sous la forme de vagues se propageant périodiquement dans un tissu ou un organe. Les études de la relation entre ces phénomènes, caractérisés par des échelles temporelles et spatiales très différentes, et les mécanismes qui en sont responsables, sont décrits dans cette revue par une approche fondée sur une interaction étroite entre expériences et modélisation, appliquée principalement à la signalisation calcique dans les hépatocytes. < des échelles de temps et d'espace très différentes sont-ils coordonnés ? Comment les cellules contrôlent-elles, ou même utilisent-elles, des fluctuations moléculaires pour réaliser des tâches dites robustes ? Pour répondre à ces questions, il est nécessaire d'associer démarches expérimentales et théoriques. Ce type d'approche pluridisciplinaire, utilisé avec succès depuis de nombreuses années en neurophysiologie, s'étend aujourd'hui à de nombreux aspects de la physiologie cellulaire. La signalisation calcique, objet de cette revue, en est un exemple frappant. Depuis les travaux de Ringer il y a près de 130 ans, l'importance du calcium dans la physiologie cellulaire n'est plus à démontrer, mais l'extraordinaire complexité de l'organisation dans le temps et dans l'espace de cette signalisation n'a été révelée que grâce aux progrès techniques réalisés depuis une petite vingtaine d'années. En combinant expériences et modélisation, nous avons pu disséquer certains aspects de cette signalisation, notamment dans les hépatocytes. En réponse à une stimulation extracellulaire, les hépatocytes augmentent leur concentration en Ca 2+ intracellulaire, celle-ci étant responsable de réponses physiologiques comme la production de glucose, la sécrétion biliaire, l'expression génétique ou la régénération du foie. Dans ces cellules, comme dans la plupart des cellules non excitables [2], des agonistes induisent la synthèse d'inositol (1, 4, 5) trisphosphate (InsP 3 ). L'InsP 3 formé se fixe sur des récepteurs (InsP 3 R)/canaux calciques présents dans la membrane du réticulum endoplasmique (RE). La libération de Ca 2+ via les récepteurs de l'InsP 3 est régulée par la concentration en Ca 2+ dans le cytoplasme ; c'est aussi le cas pour les
Biophysical Journal, 2008
Signal-induced Ca 21 oscillations have been observed in many cell types and play a primary role i... more Signal-induced Ca 21 oscillations have been observed in many cell types and play a primary role in cell physiology. Although it is the regular character of these oscillations that first catches the attention, a closer look at time series of Ca 21 increases reveals that the fluctuations on the period during individual spike trains are far from negligible. Here, we perform a statistical analysis of the regularity of Ca 21 oscillations in norepinephrine-stimulated hepatocytes and find that the coefficient of variation lies between 10% and 15%. Stochastic simulations based on Gillespie's algorithm and considering realistic numbers of Ca 21 ions and inositol trisphosphate (InsP 3 ) receptors account for this variability if the receptors are assumed to be grouped in clusters of a few tens of channels. Given the relatively small number of clusters (;200), the model predicts the existence of repetitive spikes induced by fluctuations (stochastic resonance). Oscillations of this type are found in hepatocytes at subthreshold concentrations of norepinephrine. We next predict with the model that the isoforms of the InsP 3 receptor can affect the variability of the oscillations. In contrast, possible accompanying InsP 3 oscillations have no impact on the robustness of signal-induced repetitive Ca 21 spikes.
Cell Calcium, 2011
In hepatocytes, as in other cell types, Ca(2+) signaling is subject to complex regulations, which... more In hepatocytes, as in other cell types, Ca(2+) signaling is subject to complex regulations, which result largely from the intrinsic characteristics of the different inositol 1,4,5-trisphosphate receptor (InsP(3)R) isoforms and from their interactions with other proteins. Although sigma1 receptors (Sig-1Rs) are widely expressed in the liver, their involvement in hepatic Ca(2+) signaling remains unknown. We here report that in this cell type Sig-1R interact with type 1 isoforms of the InsP(3) receptors (InsP(3)R-1). These results obtained by immunoprecipitation experiments are confirmed by the observation that Sig-1R proteins and InsP(3)R-1 colocalize in hepatocytes. However, Sig-1R ligands have no effect on InsP(3)-induced Ca(2+) release in hepatocytes. This can be explained by the rather low expression level expression of InsP(3)R-1. In contrast, we find that Sig-1R ligands can inhibit agonist-induced Ca(2+) signaling via an inhibitory effect on InsP(3) synthesis. We show that this inhibition is due to the stimulation of PKC activity by Sig-1R, resulting in the well-known down-regulation of the signaling pathway responsible for the transduction of the extracellular stimulus into InsP(3) synthesis. The PKC sensitive to Sig-1R activity belongs to the family of conventional PKC, but the precise molecular mechanism of this regulation remains to be elucidated.
Biology of the Cell, 2006
Background information. Oscillations of cytosolic Ca 2+ are well-known to rely on the regulatory ... more Background information. Oscillations of cytosolic Ca 2+ are well-known to rely on the regulatory properties of the InsP 3 R (inositol 1,4,5-trisphosphate receptor). Three isoforms of this channel have been identified. They differ in their regulatory properties by Ca 2+ and InsP 3 . Experiments in different cell types clearly indicate that the relative amounts of each isoform affect the time course of Ca 2+ changes after agonist stimulation. In the present study, we investigate whether different steady-state curves for the open probability of the InsP 3 Rs as a function of Ca 2+ imply different dynamical behaviours when these receptors are present in a cellular environment. We therefore describe by a specific phenomenological model the three main types of curves that have been reported: (i) the classical bell-shaped curve, (ii) the bell-shaped curve that is shifted towards higher Ca 2+ concentrations when InsP 3 is increased, and (iii) a monotonous increasing function of cytosolic Ca 2+ .
Chaos: An Interdisciplinary Journal of Nonlinear Science, 2009
In most cells, Ca 2+ increases in response to external stimulation are organized in the form of o... more In most cells, Ca 2+ increases in response to external stimulation are organized in the form of oscillations and waves that sometimes propagate from one cell to another. Numerous experimental and theoretical studies reveal that this spatiotemporal organization contains a non-negligible level of stochasticity. In this study, we extend the previous work based on a statistical analysis of experimental Ca 2+ traces in isolated, hormone-stimulated hepatocytes and on stochastic simulations of Ca 2+ oscillations based on the Gillespie's algorithm. Comparison of the coefficients of variation in the periods of experimental and simulated Ca 2+ spikes provides information about the clustering and the specific subtypes of the Ca 2+ channels. In hepatocytes coupled by gap junctions, the global perfusion with a hormone leads to successive Ca 2+ responses, giving the appearance of an intercellular wave. Statistical analysis of experimental Ca 2+ oscillations in coupled hepatocytes confirms that this coordinated Ca 2+ spiking corresponds to a phase wave but suggests the existence of an additional coupling mechanism.
FEBS Letters, 2003
Receptor-mediated production of inositol 1,4,5-trisphosphate (InsP 3 ) initiates Ca 2+ release an... more Receptor-mediated production of inositol 1,4,5-trisphosphate (InsP 3 ) initiates Ca 2+ release and is responsible for cytosolic Ca 2+ oscillations. InsP 3 oscillations have also been observed in some cells. One of the enzymes controlling InsP 3 catabolism, the InsP 3 3-kinase, is stimulated by Ca 2+ ; this regulation is presumably part of the reason for InsP 3 oscillations that have been observed in some cells. Here, we investigate the possible role of Ca 2+ -activated InsP 3 catabolism on the characteristics of the InsP 3 -induced Ca 2+ oscillations. Numerical simulations show that if it is assumed that the Ca 2+ -independent InsP 3 catabolism is predominant, Ca 2+ oscillations remain qualitatively unchanged although the relative amplitude of the oscillations in InsP 3 concentrations becomes minimal. We tested this prediction in hepatocytes by masking the Ca 2+ -dependent InsP 3 catabolism by 3-kinase through the injection of massive amounts of InsP 3 5-phosphatase, which is not stimulated by Ca 2+ . We ¢nd that in such injected hepatocytes, Ca 2+ oscillations generated by modest agonist levels are suppressed, presumably because of the decreased dose in InsP 3 , but that at higher doses of agonist, oscillations reappear, with characteristics similar to those of untreated cells at low agonist doses. Altogether, these results suggest that oscillations in InsP 3 concentration due to Ca 2+ -stimulated InsP 3 catabolism do not play a major role for the oscillations in Ca 2+ concentration. ß 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.
The FASEB Journal, 2000
Intercellular calcium signals are propagated in multicellular hepatocyte systems as well as in th... more Intercellular calcium signals are propagated in multicellular hepatocyte systems as well as in the intact liver. The stimulation of connected hepatocytes by glycogenolytic agonists induces reproducible sequences of intracellular calcium concentration increases, resulting in unidirectional intercellular calcium waves. Hepatocytes are characterized by a gradient of vasopressin binding sites from the periportal to perivenous areas of the cell plate in hepatic lobules. Also, coordination of calcium signals between neighboring cells requires the presence of the agonist at each cell surface as well as gap junction permeability. We present a model based on the junctional coupling of several hepatocytes differing in sensitivity to the agonist and thus in the intrinsic period of calcium oscillations. In this model, each hepatocyte displays repetitive calcium spikes with a slight phase shift with respect to neighboring cells, giving rise to a phase wave. The orientation of the apparent calcium wave is imposed by the direction of the gradient of hormonal sensitivity. Calcium spikes are coordinated by the diffusion across junctions of small amounts of inositol 1,4,5-trisphosphate (InsP 3 ). Theoretical predictions from this model are confirmed experimentally. Thus, major physiological insights may be gained from this model for coordination and spatial orientation of intercellular signals.-Dupont, G., Tordjmann, T., Clair, C., Swillens, S., Claret, M., Combettes, L. Mechanism of receptororiented intercellular calcium wave propagation in hepatocytes. FASEB J. 14, 279 -289 (2000)
Biophysical Journal, 2013
Hint2, one of the five members of the superfamily of the histidine triad AMP-lysine hydrolase pro... more Hint2, one of the five members of the superfamily of the histidine triad AMP-lysine hydrolase proteins, is expressed in mitochondria of various cell types. In human adrenocarcinoma cells, Hint2 modulates Ca(2+) handling by mitochondria. As Hint2 is highly expressed in hepatocytes, we investigated if this protein affects Ca(2+) dynamics in this cell type. We found that in hepatocytes isolated from Hint2(-/-) mice, the frequency of Ca(2+) oscillations induced by 1 μM noradrenaline was 150% higher than in the wild-type. Using spectrophotometry, we analyzed the rates of Ca(2+) pumping in suspensions of mitochondria prepared from hepatocytes of either wild-type or Hint2(-/-) mice; we found that Hint2 accelerates Ca(2+) pumping into mitochondria. We then resorted to computational modeling to elucidate the possible molecular target of Hint2 that could explain both observations. On the basis of a detailed model for mitochondrial metabolism proposed in another study, we identified the respiratory chain as the most probable target of Hint2. We then used the model to predict that the absence of Hint2 leads to a premature opening of the mitochondrial permeability transition pore in response to repetitive additions of Ca(2+) in suspensions of mitochondria. This prediction was then confirmed experimentally.