Calcium Permeable AMPA Receptor-Dependent Long Lasting Plasticity of Intrinsic Excitability in Fast Spiking Interneurons of the Dentate Gyrus Decreases Inhibition in the Granule Cell Layer

The local fast-spiking interneurons (FSINs) are considered to be crucial for the generation, maintenance, and modulation of neuronal network oscillations especially in the gamma frequency band. Gamma frequency oscillations have been associated with different aspects of behavior. But the prolonged effects of gamma frequency synaptic activity on the FSINs remain elusive. Using whole cell current clamp patch recordings, we observed a sustained decrease of intrinsic excitability in the FSINs of the dentate gyrus (DG) following repetitive stimulations of the mossy fibers at 30 Hz (gamma bursts). Surprisingly, the granule cells (GCs) did not express intrinsic plastic changes upon similar synaptic excitation of their apical dendritic inputs. Interestingly, pairing the gamma bursts with membrane hyperpolarization accentuated the plasticity in FSINs following the induction protocol, while the plasticity attenuated following gamma bursts paired with membrane depolarization. Paired pulse ratio measurement of the synaptic responses did not show significant changes during the experiments. However, the induction protocols were accompanied with postsynaptic calcium rise in FSINs. Interestingly, the maximum and the minimum increase occurred during gamma bursts with membrane hyperpolarization and depolarization respectively. Including a selective blocker of calcium-permeable AMPA receptors (CP-AMPARs) in the bath; significantly attenuated the calcium rise and blocked the membrane potential dependence of the calcium rise in the FSINs, suggesting their involvement in the observed phenomenon. Chelation of intracellular calcium, blocking HCN channel conductance or blocking CP-AMPARs during the experiment forbade the long lasting expression of the plasticity. Simultaneous dual patch recordings from FSINs and synaptically connected putative GCs confirmed the decreased inhibition in the GCs accompanying the decreased intrinsic excitability in the FSINs. Experimentally constrained network simulations using NEURON predicted increased spiking in the GC owing to decreased input resistance in the FSIN. We hypothesize that the selective plasticity in the FSINs induced by local network activity may serve to increase information throughput into the downstream hippocampal subfields besides providing neuroprotection to the FSINs. (c) 2014 Wiley Periodicals, Inc.

Calcium dependent CAMTA1 in adult stem cell commitment to a myocardial lineage.

The phenotype of somatic cells has recently been found to be reversible. Direct reprogramming of one cell type into another has been achieved with transduction and over expression of exogenous defined transcription factors emphasizing their role in specifying cell fate. To discover early and novel endogenous transcription factors that may have a role in adult-derived stem cell acquisition of a cardiomyocyte phenotype, mesenchymal stem cells from human and mouse bone marrow and rat liver were co-cultured with neonatal cardiomyocytes as an in vitro cardiogenic microenvironment. Cell-cell communications develop between the two cell types as early as 24 hrs in co-culture and are required for elaboration of a myocardial phenotype in the stem cells 8-16 days later. These intercellular communications are associated with novel Ca(2+) oscillations in the stem cells that are synchronous with the Ca(2+) transients in adjacent cardiomyocytes and are detected in the stem cells as early as 24-48 hrs in co-culture. Early and significant up-regulation of Ca(2+)-dependent effectors, CAMTA1 and RCAN1 ensues before a myocardial program is activated. CAMTA1 loss-of-function minimizes the activation of the cardiac gene program in the stem cells. While the expression of RCAN1 suggests involvement of the well-characterized calcineurin-NFAT pathway as a response to a Ca(2+) signal, the CAMTA1 up-regulated expression as a response to such a signal in the stem cells was unknown. Cell-cell communications between the stem cells and adjacent cardiomyocytes induce Ca(2+) signals that activate a myocardial gene program in the stem cells via a novel and early Ca(2+)-dependent intermediate, up-regulation of CAMTA1.

Calcium signaling in ocular arterioles

Local control of blood flow to the photoreceptors and associated neurons in the retina is largely achieved through changes in tone within the choroidal and retinal arterioles. This is primarily achieved through changes in [Ca2+] within the smooth muscle of these vessels, which regulates cell contraction and vascular constriction. Here we review some aspects of the cell physiology involved in these Ca2+-signaling processes, with particular emphasis on the molecular mechanisms involved. Ca2+-influx across the plasma membrane can occur via a variety of Ca2+-channels, including voltage-operated, store-operated, and receptor-operated channels. Ca2+ may also be released from intracellular stores via RyR-, or IP3R-gated channels in the SR membrane. Using high-speed confocal Ca2+-imaging, we have also demonstrated that the resulting signals are far from homogeneous, with spontaneous activity in retinal arterioles being characterized by both localized Ca2+-sparks and more global Ca2+-waves and oscillations. These signals may be specifically and differentially targeted, for example, to Ca2+-sensitive ion channels (stimulus-excitation coupling), or pathways regulating contraction (stimulus-contraction coupling). Exploring the role of changes in such targeting in disease states will provide exciting opportunities for future research.

Functional innervation of Guinea-pig bladder interstitial cells of cajal subtypes: neurogenic stimulation evokes in situ calcium transients

Several populations of interstitial cells of Cajal (ICC) exist in the bladder, associated with intramural nerves. Although ICC respond to exogenous agonists, there is currently no evidence of their functional innervation. The objective was to determine whether bladder ICC are functionally innervated. Guinea-pig bladder tissues, loaded with fluo-4AM were imaged with fluorescent microscopy and challenged with neurogenic electrical field stimulation (EFS). All subtypes of ICC and smooth muscle cells (SMC) displayed spontaneous Ca2+-oscillations. EFS (0.5Hz, 2Hz, 10Hz) evoked tetrodotoxin (1µM)-sensitive Ca2+-transients in lamina propria ICC (ICC-LP), detrusor ICC and perivascular ICC (PICC) associated with mucosal microvessels. EFS responses in ICC-LP were significantly reduced by atropine or suramin. SMC and vascular SMC (VSM) also responded to EFS. Spontaneous Ca2+-oscillations in individual ICC-LP within networks occurred asynchronously whereas EFS evoked coordinated Ca2+-transients in all ICC-LP within a field of view. Non-correlated Ca2+-oscillations in detrusor ICC and adjacent SMC pre-EFS, contrasted with simultaneous neurogenic Ca2+ transients evoked by EFS. Spontaneous Ca2+-oscillations in PICC were little affected by EFS, whereas large Ca2+-transients were evoked in pre-EFS quiescent PICC. EFS also increased the frequency of VSM Ca2+-oscillations. In conclusion, ICC-LP, detrusor ICC and PICC are functionally innervated. Interestingly, Ca2+-activity within ICC-LP networks and between detrusor ICC and their adjacent SMC were synchronous under neural control. VSM and PICC Ca2+-activity was regulated by bladder nerves. These novel findings demonstrate functional neural control of bladder ICC. Similar studies should now be carried out on neurogenic bladder to elucidate the contribution of impaired nerve-ICC communication to bladder pathophysiology.

T-type Ca2+ channels, SK2 channels and SERCAs gate sleep-related oscillations in thalamic dendrites.

T-type Ca2+ channels (T channels) underlie rhythmic burst discharges during neuronal oscillations that are typical during sleep. However, the Ca2+-dependent effectors that are selectively regulated by T currents remain unknown. We found that, in dendrites of nucleus reticularis thalami (nRt), intracellular Ca2+ concentration increases were dominated by Ca2+ influx through T channels and shaped rhythmic bursting via competition between Ca2+-dependent small-conductance (SK)-type K+ channels and Ca2+ uptake pumps. Oscillatory bursting was initiated via selective activation of dendritically located SK2 channels, whereas Ca2+ sequestration by sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) and cumulative T channel inactivation dampened oscillations. Sk2-/- (also known as Kcnn2) mice lacked cellular oscillations, showed a greater than threefold reduction in low-frequency rhythms in the electroencephalogram of non-rapid-eye-movement sleep and had disrupted sleep. Thus, the interplay of T channels, SK2 channels and SERCAs in nRt dendrites comprises a specialized Ca2+ signaling triad to regulate oscillatory dynamics related to sleep.

Nitric oxide suppresses cerebral vasomotion by sGC-independent effects on ryanodine receptors and voltage-gated calcium channels

Background/Aims: In cerebral arteries, nitric oxide (NO) release plays a key role in suppressing vasomotion. Our aim was to establish the pathways affected by NO in rat middle cerebral arteries. Methods: In isolated segments of artery, isometric tension and simultaneous measurements of either smooth muscle membrane potential or intracellular [Ca 2+ ] ([Ca 2+ ] SMC ) changes were recorded. Results: In the absence of L -NAME, asynchronous propagating Ca 2+ waves were recorded that were sensitive to block with ryanodine, but not nifedipine. L -NAME stimulated pronounced vasomotion and synchronous Ca 2+ oscillations with close temporal coupling between membrane potential, tone and [Ca 2+ ] SMC . If nifedipine was applied together with L -NAME, [Ca 2+ ] SMC decreased and synchronous Ca 2+ oscillations were lost, but asynchronous propagating Ca 2+ waves persisted. Vasomotion was similarly evoked by either iberiotoxin, or by ryanodine, and to a lesser extent by ODQ. Exogenous application of NONOate stimulated endothelium-independent hyperpolarization and relaxation of either L -NAME-induced or spontaneous arterial tone. NO-evoked hyperpolarization involved activation of BK Ca channels via ryanodine receptors (RYRs), with little involvement of sGC. Further, in whole cell mode, NO inhibited current through L-type voltage-gated Ca 2+ channels (VGCC), which was independent of both voltage and sGC. Conclusion: NO exerts sGC-independent actions at RYRs and at VGCC, both of which normally suppress cerebral artery myogenic tone.

Evidence both L-type and non-L-type voltage-dependent calcium channels contribute to cerebral artery vasospasm following loss of NO in the rat

We recently found block of NO synthase in rat middle cerebral artery caused spasm, associated with depolarizing oscillations in membrane potential (Em) similar in form but faster in frequency (circa 1 Hz) to vasomotion. T-type voltage-gated Ca2+ channels contribute to cerebral myogenic tone and vasomotion, so we investigated the significance of T-type and other ion channels for membrane potential oscillations underlying arterial spasm. Smooth muscle cell membrane potential (Em) and tension were measured simultaneously in rat middle cerebral artery. NO synthase blockade caused temporally coupled depolarizing oscillations in cerebrovascular Em with associated vasoconstriction. Both events were accentuated by block of smooth muscle BKCa. Block of T-type channels or inhibition of Na+/K+-ATPase abolished the oscillations in Em and reduced vasoconstriction. Oscillations in Em were either attenuated or accentuated by reducing [Ca2+]o or block of KV, respectively. TRAM-34 attenuated oscillations in both Em and tone, apparently independent of effects against KCa3.1. Thus, rapid depolarizing oscillations in Em and tone observed after endothelial function has been disrupted reflect input from T-type calcium channels in addition to L-type channels, while other depolarizing currents appear to be unimportant. These data suggest that combined block of T and L-type channels may represent an effective approach to reverse cerebral vasospasm.

Expression and characterization of LTx2, a neurotoxin from Lasiodora sp effecting on calcium channels

Here, we described the expression and characterization of the recombinant toxin LTx2, which was previously isolated from the venomous cDNA library of a Brazilian spider, Lasiodora sp. (Mygalomorphae, Theraphosidae). The recombinant toxin found in the soluble and insoluble fractions was purified by reverse phase high-performance liquid chromatography (HPLC). Ca2+ imaging analysis revealed that the recombinant LTx2 acts on calcium channels of BC3H1 cells, blocking L-type calcium channels. (C) 2008 Elsevier Inc. All rights reserved.

Palaeoceanographic changes in the North Atlantic during the Mid-Pleistocene Transition (MIS 31-19) as inferred from planktonic foraminiferal and calcium carbonate records

Marine sediments from the Integrated Ocean Drilling Project (IODP) Site U1314 (56.36°N, 27.88°W), in the subpolar North Atlantic, were studied for their planktonic foraminifera, calcium carbonate content, and Neogloboqudrina pachyderma sinistral (sin.) δ13C records in order to reconstruct surface and intermediate conditions in this region during the Mid-Pleistocene Transition (MPT). Variations in the palaeoceanography and regional dynamics of the Arctic Front were estimated by comparing CaCO3 content, planktonic foraminiferal species abundances, carbon isotopes and ice-rafted debris (IRD) data from Site U1314 with published data from other North Atlantic sites. Site U1314 exhibited high abundances of the polar planktonic foraminifera N. pachyderma sin. and low CaCO3 content until Marine Isotope Stage (MIS) 26, indicating a relatively southeastward position of the Arctic Front (AF) and penetration of colder and low-salinity surface arctic water-masses. Changing conditions after MIS 25, with oscillations in the position of the AF, caused an increase in the northward export of the warmer North Atlantic Current (NAC), indicated by greater abundances of non-polar planktonic foraminifera and higher CaCO3. The N. pachyderma sin. δ13C data indicate good ventilation of the upper part of the intermediate water layer in the eastern North Atlantic during both glacial and interglacial stages, except during Terminations 24/23, 22/21 and 20/1. In addition, for N. pachyderma (sin.) we distinguished two morphotypes: non-encrusted and heavily encrusted test. Results indicate that increases in the encrusted morphotype and lower planktonic foraminiferal diversity are related to the intensification of glacial conditions (lower sea-surface temperatures, sea-ice formation) during MIS 22 and 20.

Genetic analysis of calcium spiking responses in nodulation mutants of Medicago truncatula

The symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti results in the formation of nitrogen-fixing nodules on the roots of the host plant. The early stages of nodule formation are induced by bacteria via lipochitooligosaccharide signals known as Nod factors (NFs). These NFs are structurally specific for bacterium–host pairs and are sufficient to cause a range of early responses involved in the host developmental program. Early events in the signal transduction of NFs are not well defined. We have previously reported that Medicago sativa root hairs exposed to NF display sharp oscillations of cytoplasmic calcium ion concentration (calcium spiking). To assess the possible role of calcium spiking in the nodulation response, we analyzed M. truncatula mutants in five complementation groups. Each of the plant mutants is completely Nod− and is blocked at early stages of the symbiosis. We defined two genes, DMI1 and DMI2, required in common for early steps of infection and nodulation and for calcium spiking. Another mutant, altered in the DMI3 gene, has a similar mutant phenotype to dmi1 and dmi2 mutants but displays normal calcium spiking. The calcium behavior thus implies that the DMI3 gene acts either downstream of calcium spiking or downstream of a common branch point for the calcium response and the later nodulation responses. Two additional mutants, altered in the NSP and HCL genes, which show root hair branching in response to NF, are normal for calcium spiking. This system provides an opportunity to use genetics to study ligand-stimulated calcium spiking as a signal transduction event.

Strontium-Induced Repetitive Calcium Spikes in a Unicellular Green Alga1

The divalent cation Sr2+ induced repetitive transient spikes of the cytosolic Ca2+ activity [Ca2+]cy and parallel repetitive transient hyperpolarizations of the plasma membrane in the unicellular green alga Eremosphaera viridis. [Ca2+]cy measurements, membrane potential measurements, and cation analysis of the cells were used to elucidate the mechanism of Sr2+-induced [Ca2+]cy oscillations. Sr2+ was effectively and rapidly compartmentalized within the cell, probably into the vacuole. The [Ca2+]cy oscillations cause membrane potential oscillations, and not the reverse. The endoplasmic reticulum (ER) Ca2+-ATPase blockers 2,5-di-tert-butylhydroquinone and cyclopiazonic acid inhibited Sr2+-induced repetitive [Ca2+]cy spikes, whereas the compartmentalization of Sr2+ was not influenced. A repetitive Ca2+ release and Ca2+ re-uptake by the ER probably generated repetitive [Ca2+]cy spikes in E. viridis in the presence of Sr2+. The inhibitory effect of ruthenium red and ryanodine indicated that the Sr2+-induced Ca2+ release from the ER was mediated by a ryanodine/cyclic ADP-ribose type of Ca2+ channel. The blockage of Sr2+-induced repetitive [Ca2+]cy spikes by La3+ or Gd3+ indicated the necessity of a certain influx of divalent cations for sustained [Ca2+]cy oscillations. Based on these data we present a mathematical model that describes the baseline spiking [Ca2+]cy oscillations in E. viridis.

The generation of oscillations in networks of electrically coupled cells

In several biological systems, the electrical coupling of nonoscillating cells generates synchronized membrane potential oscillations. Because the isolated cell is nonoscillating and electrical coupling tends to equalize the membrane potentials of the coupled cells, the mechanism underlying these oscillations is unclear. Here we present a dynamic mechanism by which the electrical coupling of identical nonoscillating cells can generate synchronous membrane potential oscillations. We demonstrate this mechanism by constructing a biologically feasible model of electrically coupled cells, characterized by an excitable membrane and calcium dynamics. We show that strong electrical coupling in this network generates multiple oscillatory states with different spatio-temporal patterns and discuss their possible role in the cooperative computations performed by the system.

Oxygen-sensitive calcium channels in vascular smooth muscle and their possible role in hypoxic arterial relaxation.

We have investigated the modifications of cytosolic [Ca2+] and the activity of Ca2+ channels in freshly dispersed arterial myocytes to test whether lowering O2 tension (PO2) directly influences Ca2+ homeostasis in these cells. Unclamped cells loaded with fura-2 AM exhibit oscillations of cytosolic Ca2+ whose frequency depends on extracellular Ca2+ influx. Switching from a PO2 of 150 to 20 mmHg leads to a reversible attenuation of the Ca2+ oscillations. In voltage-clamped cells, hypoxia reversibly reduces the influx of Ca2+ through voltage-dependent channels, which can account for the inhibition of the Ca2+ oscillations. Low PO2 selectively inhibits L-type Ca2+ channel activity, whereas the current mediated by T-type channels is unaltered by hypoxia. The effect of low PO2 on the L-type channels is markedly voltage dependent, being more apparent with moderate depolarizations. These findings demonstrate the existence of O2-sensitive, voltage-dependent, Ca2+ channels in vascular smooth muscle that may critically contribute to the local regulation of circulation.

Spontaneous astrocytic Ca2+ oscillations in situ drive NMDAR-mediated neuronal excitation

Astrocytes respond to chemical, electrical and mechanical stimuli with transient increases in intracellular calcium concentration ([Ca2+]i). We now show that astrocytes in situ display intrinsic [Ca2+]i oscillations that are not driven by neuronal activity. These spontaneous astrocytic oscillations can propagate as waves to neighboring astrocytes and trigger slowly decaying NMDA receptor-mediated inward currents in neurons located along the wave path. These findings show that astrocytes in situ can act as a primary source for generating neuronal activity in the mammalian central nervous system.