An Excel-based model of Ca2+diffusion and fura 2 measurements in a spherical cell

A biophysically detailed computational model of urinary bladder small DRG neuron soma

Bladder small DRG neurons, which are putative nociceptors pivotal to urinary bladder function, express more than a dozen different ionic membrane mechanisms: ion channels, pumps and exchangers. Small-conductance Ca2+-activated K+ (SKCa) channels which were earlier thought to be gated solely by intracellular Ca2+ concentration ([Ca]i) have recently been shown to exhibit inward rectification with respect to membrane potential. The effect of SKCa inward rectification on the excitability of these neurons is unknown. Furthermore, studies on the role of KCa channels in repetitive firing and their contributions to different types of afterhyperpolarization (AHP) in these neurons are lacking. In order to study these phenomena, we first constructed and validated a biophysically detailed single compartment model of bladder small DRG neuron soma constrained by physiological data. The model includes twenty-two major known membrane mechanisms along with intracellular Ca2+ dynamics comprising Ca2+ diffusion, cytoplasmic buffering, and endoplasmic reticulum (ER) and mitochondrial mechanisms. Using modelling studies, we show that inward rectification of SKCa is an important parameter regulating neuronal repetitive firing and that its absence reduces action potential (AP) firing frequency. We also show that SKCa is more potent in reducing AP spiking than the large-conductance KCa channel (BKCa) in these neurons. Moreover, BKCa was found to contribute to the fast AHP (fAHP) and SKCa to the medium-duration (mAHP) and slow AHP (sAHP). We also report that the slow inactivating A-type K+ channel (slow KA) current in these neurons is composed of 2 components: an initial fast inactivating (time constant ∼ 25-100 ms) and a slow inactivating (time constant ∼ 200-800 ms) current. We discuss the implications of our findings, and how our detailed model can help further our understanding of the role of C-fibre afferents in the physiology of urinary bladder as well as in certain disorders.

From synaptically localized to volume transmission by nitric oxide

Nitric oxide (NO) functions widely as a transmitter/diffusible second messenger in the central nervous system, exerting physiological effects in target cells by binding to specialized guanylyl cyclase-coupled receptors, resulting in cGMP generation. Despite having many context-dependent physiological roles and being implicated in numerous disease states, there has been a lack of clarity about the ways that NO operates at the cellular and subcellular levels. Recently, several approaches have been used to try to gain a more concrete, quantitative understanding of this unique signalling pathway. These approaches have included analysing the kinetics of NO receptor function, real-time imaging of cellular NO signal transduction in target cells, and the use of ultrasensitive detector cells to record NO as it is being generated from native sources in brain tissue. The current picture is that, when formed in a synapse, NO is likely to act only very locally, probably mostly within the confines of that synapse, and to exist only in picomolar concentrations. Nevertheless, closely neighbouring synapses may also be within reach, raising the possibility of synaptic crosstalk. By engaging its enzyme-coupled receptors, the low NO concentrations are able to stimulate physiological (submicromolar) increases in cGMP concentration in an activity-dependent manner. When many NO-emitting neurones or synapses are active simultaneously in a tissue region, NO can act more like a volume transmitter to influence, and perhaps coordinate, the behaviour of cells within that region, irrespective of their identity and anatomical connectivity.

Light adaptation increases response latency of alpha ganglion cells via a threshold-like nonlinearity

Adaptation is an important process of sensory systems to adjust sensitivity to ensure the appropriate information encoding. Sensitivity and kinetics of retinal ganglion cell (RGC) responses have been studied extensively using a brief flash superimposed on different but steady backgrounds. However, it is still unclear if light adaptation exerts any effect on more complex response properties, such as response nonlinearity. In this study, we found that the latency of spike responses to a repeated flashing spot stimulation increased by 30 ms in the mouse ON α RGCs (An ON-type RGC is excited when a spot is turned on in the center of its receptive field). A single dimming event preceding the test flash on a steady adapting background could also produce similar effect in increasing latency of light responses. A simple computational model with a linear transformation of the light stimulus and a threshold-like nonlinearity could account for the experimental data. Moreover, the strength of the measured nonlinearity and the response latency were affected by the duration of light adaptation. The possible biological processes underlying this nonlinearity were explored. Voltage clamp recording revealed the presence of the increase in latency and threshold-like nonlinearity in the excitatory input of RGCs. However, no comparable nonlinearity was observed in the light responses of the ON cone bipolar cells. We further excluded GABAergic and glycinergic inhibition, N-methyl-D-aspartate receptor rectification and voltage-gated Na(+) channels as potential sources of this nonlinearity by pharmacological experiments. Our results indicate the bipolar cell terminals as the potential site of nonlinearity. Computational modeling constrained by experimental data supports that conclusion and suggests the voltage-sensitive Ca(++) channels and Ca(++)-dependent vesicle release in the bipolar cell terminals as mechanistic basis.

Modulation of cardiac ryanodine receptor channels by alkaline earth cations

Cardiac ryanodine receptor (RyR2) function is modulated by Ca(2+) and Mg(2+). To better characterize Ca(2+) and Mg(2+) binding sites involved in RyR2 regulation, the effects of cytosolic and luminal earth alkaline divalent cations (M(2+): Mg(2+), Ca(2+), Sr(2+), Ba(2+)) were studied on RyR2 from pig ventricle reconstituted in bilayers. RyR2 were activated by M(2+) binding to high affinity activating sites at the cytosolic channel surface, specific for Ca(2+) or Sr(2+). This activation was interfered by Mg(2+) and Ba(2+) acting at low affinity M(2+)-unspecific binding sites. When testing the effects of luminal M(2+) as current carriers, all M(2+) increased maximal RyR2 open probability (compared to Cs(+)), suggesting the existence of low affinity activating M(2+)-unspecific sites at the luminal surface. Responses to M(2+) vary from channel to channel (heterogeneity). However, with luminal Ba(2+)or Mg(2+), RyR2 were less sensitive to cytosolic Ca(2+) and caffeine-mediated activation, openings were shorter and voltage-dependence was more marked (compared to RyR2 with luminal Ca(2+)or Sr(2+)). Kinetics of RyR2 with mixtures of luminal Ba(2+)/Ca(2+) and additive action of luminal plus cytosolic Ba(2+) or Mg(2+) suggest luminal M(2+) differentially act on luminal sites rather than accessing cytosolic sites through the pore. This suggests the presence of additional luminal activating Ca(2+)/Sr(2+)-specific sites, which stabilize high P(o) mode (less voltage-dependent) and increase RyR2 sensitivity to cytosolic Ca(2+) activation. In summary, RyR2 luminal and cytosolic surfaces have at least two sets of M(2+) binding sites (specific for Ca(2+) and unspecific for Ca(2+)/Mg(2+)) that dynamically modulate channel activity and gating status, depending on SR voltage.

A biophysically based mathematical model of unitary potential activity in interstitial cells of Cajal

Unitary potential (UP) depolarizations are the basic intracellular events responsible for pacemaker activity in interstitial cells of Cajal (ICCs), and are generated at intracellular sites termed "pacemaker units". In this study, we present a mathematical model of the transmembrane ion flows and intracellular Ca(2+) dynamics from a single ICC pacemaker unit acting at near-resting membrane potential. This model quantitatively formalizes the framework of a novel ICC pacemaking mechanism that has recently been proposed. Model simulations produce spontaneously rhythmic UP depolarizations with an amplitude of approximately 3 mV at a frequency of 0.05 Hz. The model predicts that the main inward currents, carried by a Ca(2+)-inhibited nonselective cation conductance, are activated by depletion of sub-plasma-membrane [Ca(2+)] caused by sarcoendoplasmic reticulum calcium ATPase Ca(2+) sequestration. Furthermore, pacemaker activity predicted by our model persists under simulated voltage clamp and is independent of [IP(3)] oscillations. The model presented here provides a basis to quantitatively analyze UP depolarizations and the biophysical mechanisms underlying their production.

Depolarization-induced calcium responses in sympathetic neurons: relative contributions from Ca2+ entry, extrusion, ER/mitochondrial Ca2+ uptake and release, and Ca2+ buffering

Many models have been developed to account for stimulus-evoked [Ca(2+)] responses, but few address how responses elicited in specific cell types are defined by the Ca(2+) transport and buffering systems that operate in the same cells. In this study, we extend previous modeling studies by linking the time course of stimulus-evoked [Ca(2+)] responses to the underlying Ca(2+) transport and buffering systems. Depolarization-evoked [Ca(2+)](i) responses were studied in sympathetic neurons under voltage clamp, asking how response kinetics are defined by the Ca(2+) handling systems expressed in these cells. We investigated five cases of increasing complexity, comparing observed and calculated responses deduced from measured Ca(2+) handling properties. In Case 1, [Ca(2+)](i) responses were elicited by small Ca(2+) currents while Ca(2+) transport by internal stores was inhibited, leaving plasma membrane Ca(2+) extrusion intact. In Case 2, responses to the same stimuli were measured while mitochondrial Ca(2+) uptake was active. In Case 3, responses were elicited as in Case 2 but with larger Ca(2+) currents that produce larger and faster [Ca(2+)](i) elevations. Case 4 included the mitochondrial Na/Ca exchanger. Finally, Case 5 included ER Ca(2+) uptake and release pathways. We found that [Ca(2+)](i) responses elicited by weak stimuli (Cases 1 and 2) could be quantitatively reconstructed using a spatially uniform model incorporating the measured properties of Ca(2+) entry, removal, and buffering. Responses to strong depolarization (Case 3) could not be described by this model, but were consistent with a diffusion model incorporating the same Ca(2+) transport and buffering descriptions, as long as endogenous buffers have low mobility, leading to steep radial [Ca(2+)](i) gradients and spatially nonuniform Ca(2+) loading by mitochondria. When extended to include mitochondrial Ca(2+) release (Case 4) and ER Ca(2+) transport (Case 5), the diffusion model could also account for previous measurements of stimulus-evoked changes in total mitochondrial and ER Ca concentration.

Dynamic and static calcium gradients inside large snail (Helix aspersa) neurones detected with calcium-sensitive microelectrodes

We have used quartz Ca²⁺-sensitive microelectrodes (CASMs) in large voltage-clamped snail neurones to investigate the inward spread of Ca²⁺ after a brief depolarisation. Both steady state and [Ca²⁺]_i transients changed with depth of penetration. When the CASM tip was within 20 μm of the far side of the cell the [Ca²⁺]_i transient time to peak was 4.4 ± 0.5 s, rising to 14.7 ± 0.7 s at a distance of 80 μm. We estimate that the Ca²⁺ transients travelled centripetally at an average speed of 6 μm² s⁻¹ and decreased in size by half over a distance of about 45 μm. Cyclopiazonic acid had little effect on the size and time to peak of Ca²⁺ transients but slowed their recovery significantly. This suggests that the endoplasmic reticulum curtails rather than reinforces the transients. Injecting the calcium buffer BAPTA made the Ca²⁺ transients more uniform in size and increased their times to peak and rates of recovery near the membrane. We have developed a computational model for the transients, which includes diffusion, uptake and Ca²⁺ extrusion. Good fits were obtained with a rather large apparent diffusion coefficient of about 90 ± 20 μm² s⁻¹.This may assist fast recovery by extrusion.

Software for simulating calcium-triggered exocytotic processes

We describe a software package for the simulation of exocytotic events from readily releasable pools of secretory vesicles in neuroendocrine cells and presynaptic terminals. The visual package Ca3D_Exolab simulates the entry of Ca(2+) through the calcium channels, the kinetic reactions of calcium with buffers, the diffusion of calcium and mobile buffers, and the kinetic reactions of calcium with the secretory vesicles. The location of both channels and secretory vesicles can be set by using a graphical interface. Calcium and buffer concentrations at different depths from the cellular membrane and capacitance time courses are obtained as outputs. The software package also provides a descriptive statistical data analysis of the different output data.

AS160 regulates insulin- and contraction-stimulated glucose uptake in mouse skeletal muscle

Insulin and contraction are potent stimulators of GLUT4 translocation and increase skeletal muscle glucose uptake. We recently identified the Rab GTPase-activating protein (GAP) AS160 as a putative point of convergence linking distinct upstream signaling cascades induced by insulin and contraction in mouse skeletal muscle. Here, we studied the functional implications of these AS160 signaling events by using an in vivo electroporation technique to overexpress wild type and three AS160 mutants in mouse tibialis anterior muscles: 1) AS160 mutated to prevent phosphorylation on four regulatory phospho-Akt-substrate sites (4P); 2) AS160 mutated to abolish Rab GTPase activity (R/K); and 3) double mutant AS160 containing both 4P and R/K mutations (2M). One week following gene injection, protein expression for all AS160 isoforms was elevated over 7-fold. To determine the effects of AS160 on insulin- and contraction-stimulated glucose uptake in transfected muscles, we measured [3H]2-deoxyglucose uptake in vivo following intravenous glucose administration and in situ muscle contraction, respectively. Insulin-stimulated glucose uptake was significantly inhibited in muscles overexpressing 4P mutant AS160. However, this inhibition was completely prevented by concomitant disruption of AS160 Rab GAP activity. Transfection with 4P mutant AS160 also significantly impaired contraction-stimulated glucose uptake, as did overexpression of wild type AS160. In contrast, overexpressing mutant AS160 lacking Rab GAP activity resulted in increases in both sham and contraction-stimulated muscles. These data suggest that AS160 regulates both insulin- and contraction-stimulated glucose metabolism in mouse skeletal muscle in vivo and that the effects of mutant AS160 on the actions of insulin and contraction are not identical. Our findings directly implicate AS160 as a critical convergence factor for independent stimulators of skeletal muscle glucose uptake.

Dynamic imaging with MRI contrast agents: quantitative considerations

Time-resolved MRI has had enormous impact in cognitive science and may become a significant tool in basic biological research with the application of new molecular imaging agents. In this paper, we examine the temporal characteristics of MRI contrast agents that could be used in dynamic studies. We consider "smart" T1 contrast agents, T2 agents based on reversible aggregation of superparamagnetic nanoparticles and sensors that produce changes in saturation transfer effects (chemical exchange saturation transfer, CEST). We discuss response properties of several agents with reference to available experimental data, and we develop a new theoretical model that predicts the response rates and relaxivity changes of aggregation-based sensors. We also perform calculations to define the extent to which constraints on temporal resolution are imposed by the imaging methods themselves. Our analysis confirms that some small T1 agents may be compatible with MRI temporal resolution on the order of 100 ms. Nanoparticle aggregation T2 sensors are applicable at much lower concentrations, but are likely to respond on a single second or slower timescale. CEST agents work at high concentrations and temporal resolutions of 1-10 s, limited by a requirement for long presaturation periods in the MRI pulse sequence.

Induction of Apoptosis in Renal Cell Carcinoma by Reactive Oxygen Species: Involvement of Extracellular Signal-Regulated Kinase 1/2, p38δ/γ, Cyclooxygenase-2 Down-Regulation, and Translocation of Apoptosis-Inducing Factor

Renal cell carcinoma (RCC) is the most common malignancy of the kidney. Unfortunately, RCCs are highly refractory to conventional chemotherapy, radiation therapy, and even immunotherapy. Thus, novel therapeutic targets need to be sought for the successful treatment of RCCs. We now report that 6-anilino-5,8-quinolinequinone (LY83583), an inhibitor of cyclic GMP production, induced growth arrest and apoptosis of the RCC cell line 786-0. It did not prove deleterious to normal renal epithelial cells, an important aspect of chemotherapy. To address the cellular mechanism(s), we used both genetic and pharmacological approaches. LY83583 induced a time- and dose-dependent increase in RCC apoptosis through dephosphorylation of mitogen-activated protein kinase kinase 1/2 and its downstream extracellular signal-regulated kinases (ERK) 1 and -2. In addition, we observed a decrease in Elk-1 phosphorylation and cyclooxygenase-2 (COX-2) down-regulation. We were surprised that we failed to observe an increase in either c-Jun NH(2)-terminal kinase or p38alpha and -beta mitogen-activated protein kinase activation. In contradiction, reintroduction of p38delta by stable transfection or overexpression of p38gamma dominant negative abrogated the apoptotic effect. Cell death was associated with a decrease and increase in Bcl-x(L) and Bax expression, respectively, as well as release of cytochrome c and translocation of apoptosis-inducing factor. These events were associated with an increase in reactive oxygen species formation. The antioxidant N-acetyl l-cysteine, however, opposed LY83583-mediated mitochondrial dysfunction, ERK1/2 inactivation, COX-2 down-regulation, and apoptosis. In conclusion, our results suggest that LY83583 may represent a novel therapeutic agent for the treatment of RCC, which remains highly refractory to antineoplastic agents. Our data provide a molecular basis for the anticancer activity of LY83583.

Calcium3D: a visual software package for the simulation of calcium buffered diffusion in neuroendocrine cells

We present Calcium3D, a user-friendly software package for simulating calcium triggered processes in neuroendocrine cells. We use Monte Carlo methods for the simulation of the basic processes involved: entry of calcium into the cytoplasm, the diffusion of ions and mobile intracellular calcium buffers inside the intracellular medium, and the kinetics of the reaction of calcium with these buffers. The outputs of the simulation are calcium and buffer concentrations as a function of time and for different depths from the cellular membrane.

Passive stretching produces Akt- and MAPK-dependent augmentations of GLUT4 translocation and glucose uptake in skeletal muscles of mice

Muscle contraction is accompanied by passive stretching or deformation of cells and tissues. The present study aims to clarify whether or not acute passive stretching evokes glucose transporter 4 (GLUT4) translocation and glucose uptake in skeletal muscles of mice. Passive stretching mainly induced GLUT4 translocation from an intracellular membrane-rich fraction (PF5) to a plasma membrane-rich fraction (F2) and accelerated glucose uptake in hindlimb muscles; whereas electrical stimulation, which mimics physical exercise in vivo, and insulin, each induced GLUT4 translocation from an intracellular membrane-rich fraction (PF5) to a fraction rich in plasma membrane (F2), and to one rich in transverse tubules (PF3), along with subsequent glucose uptake. Mechanical stretching increased phosphorylation of Akt and p38 mitogen-activated protein kinase (p38 MAPK), but it had no apparent effect on the activity of AMP-activated protein kinase (AMPK). Electrical stimulation augmented the activity of not only AMPK but also phosphorylation of Akt and p38 MAPK. Our results suggest that passive stretching produces translocation of GLUT4 mainly from the fraction rich in intracellular membrane to that rich in plasma membrane, and that the glucose uptake could be Akt- and p38 MAPK-dependent, but AMPK-independent manners.

AMP-activated protein kinase alpha2 activity is not essential for contraction- and hyperosmolarity-induced glucose transport in skeletal muscle

To examine the role of AMP-activated protein kinase (AMPK) in muscle glucose transport, we generated muscle-specific transgenic mice (TG) carrying cDNAs of inactive alpha2 (alpha2i TG) and alpha1 (alpha1i TG) catalytic subunits. Extensor digitorum longus (EDL) muscles from wild type and TG mice were isolated and subjected to a series of in vitro incubation experiments. In alpha2i TG mice basal alpha2 activity was barely detectable, whereas basal alpha1 activity was only partially reduced. Known AMPK stimuli including 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR), rotenone (a Complex I inhibitor), dinitrophenol (a mitochondrial uncoupler), muscle contraction, and sorbitol (producing hyperosmolar shock) did not increase AMPK alpha2 activity in alpha2i TG mice, whereas alpha1 activation was attenuated by only 30-50%. Glucose transport was measured in vitro using isolated EDL muscles from alpha2i TG mice. AICAR- and rotenone-stimulated glucose transport was fully inhibited in alpha2i TG mice; however, the lack of AMPK alpha2 activity had no effect on contraction- or sorbitol-induced glucose transport. Similar to these observations in vitro, contraction-stimulated glucose transport, assessed in vivo by 2-deoxy-d-[(3)H]glucose incorporation into EDL, tibialis anterior, and gastrocnemius muscles, was normal in alpha2i TG mice. Thus, AMPK alpha2 activation is essential for some, but not all, insulin-independent glucose transport. Muscle contraction- and hyperosmolarity-induced glucose transport may be regulated by a redundant mechanism in which AMPK alpha2 is one of multiple signaling pathways.

Counting on mitogen-activated protein kinases—ERKs 3, 4, 5, 6, 7 and 8

Signal transduction pathways in eukaryotic cells integrate diverse extracellular signals, and regulate complex biological responses such as growth, differentiation and death. One group of proline-directed Ser/Thr protein kinases, the mitogen-activated protein kinases (MAPKs), plays a central role in these signalling pathways. Much attention has focused in recent years on three subfamilies of MAPKs, the extracellular signal regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. However, the ERK family is broader than the ERK1 and ERK2 proteins that have been the subject of most studies in this area. Here we overview the work on ERKs 3 to 8, emphasising where possible their biological activities as well as distinctive biochemical properties. It is clear from these studies that these additional ERKs show similarities to ERK1 and ERK2, but with some interesting differences that challenge the paradigm of the archetypical ERK1/2 MAPK pathway.