Signaling microdomains regulate inositol 1,4,5-trisphosphate-mediated intracellular calcium transients in cultured neurons

Abnormal neurovascular coupling exists in patients with peritoneal dialysis and hemodialysis: evidence from a multi-mode MRI study

Background

Neurovascular coupling (NVC), as indicated by a comprehensive analysis of the amplitude of low-frequency fluctuation (ALFF) and cerebral blood flow (CBF), provides mechanistic insights into neurological disorders. Patients undergoing peritoneal dialysis (PD) and hemodialysis (HD) often face cognitive impairment, the causes of which are not fully understood.

Methods

ALFF was derived from functional magnetic resonance imaging, and CBF was quantified using arterial spin labeling in a cohort comprising 58 patients with PD, 60 patients with HD and 62 healthy controls. Voxel-based global analysis for both ALFF and CBF, alongside region-based analyses of ALFF-CBF coupling coefficients, were conducted. Additionally, the study explored the correlation between clinical laboratory indices and imaging metrics.

Results

Compared with HC, NVC was reduced in the bilateral medial superior frontal gyrus (SFGmed), insula, posterior cingulate cortex (PCC) and caudate (CAU) among dialysis patients. Furthermore, the PD group exhibited lower NVC in the bilateral SFGmed, bilateral PCC and left CAU compared with the HD group. Within the PD group, sodium level was negatively correlated with the ALFF-CBF coupling coefficient in the right insula. Additionally, a positive correlation emerged between the ALFF-CBF coupling coefficient in bilateral SFGmed and the dialysis adequacy.

Conclusion

While Montreal Cognitive Assessment scores did not significantly differ between patients with PD and HD, PD group demonstrated poorer NVC in the bilateral SFGmed, bilateral PCC and left CAU. Sodium level and dialysis adequacy may affect NVC in patients with PD.

Tuning Protein Dynamics to Sense Rapid Endoplasmic-Reticulum Calcium Dynamics

Multi-scale calcium (Ca²⁺ ) dynamics, exhibiting wide-ranging temporal kinetics, constitutes a ubiquitous mode of signal transduction. We report a novel endoplasmic-reticulum (ER)-targeted Ca²⁺ indicator, R-CatchER, which showed superior kinetics in vitro (k_off ≥2×10³  s^-1 , k_on ≥7×10⁶  M^-1  s^-1 ) and in multiple cell types. R-CatchER captured spatiotemporal ER Ca²⁺ dynamics in neurons and hotspots at dendritic branchpoints, enabled the first report of ER Ca²⁺ oscillations mediated by calcium sensing receptors (CaSRs), and revealed ER Ca²⁺ -based functional cooperativity of CaSR. We elucidate the mechanism of R-CatchER and propose a principle to rationally design genetically encoded Ca²⁺ indicators with a single Ca²⁺ -binding site and fast kinetics by tuning rapid fluorescent-protein dynamics and the electrostatic potential around the chromophore. The design principle is supported by the development of G-CatchER2, an upgrade of our previous (G-)CatchER with improved dynamic range. Our work may facilitate protein design, visualizing Ca²⁺ dynamics, and drug discovery.

Comparison of Ca2+ puffs evoked by extracellular agonists and photoreleased IP3

The inositol trisphosphate (IP3) signaling pathway evokes local Ca2+ signals (Ca2+ puffs) that arise from the concerted openings of clustered IP3 receptor/channels in the ER membrane. Physiological activation is triggered by binding of agonists to G-protein-coupled receptors (GPCRs) on the cell surface, leading to cleavage of phosphatidyl inositol bisphosphate and release of IP3 into the cytosol. Photorelease of IP3 from a caged precursor provides a convenient and widely employed means to study the final stage of IP3-mediated Ca2+ liberation, bypassing upstream signaling events to enable more precise control of the timing and relative concentration of cytosolic IP3. Here, we address whether Ca2+ puffs evoked by photoreleased IP3 fully replicate those arising from physiological agonist stimulation. We imaged puffs in individual SH-SY5Y neuroblastoma cells that were sequentially stimulated by picospritzing extracellular agonist (carbachol, CCH or bradykinin, BK) followed by photorelease of a poorly-metabolized IP3 analog, i-IP3. The centroid localizations of fluorescence signals during puffs evoked in the same cells by agonists and photorelease substantially overlapped (within ∼1μm), suggesting that IP3 from both sources accesses the same, or closely co-localized clusters of IP3Rs. Moreover, the time course and spatial spread of puffs evoked by agonists and photorelease matched closely. Because photolysis generates IP3 uniformly throughout the cytoplasm, our results imply that IP3 generated in SH-SY5Y cells by activation of receptors to CCH and BK also exerts broadly distributed actions, rather than specifically activating a subpopulation of IP3Rs that are scaffolded in close proximity to cell surface receptors to form a signaling nanodomain.

Age- and location-dependent differences in store depletion-induced h-channel plasticity in hippocampal pyramidal neurons

Disruptions of endoplasmic reticulum (ER) Ca(2+) homeostasis are heavily linked to neuronal pathology. Depletion of ER Ca(2+) stores can result in cellular dysfunction and potentially cell death, although adaptive processes exist to aid in survival. We examined the age and region dependence of one postulated, adaptive response to ER store-depletion (SD), hyperpolarization-activated cation-nonspecific (h)-channel plasticity in neurons of the dorsal and ventral hippocampus (DHC and VHC, respectively) from adolescent and adult rats. With the use of whole-cell patch-clamp recordings from the soma and dendrites of CA1 pyramidal neurons, we observed a change in h-sensitive measurements in response to SD, induced by treatment with cyclopiazonic acid, a sarcoplasmic reticulum/ER Ca(2+)-ATPase blocker. We found that whereas DHC and VHC neurons in adolescent animals respond to SD with a perisomatic expression of SD h plasticity, adult animals express SD h plasticity with a dendritic and somatodendritic locus of plasticity in DHC and VHC neurons, respectively. Furthermore, SD h plasticity in adults was dependent on membrane potential and on the activation of L-type voltage-gated Ca(2+) channels. These results suggest that cellular responses to the impairment of ER function, or ER stress, are dependent on brain region and age and that the differential expression of SD h plasticity could provide a neural basis for region- and age-dependent disease vulnerabilities.

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

The role of chromogranin B in an animal model of multiple sclerosis

Chromogranin B (CGB) is a high capacity, low affinity calcium binding protein in the endoplasmic reticulum (ER) that binds to the inositol 1,4,5 trisphosphate receptor (InsP3R) and amplifies calcium release from ER stores. Recently, it was discovered that levels of CGB-derived peptides are decreased in the cerebrospinal fluid of multiple sclerosis (MS) patients. One of the mechanisms by which neurodegeneration in MS is thought to occur is through increased levels of intra-axonal calcium. The combination of excess intracellular calcium and dysregulated levels of CGB in MS led us to hypothesize that CGB may be involved in MS pathophysiology. Here, we show in a mouse model of MS that CGB levels are elevated in neurons prior to onset of symptoms. Once symptoms develop, CGB protein levels increase with disease severity. Additionally, we show that elevated levels of CGB may have a role in the pathophysiology of MS and suggest that the initial elevation of CGB, prior to symptom onset, is due to inflammatory processes. Upon development of symptoms, CGB accumulation in neurons results from decreased ubiquitination and decreased secretion. Furthermore, we show that calpain activity is increased and levels of InsP3R are decreased. From these results, we suggest that the elevated levels of CGB and altered InsP3R levels may contribute to the axonal/neuronal damage and dysregulated calcium homeostasis observed in MS. Additionally, we propose that CGB can be a biomarker that predicts the onset and severity of disease in patients with MS.

Three dimensional neuronal cell cultures more accurately model voltage gated calcium channel functionality in freshly dissected nerve tissue

It has been demonstrated that neuronal cells cultured on traditional flat surfaces may exhibit exaggerated voltage gated calcium channel (VGCC) functionality. To gain a better understanding of this phenomenon, primary neuronal cells harvested from mice superior cervical ganglion (SCG) were cultured on two dimensional (2D) flat surfaces and in three dimensional (3D) synthetic poly-L-lactic acid (PLLA) and polystyrene (PS) polymer scaffolds. These 2D- and 3D-cultured cells were compared to cells in freshly dissected SCG tissues, with respect to intracellular calcium increase in response to high K⁺ depolarization. The calcium increases were identical for 3D-cultured and freshly dissected, but significantly higher for 2D-cultured cells. This finding established the physiological relevance of 3D-cultured cells. To shed light on the mechanism behind the exaggerated 2D-cultured cells’ functionality, transcriptase expression and related membrane protein distributions (caveolin-1) were obtained. Our results support the view that exaggerated VGCC functionality from 2D cultured SCG cells is possibly due to differences in membrane architecture, characterized by uniquely organized caveolar lipid rafts. The practical implication of use of 3D-cultured cells in preclinical drug discovery studies is that such platforms would be more effective in eliminating false positive hits and as such improve the overall yield from screening campaigns.

Microdomains of muscarinic acetylcholine and Ins(1,4,5)P₃ receptors create 'Ins(1,4,5)P₃ junctions' and sites of Ca²+ wave initiation in smooth muscle

Increases in cytosolic Ca²⁺ concentration ([Ca²⁺]_c) mediated by inositol (1,4,5)-trisphosphate [Ins(1,4,5)P₃, hereafter InsP₃] regulate activities that include division, contraction and cell death. InsP₃-evoked Ca²⁺ release often begins at a single site, then regeneratively propagates through the cell as a Ca²⁺ wave. The Ca²⁺ wave consistently begins at the same site on successive activations. Here, we address the mechanisms that determine the Ca²⁺ wave initiation site in intestinal smooth muscle cells. Neither an increased sensitivity of InsP₃ receptors (InsP₃R) to InsP₃ nor regional clustering of muscarinic receptors (mAChR3) or InsP₃R1 explained the selection of an initiation site. However, examination of the overlap of mAChR3 and InsP₃R1 localisation, by centre of mass analysis, revealed that there was a small percentage (∼10%) of sites that showed colocalisation. Indeed, the extent of colocalisation was greatest at the Ca²⁺ wave initiation site. The initiation site might arise from a selective delivery of InsP₃ from mAChR3 activity to particular InsP₃Rs to generate faster local [Ca²⁺]_c increases at sites of colocalisation. In support of this hypothesis, a localised subthreshold ‘priming’ InsP₃ concentration applied rapidly, but at regions distant from the initiation site, shifted the wave to the site of the priming. Conversely, when the Ca²⁺ rise at the initiation site was rapidly and selectively attenuated, the Ca²⁺ wave again shifted and initiated at a new site. These results indicate that Ca²⁺ waves initiate where there is a structural and functional coupling of mAChR3 and InsP₃R1, which generates junctions in which InsP₃ acts as a highly localised signal by being rapidly and selectively delivered to InsP₃R1.

Imaging adhesion and signaling dynamics in Xenopus laevis growth cones

Xenopus laevis provides a robust model system to study cellular signaling and downstream processes during development both in vitro and in vivo. Intracellular signals must function within highly restricted spatial and temporal domains to activate specific downstream targets and cellular processes. Combining the versatility of developing Xenopus neurons with advances in fluorescent protein biosensors and imaging technologies has allowed many dynamic cellular processes to be visualized. This review will focus on the techniques we use to visualize and measure cell signaling, motility and adhesion by quantitative fluorescence microscopy in vitro and in vivo.

Understanding calcium waves and sparks in central neurons

All cells use changes in intracellular calcium concentration ([Ca(2+)](i)) to regulate cell signalling events. In neurons, with their elaborate dendritic and axonal arborizations, there are clear examples of both localized and widespread Ca(2+) signals. [Ca(2+)](i) changes that are generated by Ca(2+) entry through voltage- and ligand-gated channels are the best characterized. In addition, the release of Ca(2+) from intracellular stores can result in increased [Ca(2+)](i); the signals that trigger this release have been less well-studied, in part because they are not usually associated with specific changes in membrane potential. However, recent experiments have revealed dramatic widespread Ca(2+) waves and localized spark-like events, particularly in dendrites. Here we review emerging data on the nature of these signals and their functions.

C-terminal domain of chromogranin B regulates intracellular calcium signaling

The versatility of intracellular calcium as a second messenger is seen in its ability to mediate opposing events such as neuronal cell growth and apoptosis. A leading hypothesis used to explain how calcium regulates such divergent signaling pathways is that molecules responsible for maintaining calcium homeostasis have multiple roles. For example, chromogranin B (CGB), a calcium binding protein found in secretory granules and in the lumen of the endoplasmic reticulum, buffers calcium and also binds to and amplifies the activity of the inositol 1,4,5-trisphosphate receptor (InsP(3)R). Previous studies have identified two conserved domains of CGB, an N-terminal domain (N-CGB) and a C-terminal domain (C-CGB). N-CGB binds to the third intraluminal loop of the InsP(3)R and inhibits binding of full-length CGB. This displacement of CGB decreases InsP(3)R-dependent calcium release and alters normal signaling patterns. In the present study, we further characterized the role of N-CGB and identified roles for C-CGB. The effect of N-CGB on calcium release depended upon endogenous levels of cellular CGB, whereas the regulatory effect of C-CGB was apparent regardless of endogenous levels of CGB. When either full-length CGB or C-CGB was expressed in cells, calcium transients were increased. Additionally, the calcium signal initiation site was altered upon C-CGB expression in neuronally differentiated PC12 and SHSY5Y cells. These results show that CGB has numerous regulatory roles and that CGB is a critical component in modulating InsP(3)R-dependent calcium signaling.

Layer-specific serotonergic facilitation of IPSC in layer 2/3 pyramidal neurons of the visual cortex

Serotonin (5-hydroxytryptamine, 5-HT) inhibits the induction of long-term synaptic plasticity in layer 2/3 of the visual cortex at the end of its critical period in rats. However, the cellular and molecular mechanisms remain unclear. Since inhibitory influence is crucial in the induction of synaptic plasticity, the effect of 5-HT on inhibitory transmission was investigated in layer 2/3 pyramidal neurons of the primary visual cortex. The amplitude of inhibitory postsynaptic current (IPSC), but not excitatory postsynaptic current, evoked by stimulation of the underlying layer 4, was increased by ∼20% with a bath application of 5-HT. The amplitude of miniature IPSC was also increased by the application of 5-HT, while the paired-pulse ratio was not changed. The facilitating effect of 5-HT on IPSC was mediated by the activation of 5-HT(2) receptors. An increase in intracellular Ca(2+) via release from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores, which was confirmed by confocal Ca(2+) imaging, and activation of Ca(2+)/calmodulin-dependent kinase II (CaMKII) were involved in the facilitation of IPSC by 5-HT. However, 5-HT failed to facilitate IPSC evoked by the stimulation of layer 1. These results suggest that activation of 5-HT(2) receptors releases intracellular Ca(2+) via IP(3)-sensitive stores, which facilitates GABA(A)ergic transmission via the activation of CaMKII in layer 2/3 pyramidal neurons of the visual cortex in a layer-specific manner. Thus facilitation of inhibitory transmission by 5-HT might be involved in regulating the information flow and the induction of long-term synaptic plasticity, in a pathway-specific manner.

The type I inositol 1,4,5-trisphosphate receptor interacts with protein 4.1N to mediate neurite formation through intracellular Ca waves

Ca²⁺ waves are an important mechanism for encoding Ca²⁺ signaling information, but the molecular basis for wave formation and how this regulates neuronal function is not entirely understood. Using nerve growth factor-differentiated PC12 cells as a model system, we investigated the interaction between the type I inositol 1,4,5-trisphosphate receptor (IP3R1) and the cytoskeletal linker, protein 4.1N, to examine the relationship between Ca²⁺ wave formation and neurite development. This was examined using RNAi and overexpressed dominant negative binding regions of each protein. Confocal microscopy was used to monitor neurite formation and Ca²⁺ waves. Knockdown of IP3R1 or 4.1N attenuated neurite formation, as did binding regions of IP3R1 and 4.1N, which colocalized with endogenous 4.1N and IP3R1, respectively. Upon stimulation with the IP3-producing agonist carbachol, both RNAi and dominant negative molecules shifted signaling events from waves to homogeneous patterns of Ca²⁺ release. These findings provide evidence that IP3R1 localization, via protein 4.1N, is necessary for Ca²⁺ wave formation, which in turn mediates neurite formation.

Stimulation of bursting in pre-Bötzinger neurons by Epac through calcium release and modulation of TRPM4 and K-ATP channels

The exchange factor directly activated by cAMP (Epac) can couple cAMP production to the activation of particular membrane and cytoplasmic targets. Using patch-clamp recordings and calcium imaging in organotypic brainstem slices, we examined the role of Epac in pre-Bötzinger complex, an essential part of the respiratory network. The selective agonist 8-(4-chlorophenylthio)-2'-O-methyl-cAMP (8-pCPT) sensitized calcium mobilisation from inositol-1,4,5-trisphosphate-sensitive internal stores that stimulated TRPM4 (transient receptor potential cation channel, subfamily M, Melastatin) channels and potentiated the bursts of action potentials. 8-pCPT actions were abolished after inhibition of phospholipase C with U73122 and depletion of calcium stores with thapsigargin. Caffeine-sensitive release channels were not modulated by 8-pCPT. Epac inhibited ATP-sensitive K(+) channels that also led to the enhancement of bursting by 8-pCPT. Bursting activity, spontaneous calcium transients and activity of TRPM4 and ATP-sensitive K(+) channels were potentiated after brief exposures to bradykinin and incubation with wortmannin produced opposite effects that can be explained by changes in phosphatidylinositol 4,5-bisphosphate levels. 8-pCPT stimulated the respiratory motor output in functionally intact preparations and the effects of bradykinin and wortmannin were identical to those observed in organotypic slices. The data thus indicate a novel pathway of controlling bursting activity in pre-Bötzinger complex neurons through Epac that can involved in reinforcement of the respiratory activity by cAMP.

Unloading intracellular calcium stores reveals regionally specific functions

Neuronal excitability can be modulated by release of intracellular calcium but the impact of calcium store depletion on intrinsic neuronal properties is unknown. In this issue of Neuron, Narayanan et al. describe an intrinsic plasticity that is depletion induced, is regionally restricted, and may protect neurons from pathological alterations in calcium signaling.

Inositol 1,4,5-trisphosphate receptor in chromaffin secretory granules and its relation to chromogranins

The inositol 1,4,5-trisphosphate (IP(3))-mediated intracellular Ca(2+) releases in secretory cells play vital roles in controlling not only the intracellular Ca(2+) concentrations but also the Ca(2+)-dependent exocytotic processes. Of intracellular organelles that release Ca(2+) in response to IP(3), secretory granules stand out as the most prominent organelle and are responsible for the majority of IP(3)-dependent Ca(2+) releases in the cytoplasm of chromaffin cells. Bovine chromaffin granules were the first granules that demonstrated the IP(3)-mediated Ca(2+) release as well as the presence of the IP(3) receptor (IP(3)R) in granule membranes. Secretory granules contain all three (type 1, 2, and 3) IP(3)R isoforms, and 58-69% of total cellular IP(3)R isoforms are expressed in bovine chromaffin granules. Moreover, secretory granules contain large amounts (2-4 mM) of chromogranins and secretogranins; chromogranins A and B, and secretogranin II being the major species. Chromogranins A and B, and secretogranin II are high-capacity, low-affinity Ca(2+) binding proteins, binding 30-93 mol of Ca(2+)/mol of protein with dissociation constants of 1.5-4.0 mM. Due to this high Ca(2+) storage properties of chromogranins secretory granules contain ~40 mM Ca(2+). Furthermore, chromogranins A and B directly interact with the IP(3)Rs and modulate the IP(3)R/Ca(2+) channels, i.e., increasing the open probability and the mean open time of the channels 8- to 16-fold and 9- to 42-fold, respectively. Coupled chromogranins change the IP(3)R/Ca(2+) channels to a more ordered, release-ready state, whereby making the IP(3)R/Ca(2+) channels significantly more sensitive to IP(3).

Inositol trisphosphate 3-kinases: focus on immune and neuronal signaling

The localized control of second messenger levels sculpts dynamic and persistent changes in cell physiology and structure. Inositol trisphosphate [Ins(1,4,5)P(3)] 3-kinases (ITPKs) phosphorylate the intracellular second messenger Ins(1,4,5)P(3). These enzymes terminate the signal to release Ca(2+) from the endoplasmic reticulum and produce the messenger inositol tetrakisphosphate [Ins(1,3,4,5)P(4)]. Independent of their enzymatic activity, ITPKs regulate the microstructure of the actin cytoskeleton. The immune phenotypes of ITPK knockout mice raise new questions about how ITPKs control inositol phosphate lifetimes within spatial and temporal domains during lymphocyte maturation. The intense concentration of ITPK on actin inside the dendritic spines of pyramidal neurons suggests a role in signal integration and structural plasticity in the dendrite, and mice lacking neuronal ITPK exhibit memory deficits. Thus, the molecular and anatomical features of ITPKs allow them to regulate the spatiotemporal properties of intracellular signals, leading to the formation of persistent molecular memories.

Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca2+ signaling in the cytoplasm of neuroendocrine cells

Of all the intracellular organelles, secretory granules contain by far the highest calcium concentration; secretory granules of typical neuroendocrine chromaffin cells contain approximately 40 mM Ca(2+) and occupy approximately 20% cell volume, accounting for >60% of total cellular calcium. They also contain the majority of cellular inositol 1,4,5-trisphosphate receptors (IP(3)Rs) in addition to the presence of >2 mM of chromogranins A and B that function as high-capacity, low-affinity Ca(2+) storage proteins. Chromogranins A and B also interact with the IP(3)Rs and activate the IP(3)R/Ca(2+) channels. In experiments with both neuroendocrine PC12 and nonneuroendocrine NIH3T3 cells, in which the number of secretory granules present was changed by either suppression or induction of secretory granule formation, secretory granules were demonstrated to account for >70% of the IP(3)-induced Ca(2+) releases in the cytoplasm. Moreover, the IP(3) sensitivity of secretory granule IP(3)R/Ca(2+) channels is at least approximately 6- to 7-fold more sensitive than those of the endoplasmic reticulum, thus enabling secretory granules to release Ca(2+) ahead of the endoplasmic reticulum. Further, there is a direct correlation between the number of secretory granules and the IP(3) sensitivity of cytoplasmic IP(3)R/Ca(2+) channels and the increased ratio of IP(3)-induced cytoplasmic Ca(2+) release, highlighting the importance of secretory granules in the IP(3)-dependent Ca(2+) signaling. Given that secretory granules are present in all secretory cells, these results presage critical roles of secretory granules in the control of cytoplasmic Ca(2+) concentrations in other secretory cells.-Yoo, S. H. Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca(2+) signaling in the cytoplasm of neuroendocrine cells.

Sustained depolarization-induced propagation of [Ca2+]i oscillations in cultured DRG neurons: the involvement of extracellular ATP and P2Y receptor activation

Recently emerging evidence implicates a number of neuroactive substances and their receptors in mediating complex cell-to-cell communications in the ganglia. In the present study, we characterized the nonsynaptic chemical coupling mediated by extracellular ATP in dorsal root ganglia (DRG) neuron cultures by using the real time imaging of ATP, whole-cell patch clamping, in conjunction with confocal calcium imaging. Sustained depolarization by electrical stimulation evoked intracellular Ca2+ concentrations ([Ca2+]i) oscillations in individual DRG neurons, and subsequent ATP-dependent propagation [Ca2+]i oscillations to surrounding non-stimulated neighbors. [Ca2+]i oscillations were suppressed by inositol-1,4,5-trisphosphate (IP3) receptor antagonist 2-APB, but not ryanodine. The propagation of [Ca2+]i oscillations was prevented by the presence of the ATP-degrading enzyme, apyrase, and completely abolished by the blockase of G protein-coupled purinergic receptors-PLC-IP3 pathway with suramin, U73122 or 2-APB. In parallel, sustained depolarization elicited robust ATP release and diffusion from the stimulation site. Moreover, exogenous application of ATP to DRG cultures in large concentration elicits the [Ca2+]i oscillations in most neurons. Taken together, this data demonstrates that sustained membrane depolarization elicited ATP release, acting through a highly sensitive P2Y receptors/IP3-mediated signaling pathway to mediate the propagation of intercellular Ca2+ signaling, which suggest a novel signaling pathway for neuronal communication in DRG.

Crosstalk between calcium and redox signaling: from molecular mechanisms to health implications

Studies done many years ago established unequivocally the key role of calcium as a universal second messenger. In contrast, the second messenger roles of reactive oxygen and nitrogen species have emerged only recently. Therefore, their contributions to physiological cell signaling pathways have not yet become universally accepted, and many biological researchers still regard them only as cellular noxious agents. Furthermore, it is becoming increasingly apparent that there are significant interactions between calcium and redox species, and that these interactions modify a variety of proteins that participate in signaling transduction pathways and in other fundamental cellular functions that determine cell life or death. This review article addresses first the central aspects of calcium and redox signaling pathways in animal cells, and continues with the molecular mechanisms that underlie crosstalk between calcium and redox signals under a number of physiological or pathological conditions. To conclude, the review focuses on conditions that, by promoting cellular oxidative stress, lead to the generation of abnormal calcium signals, and how this calcium imbalance may cause a variety of human diseases including, in particular, degenerative diseases of the central nervous system and cardiac pathologies.

Chromogranin B regulates calcium signaling, nuclear factor kappaB activity, and brain natriuretic peptide production in cardiomyocytes

Altered regulation of signaling pathways can lead to pathologies including cardiac hypertrophy and heart failure. We report that neonatal and adult cardiomyocytes express chromogranin B (CGB), a Ca(2+) binding protein that modulates Ca(2+) release by the inositol 1,4,5-trisphosphate receptor (InsP(3)R). Using fluorescent Ca(2+) indicator dyes, we found that CGB regulates InsP(3)-dependent Ca(2+) release in response to angiotensin II, an octapeptide hormone that promotes cardiac hypertrophy. ELISA experiments and luciferase reporter assays identified angiotensin II as a potent inducer of brain natriuretic peptide (BNP), a hormone that recently emerged as an important biomarker in cardiovascular disease. CGB was found to regulate angiotensin II-stimulated and basal secretion, expression and promoter activity of BNP that depend on the InsP(3)R. Moreover, we provide evidence that CGB acts via the transcription activity of nuclear factor kappaB in an InsP(3)/Ca(2+)-dependent manner but independent of nuclear factor of activated T cells. In vivo experiments further showed that cardiac hypertrophy induced by angiotensin II, a condition characterized by increased ventricular BNP production, is associated with upregulation of ventricular CGB expression. Overexpression of CGB in cardiomyocytes, in turn, induced the BNP promoter. The evidence presented in this study identifies CGB as a novel regulator of cardiomyocyte InsP(3)/Ca(2+)-dependent signaling, nuclear factor kappaB activity, and BNP production.

Differential cholinergic modulation of Ca2+ transients evoked by backpropagating action potentials in apical and basal dendrites of cortical pyramidal neurons

The effect of the cholinergic agonist carbachol (CCh) on backpropagating action potential (bAP)-evoked Ca2+ transients in distal apical and basal dendrites of layer 2/3 pyramidal neurons in the primary visual cortex of rats was studied using whole cell recordings and confocal Ca2+ imaging. In the presence of CCh (20 microM), initial bAP-evoked Ca2+ transients were followed by large propagating secondary Ca2+ transients that were restricted to proximal apical dendrites < or =40 microm from the soma. In middle apical dendrites (41-100 microm from the soma), Ca2+ transients evoked by AP bursts at 20 Hz, but not by single APs, were increased by CCh without secondary transients. CCh failed to increase the bAP-evoked Ca2+ transients in distal apical dendrites (101-270 microm from the soma). In contrast, in basal dendrites, CCh increased Ca2+ transients evoked by AP bursts, but not by single APs, and these transients were relatively constant over the entire length of the dendrites. CCh further increased the enhanced bAP-evoked Ca2+ transients in the presence of 4-aminopyridine (200 microM), an A-type K+ channel blocker, in basal and apical dendrites, except in distal apical dendrites. CCh increased large Ca2+ transients evoked by high-frequency AP bursts in basal dendrites, but not in distal apical dendrites. CCh-induced increase in Ca2+ transients was mediated by InsP3-dependent Ca2+-induced Ca2+-release. These results suggest that cholinergic stimulation differentially increases the bAP-evoked increase in [Ca2+]i in apical and basal dendrites, which may modulate synaptic activities in a location-dependent manner.

Initiation and propagation of a neuronal intracellular calcium wave

The ability to image calcium movement within individual neurons inspires questions of functionality including whether calcium entry into the nucleus is related to genetic regulation for phenomena such as long term potentiation. Calcium waves have been initiated in hippocampal pyramidal cells with glutmatergic signals both in the presence and absence of back propagating action potentials (BPAPs). The dendritic sites of initiation of these calcium waves within about 100 microm of the soma are thought to be localized near oblique junctions. Stimulation of synapses on oblique dendrites leads to production of inositol 1,4,5-trisphosphate (IP(3)) which diffuses to the apical dendrite igniting awaiting IP(3) receptors (IP(3)Rs) and initiating and propagating catalytic calcium release from the endoplasmic reticulum. We construct a reduced mathematical system which accounts for calcium wave initiation and propagation due to elevated IP(3). Inhomogeneity in IP(3) distribution is responsible for calcium wave initiation versus subthreshold or spatially uniform suprathreshold activation. However, the likelihood that a calcium wave is initiated does not necessarily increase with more calcium entering from BPAPs. For low transient synaptic stimuli, timing between IP(3) generation and BPAPs is critical for calcium wave initiation. We also show that inhomogeneity in IP(3)R density can account for calcium wave directionality. Simulating somatic muscarinic receptor production of IP(3), we can account for the critical difference between calcium wave entry into the soma and failure to do so.

A crucial role for cAMP and protein kinase A in D1 dopamine receptor regulated intracellular calcium transients

D1-like dopamine receptors stimulate Ca(2+) transients in neurons but the effector coupling and signaling mechanisms underlying these responses have not been elucidated. Here we investigated potential mechanisms using both HEK 293 cells that stably express D1 receptors (D1HEK293) and hippocampal neurons in culture. In D1HEK293 cells, the full D1 receptor agonist SKF 81297 evoked a robust dose-dependent increase in Ca(2+)(i) following 'priming' of endogenous G(q/11)-coupled muscarinic or purinergic receptors. The effect of SKF81297 could be mimicked by forskolin or 8-Br-cAMP. Further, cholera toxin and the cAMP-dependent protein kinase (PKA) inhibitors, KT5720 and H89, as well as thapsigargin abrogated the D1 receptor evoked Ca(2+) transients. Removal of the priming agonist and treatment with the phospholipase C inhibitor U73122 also blocked the SKF81297-evoked responses. D1R agonist did not stimulate IP(3) production, but pretreatment of cells with the D1R agonist potentiated G(q)-linked receptor agonist mobilization of intracellular Ca(2+) stores. In neurons, SKF81297 and SKF83959, a partial D1 receptor agonist, promoted Ca(2+) oscillations in response to G(q/11)-coupled metabotropic glutamate receptor (mGluR) stimulation. The effects of both D1R agonists on the mGluR-evoked Ca(2+) responses were PKA dependent. Altogether the data suggest that dopamine D1R activation and ensuing cAMP production dynamically regulates the efficiency and timing of IP(3)-mediated intracellular Ca(2+) store mobilization.

Regulation of inositol 1,4,5-trisphosphate receptor-mediated calcium release by the Na/K-ATPase in cultured renal epithelial cells

It is known that the Na/K-ATPase alpha1 subunit interacts directly with inositol 1,4,5-triphosphate (IP(3)) receptors. In this study we tested whether this interaction is required for extracellular stimuli to efficiently regulate endoplasmic reticulum (ER) Ca(2+) release. Using cultured pig kidney LLC-PK1 cells as a model, we demonstrated that graded knockdown of the cellular Na/K-ATPase alpha1 subunit resulted in a parallel attenuation of ATP-induced ER Ca(2+) release. When the knockdown cells were rescued by knocking in a rat alpha1, the expression of rat alpha1 restored not only the cellular Na/K-ATPase but also ATP-induced ER Ca(2+) release. Mechanistically, this defect in ATP-induced ER Ca(2+) release was neither due to the changes in the amount or the function of cellular IP(3) and P2Y receptors nor the ER Ca(2+) content. However, the alpha1 knockdown did redistribute cellular IP(3) receptors. The pool of IP(3) receptors that resided close to the plasma membrane was abolished. Because changes in the plasma membrane proximity could reduce the efficiency of signal transmission from P2Y receptors to the ER, we further determined the dose-dependent effects of ATP on protein kinase Cepsilon activation and ER Ca(2+) release. The data showed that the alpha1 knockdown de-sensitized the ATP-induced ER Ca(2+) release but not PKCepsilon activation. Moreover, expression of the N terminus of Na/K-ATPase alpha1 subunit not only disrupted the formation of the Na/K-ATPase-IP(3) receptor complex but also abolished the ATP-induced Ca(2+) release. Finally, we observed that the alpha1 knockdown was also effective in attenuating ER Ca(2+) release provoked by angiotensin II and epidermal growth factor.

Inositol 1,4,5 trisphosphate receptor and chromogranin B are concentrated in different regions of the hippocampus

Calcium (Ca(2+)) release from intracellular stores plays a crucial role in many cellular functions in the brain. These intracellular signals have been shown to be transmitted within and between cells. We report a non-uniform distribution of proteins essential for Ca(2+) signaling in acutely prepared brain slice preparations and organotypic slice cultures, both made from rat hippocampus. The Type I inositol-1,4,5 trisphosphate receptor (InsP(3)R1) is the main InsP(3)R subtype in neurons. Immunohistochemistry experiments showed a prominent expression of InsP(3)R1 in the CA1 region of the hippocampus whereas the CA3 region and dentate gyrus (DG) showed only moderate immunoreactivity. In contrast, chromogranin B (CGB), a protein binding to the InsP(3)R1 on the luminal side of the endoplasmic reticular membrane was enriched in the CA3 region whereas DG and the CA1 region showed only faint CGB signals. The neuronal kinases leading to the formation of inositol-1,4,5 trisphosphate (InsP(3)), phosphatidylinositol-4-kinase (PI4K), and phosphatidylinositol-4-phosphate-5-kinase (PIPK), showed strong immunoreactivity throughout all hippocampal cell fields with differences in the subcellular distribution. Moreover, a distinct band of strong CGB and PIPK immunoreactivity was observed in the CA3 region that coincides with the mossy fiber tract (stratum lucidum). These data show differential expression of the components of the signaling toolkit leading to InsP(3)-mediated Ca(2+) release in cells of the hippocampus. The regulation of these differences may play an important role in various neuropathologic conditions such as Alzheimer's disease, epilepsy, or schizophrenia.

Identification of residues participating in the interaction between an intraluminal loop of inositol 1,4,5-trisphosphate receptor and a conserved N-terminal region of chromogranin B

The inositol 1,4,5-trisphosphate receptor (IP3R) is a membrane channel that conducts calcium ions from the intracellular calcium stores. Despite a wealth of information on the cytoplasmic regulation of the IP3R, little is known about its regulation on the luminal side of the calcium stores. Here, we report studies on the IP3R intraluminal loop L3-2 and a conserved N-terminal region of chromogranin B. The IP3R loop is an important part of the channel's pore-forming region, and the chromogranin peptide has been shown to competitively inhibit calcium signaling by IP3R. Using the NMR titration approach, we showed that a part of the L3-2 is involved in a specific interaction with the chromogranin B peptide. Further NMR resonance assignments revealed that the 14th-20th residues of L3-2 are the keys to the binding to the chromogranin B peptide. Through detailed analysis of the data, we suggest a mechanism of IP3R regulation by chromogranin B involving conformational exchanges of the L3-2 region. Our report presents the findings of the first study on the interaction between the luminal loop of the IP3 receptor and its regulator at residue-resolution. The approaches described here should help to guide further studies on the interactions between the IP3R and other luminal side regulators.

Changes in functional properties of the caffeine-sensitive Ca2+ store during differentiation of human SH-SY5Y neuroblastoma cells

We have used single cell fluorescence imaging techniques to examine how functional properties of the caffeine-sensitive Ca(2+) store change during differentiation of a sub-population of caffeine-sensitive SH-SY5Y cells. Application of caffeine (30 mM) 1-10.5 min after a 'priming' depolarisation pulse of 55 mM K(+) revealed that the caffeine-sensitive store in undifferentiated cells remained replete, whereas that in 9-cis retinoic acid (9cRA)-differentiated cells spontaneously dissipated with a t(1/2) of 2.8 min, and was essentially completely depleted approximately 10 min after priming. In 9cRA-differentiated cells that were stimulated with methacholine (10 microM) 1 min after priming, the amplitude, rate of rise and propagation velocity of the Ca(2+) wave in the neurites were all constant, whereas these kinetic parameters all progressively decreased as the wave travelled along the neurites in cells that were stimulated 10 min after priming. Use-dependent block with ryanodine inhibited the global Ca(2+) signal in 9cRA-differentiated cells stimulated with methacholine 1 min after priming (71+/-8%) but not 10 min after priming. Depolarisation was more effective at priming the caffeine-sensitive Ca(2+) store in 9cRA-differentiated cells, which lack a functional store-operated Ca(2+) entry pathway. We conclude that differentiation of caffeine-sensitive SH-SY5Y cells is accompanied by an increase in lability of the caffeine-sensitive Ca(2+) store, and that spontaneous dissipation of Ca(2+) from the store limits the time course of its molecular 'memory' during which it can amplify the hormone-induced Ca(2+) signal by Ca(2+)-induced Ca(2+) release.

Perisynaptic organization of plasma membrane calcium pumps in cerebellar cortex

Calcium, a ubiquitous intracellular messenger, regulates numerous intracellular signaling pathways. To permit specificity of signal transduction and prevent unwanted cross-talk between pathways, sites of calcium entry in neurons are localized to specific membrane domains. To test whether Ca(2+) extrusion pumps might exhibit analogous compartmentalization, we used immunohistochemistry to determine the subcellular localization of the two main plasma membrane Ca(2+)-ATPase (PMCA) isoforms in the cortex of the rat cerebellum. We find that both PMCA2 and PMCA3 are targeted to distinct compartments within the plasma membrane. In the molecular layer, both isoforms were at highest levels within synaptic profiles, but PMCA2 was postsynaptic and PMCA3 was presynaptic. Moreover, inside these compartments, both pumps exhibited nonuniform distributions. These data imply that cerebellar neurons possess remarkably effective mechanisms to target and restrict PMCA2 and -3 to specific membrane domains, raising the possibility that calcium pumps contribute to local Ca(2+) signaling.

Distribution of IP3-mediated calcium responses and their role in nuclear signalling in rat basolateral amygdala neurons

Metabotropic receptor activation is important for learning, memory and synaptic plasticity in the amygdala and other brain regions. Synaptic stimulation of metabotropic receptors in basolateral amygdala (BLA) projection neurons evokes a focal rise in free Ca(2+) in the dendrites that propagate as waves into the soma and nucleus. These Ca(2+) waves initiate in the proximal dendrites and show limited propagation centrifugally away from the soma. In other cell types, Ca(2+) waves have been shown to be mediated by either metabotropic glutamate receptor (mGluR) or muscarinic receptor (mAChR) activation. Here we show that mGluRs and mAChRs act cooperatively to release Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular Ca(2+) stores. Whereas action potentials (APs) alone were relatively ineffective in raising nuclear Ca(2+), their pairing with metabotropic receptor activation evoked an IP(3)-receptor-mediated Ca(2+)-induced Ca(2+) release, raising nuclear Ca(2+) into the micromolar range. Metabotropic-receptor-mediated Ca(2+)-store release was highly compartmentalized. When coupled with metabotropic receptor stimulation, large robust Ca(2+) rises and AP-induced amplification were observed in the soma, nucleus and sparsely spiny dendritic segments with metabotropic stimulation. In contrast, no significant amplification of the Ca(2+) transient was detected in spine-dense high-order dendritic segments. Ca(2+) rises evoked by photolytic uncaging of IP(3) showed the same distribution, suggesting that IP(3)-sensitive Ca(2+) stores are preferentially located in the soma and proximal dendrites. This distribution of metabotropic-mediated store release suggests that the neuromodulatory role of metabotropic receptor stimulation in BLA-dependent learning may result from enhanced nuclear signalling.

Calcium-triggered exit of F-actin and IP(3) 3-kinase A from dendritic spines is rapid and reversible

The structure of the actin cytoskeleton in dendritic spines is thought to underlie some forms of synaptic plasticity. We have used fixed and live-cell imaging in rat primary hippocampal cultures to characterize the synaptic dynamics of the F-actin binding protein inositol trisphosphate 3-kinase A (IP3K), which is localized in the spines of pyramidal neurons derived from the CA1 region. IP3K was intensely concentrated as puncta in spine heads when Ca(2+) influx was low, but rapidly and reversibly redistributed to a striated morphology in the main dendrite when Ca(2+) influx was high. Glutamate stimulated the exit of IP3K from spines within 10 s, and re-entry following blockage of Ca(2+) influx commenced within a minute; IP3K appeared to remain associated with F-actin throughout this process. Ca(2+)-triggered F-actin relocalization occurred in about 90% of the cells expressing IP3K endogenously, and was modulated by the synaptic activity of the cultures, suggesting that it is a physiological process. F-actin relocalization was blocked by cytochalasins, jasplakinolide and by the over-expression of actin fused to green fluorescent protein. We also used deconvolution microscopy to visualize the relationship between F-actin and endoplasmic reticulum inside dendritic spines, revealing a delicate microorganization of IP3K near the Ca(2+) stores. We conclude that Ca(2+) influx into the spines of CA1 pyramidal neurons triggers the rapid and reversible retraction of F-actin from the dendritic spine head. This process contributes to changes in spine F-actin shape and content during synaptic activity, and might also regulate spine IP3 signals.

The inositol 1,4,5-trisphosphate receptor (IP3R) and its regulators: sometimes good and sometimes bad teamwork

In both nonexcitable and excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3)R) is the primary cytosolic target responsible for the initiation of intracellular calcium (Ca(2+)) signaling. To fulfill this function, the IP(3)R depends on interaction with accessory subunits and regulatory proteins. These include proteins that reside in the lumen of the endoplasmic reticulum (ER), such as chromogranin A and B and ERp44, and cytosolic proteins, such as neuronal Ca(2+) sensor 1, huntingtin, cytochrome c, IP(3)R-binding protein released with inositol 1,4,5-trisphosphate, Homer, and 4.1N. Specific interactions between these modulatory proteins and the IP(3)R have been described, making it clear that the controlled modulation of the IP(3)R by its binding partners is necessary for physiological cell regulation. The functional coupling of these modulators with the IP(3)R can control apoptosis, intracellular pH, the initiation and regulation of neuronal Ca(2+) signaling, exocytosis, and gene expression. The pathophysiological relevance of IP(3)R modulation is apparent when the functional interaction of these proteins is enhanced or abolished by mutation or overexpression. The subsequent deregulation of the IP(3)R leads to pathological changes in Ca(2+) signaling, signal initiation, the amplitude and frequency of Ca(2+) signals, and the duration of the Ca(2+) elevation. Consequences of this deregulation include abnormal growth and apoptosis. Complex regulation of Ca(2+) signaling is required for the cell to live and function, and this difficult task can only be managed when the IP(3)R teams up and acts properly with its numerous binding partners.

Molecular basis of neuroendocrine cell type-specific expression of the chromogranin B gene: Crucial role of the transcription factors CREB, AP-2, Egr-1 and Sp1

The molecular basis of neuroendocrine-specific expression of chromogranin B gene (Chgb) has remained elusive. Utilizing wild-type and mutant Chgb promoter/luciferase reporter constructs, this study established a crucial role for the cAMP response element (CRE) box at -102/-95 bp in endocrine [rat pheochromocytoma (chromaffin) cell line (PC12) and rat pituitary somatotrope cell line (GC)] and neuronal [rat dorsal root ganglion/mouse neuroblastoma hybrid cell line (F-11), cortical and hippocampal primary neurons] cells. Additionally, G/C-rich domains at -134/-127, -125/-117 and -115/-110 bp played especially important roles for endocrine-specific expression of the Chgb gene. Co-transfection of expression plasmids for CREB, activator protein-2 (transcription factor) (AP-2), early growth response protein (transcription factor) (Egr-1) or specificity protein 1 (transcription factor) (Sp1) with the Chgb promoter constructs trans-activated expression of the Chgb gene. Nuclear extracts from either PC12 or F-11 cells formed specific complexes with the Chgb (-110/-87 bp) (CRE) oligonucleotide, which were either supershifted or disrupted by anti-CREB antibodies. In addition PC12 nuclear extracts also formed a specific complex with a Chgb (-140/-104-bp) oligonucleotide containing three G/C-rich regions, which was dose-dependently disrupted by anti-AP-2, anti-Egr-1 or anti-Sp1 antibodies; indeed, any one of these three antibodies completely abolished the complex, suggesting that all three factors bind the region simultaneously, at least in vitro. Chromatin immunoprecipitation assays documented the binding of the transcription factors CREB, AP-2, Egr-1 and Sp1 to the chromosomal Chgb gene promoter in vivo in PC12 cells within the context of chromatin. We conclude that the neuroendocrine-specific expression of Chgb is mediated by the CRE and G/C boxes in cis and the transcription factors CREB, AP-2, Egr-1 and Sp1 in trans.

Calcium microdomains and gene expression in neurons and skeletal muscle cells

Neurons generate particular calcium microdomains in response to different stimuli. Calcium microdomains have a central role in a variety of neuronal functions. In particular, calcium microdomains participate in long-lasting synaptic plasticity--a neuronal response presumably correlated with cognitive brain functions that requires expression of new gene products. Stimulation of skeletal muscle generates - with few milliseconds delay - calcium microdomains that have a central role in the ensuing muscle contraction. In addition, recent evidence indicates that sustained stimulation of skeletal muscle cells in culture generates calcium microdomains, which stimulate gene expression but not muscle contraction. The mechanisms whereby calcium microdomains activate signaling cascades that lead to the transcription of genes known to participate in specific cellular responses are the central topic of this review. Thus, we will discuss here the signaling pathways and molecular mechanisms, which via activation of particular calcium-dependent transcription factors regulate the expression of specific genes or set of genes in neurons or skeletal muscle cells.

Modulation of calcium wave propagation in the dendrites and to the soma of rat hippocampal pyramidal neurons

Repetitive synaptic stimulation in the stratum radiatum (SR) evokes large amplitude Ca2+ waves in the thick apical dendrites of hippocampal CA1 pyramidal neurons. These waves are initiated by activation of metabotropic glutamate receptors (mGluRs), which mobilize inositol-1,4,5-trisphospate (IP3) and release Ca2+ from intracellular stores. We explored mechanisms that modulate the spatial properties of these waves. Higher stimulus current evoked waves of increasing spatial extent. Most waves did not propagate through the soma; the majority stopped close to the junction of the soma and apical dendrite. Pairing strong stimulation with one electrode and subthreshold stimulation with another (associative activation) extended the waves distally but failed to extend waves into the cell body. Pairing synaptic stimulation with backpropagating action potentials enhanced the likelihood of wave generation but did not extend the waves to the somatic region. Priming the stores with Ca2+ entry through voltage dependent channels modulated wave properties but did not extend them past the dendrites. These results are consistent with propagation failing due to the dilution of synaptically generated IP3 as it diffuses into the large volume of the soma (impedance mismatch). Synaptically activating waves in the presence of low concentrations of carbachol, which probably increased the tonic level of IP3 throughout the cell, enhanced the extent of propagation and generated waves that invaded the soma, as long as low-affinity indicators were used to detect the [Ca2+]i changes. Consistent with this explanation direct injection of IP3 into the soma promoted wave propagation into this region. Ca2+ waves that propagated through the cell body were interesting because they did not fill the volume of the soma, but passed through the centre, often with large amplitude. These waves may be particularly effective in activating gene expression and protein synthesis.

Signaling proteins in raft-like microdomains are essential for Ca2+ wave propagation in glial cells

The hypothesis that calcium signaling proteins segregate into lipid raft-like microdomains was tested in isolated membranes of rat oligodendrocyte progenitor (OP) cells and astrocytes using Triton X-100 solubilization and density gradient centrifugation. Western blot analysis of gradient fractions showed co-localization of caveolin-1 with proteins involved in the Ca2+ signaling cascade. These included agonist receptors, P2Y1, and M1, TRPC1, IP3R2, ryanodine receptor, as well as the G protein Galphaq and Homer. Membranes isolated from agonist-stimulated astrocytes showed an enhanced recruitment of phospholipase C (PLCbeta1), IP3R2 and protein kinase C (PKC-alpha) into lipid raft fractions. IP3R2, TRPC1 and Homer co-immunoprecipitated, suggesting protein-protein interactions. Disruption of rafts by cholesterol depletion using methyl-beta-cyclodextrin (beta-MCD) altered the distribution of caveolin-1 and GM1 to non-raft fractions with higher densities. beta-MCD-induced disruption of rafts inhibited agonist-evoked Ca2+ wave propagation in astrocytes and attenuated wave speeds. These results indicate that in glial cells, Ca2+ signaling proteins might exist in organized membrane microdomains, and these complexes may include proteins from different cellular membrane systems. Such an organization is essential for Ca2+ wave propagation.

Insulin-like growth factor-1 modulation of CaV1.3 calcium channels depends on Ca2+ release from IP3-sensitive stores and calcium/calmodulin kinase II phosphorylation of the alpha1 subunit EF hand

In neurons, L-type calcium channels (CaV1.2 and CaV1.3) regulate an extensive range of functions. However, the roles of CaV1.3-containing L channels, which are physiologically and pharmacologically distinct from the better understood CaV1.2 channels, are only beginning to be determined. We find that CaV1.3 channels are modulated by the insulin-like growth factor-1/receptor tyrosine kinase (IGF-1/RTK) through a signaling pathway that involves phospholipase C, calcium release from IP3-sensitive internal stores, and calcium/calmodulin kinase II. In addition, we find that the IGF-1-induced modulation requires phosphorylation of a specific serine residue, S1486, in the EF hand motif of the CaV1.3 subunit. This modulation alters CaV1.3 activity, causing a left shift in the current-voltage relationship and strongly potentiating peak currents at hyperpolarized membrane potentials. We also find that CaV1.3 channels and their RTK-dependent potentiation contribute to the regulation of the survival-promoting transcription factor cAMP response element-binding protein (CREB): in both cortical and hippocampal neurons, depolarization and IGF-1 rapidly increase phospho-CREB levels in a manner that requires CaV1.3 activity and the S1486 phosphorylation site to achieve a full effect. Although the full effects of CaV1.3 channels remain to be determined, their preferential localization to dendritic shafts and spine heads coupled with their ability to activate at relatively hyperpolarized and even subthreshold potentials suggests that CaV1.3 activity may subserve different cellular functions from CaV1.2 and, in particular, may be important in transducing signals initiated by excitatory neurotransmission.

Elevated testosterone induces apoptosis in neuronal cells

Testosterone plays a crucial role in neuronal function, but elevated concentrations can have deleterious effects. Here we show that supraphysiological levels of testosterone (micromolar range) initiate the apoptotic cascade. We used three criteria, annexin V labeling, caspase activity, and DNA fragmentation, to determine that apoptotic pathways were activated by testosterone. Micromolar, but not nanomolar, testosterone concentrations increased the response in all three assays of apoptosis. In addition, testosterone induced different concentration-dependent Ca2+ signaling patterns: at low concentrations of testosterone (100 nm), Ca2+ oscillations were produced, whereas high concentrations (1-10 microm) induced a sustained Ca2+ increase. Elevated testosterone concentrations increase cell death, and this effect was abolished in the presence of either inhibitors of caspases or the inositol 1,4,5-trisphosphate receptor (InsP3R)-mediated Ca2+ release. Knockdown of InsP3R type 1 with specific small interfering RNA also abolished the testosterone-induced cell death and the prolonged Ca2+ signals. In contrast, knockdown of InsP3R type 3 modified neither the apoptotic response nor the Ca2+ signals. These results support our hypothesis that elevated testosterone alters InsP3R type 1-mediated intracellular Ca2+ signaling and that the prolonged Ca2+ signals lead to apoptotic cell death. These effects of testosterone on neurons will have long term effects on brain function.