Tracking transmitter-gated P2X cation channel activation in vitro and in vivo

Extracellular ATP/adenosine dynamics in the brain and its role in health and disease

Extracellular ATP and adenosine are neuromodulators that regulate numerous neuronal functions in the brain. Neuronal activity and brain insults such as ischemic and traumatic injury upregulate these neuromodulators, which exert their effects by activating purinergic receptors. In addition, extracellular ATP/adenosine signaling plays a pivotal role in the pathogenesis of neurological diseases. Virtually every cell type in the brain contributes to the elevation of ATP/adenosine, and various mechanisms underlying this increase have been proposed. Extracellular adenosine is thought to be mainly produced via the degradation of extracellular ATP. However, adenosine is also released from neurons and glia in the brain. Therefore, the regulation of extracellular ATP/adenosine in physiological and pathophysiological conditions is likely far more complex than previously thought. To elucidate the complex mechanisms that regulate extracellular ATP/adenosine levels, accurate methods of assessing their spatiotemporal dynamics are needed. Several novel techniques for acquiring spatiotemporal information on extracellular ATP/adenosine, including fluorescent sensors, have been developed and have started to reveal the mechanisms underlying the release, uptake and degradation of ATP/adenosine. Here, we review methods for analyzing extracellular ATP/adenosine dynamics as well as the current state of knowledge on the spatiotemporal dynamics of ATP/adenosine in the brain. We focus on the mechanisms used by neurons and glia to cooperatively produce the activity-dependent increase in ATP/adenosine and its physiological and pathophysiological significance in the brain.

Photoswitching alters fluorescence readout of jGCaMP8 Ca2+ indicators tethered to Orai1 channels

We used electrophysiology and Ca2+ channel tethering to evaluate the performance of jGCaMP8 genetically encoded Ca2+ indicators (GECIs). Orai1 Ca2+ channel-jGCaMP8 fusions were transfected into HEK 293A cells and jGCaMP8 fluorescence responses recorded by simultaneous total internal reflection fluorescence microscopy and whole-cell patch clamp electrophysiology. Noninactivating currents from the Orai1 Y80E mutant provided a steady flux of Ca2+ controlled on a millisecond time scale by step changes in membrane potential. Test pulses to -100 mV produced Orai1 Y80E-jGCaMP8f fluorescence traces that unexpectedly declined by ~50% over 100 ms before reaching a stable plateau. Testing of Orai1-jGCaMP8f using unroofed cells further demonstrated that rapid and partial fluorescence inactivation is a property of the indicator itself, rather than channel function. Photoinactivation spontaneously recovered over 5 min in the dark, and recovery was accelerated in the absence of Ca2+. Mutational analysis of residues near the tripeptide fluorophore of jGCaMP8f pointed to a mechanism: Q69M/C70V greatly increased (~90%) photoinactivation, reminiscent of fluorescent protein fluorophore cis-trans photoswitching. Indeed, 405-nm illumination of jGCaMP8f or 8m/8s/6f led to immediate photorecovery, and simultaneous illumination with 405 and 488-nm light blocked photoinactivation. Subsequent mutagenesis produced a variant, V203Y, that lacks photoinactivation but largely preserves the desirable properties of jGCaMP8f. Our results point to caution in interpreting rapidly changing Ca2+ signals using jGCaMP8 and earlier series GECIs, suggest strategies to avoid photoswitching, and serve as a starting point to produce more photostable, and thus more accurate, GECI derivatives.

A sensitive GRAB sensor for detecting extracellular ATP in vitro and in vivo

The purinergic transmitter ATP (adenosine 5'-triphosphate) plays an essential role in both the central and peripheral nervous systems, and the ability to directly measure extracellular ATP in real time will increase our understanding of its physiological functions. Here, we developed a sensitive GPCR activation-based ATP sensor called GRAB_ATP1.0, with a robust fluorescence response to extracellular ATP when expressed in several cell types. This sensor has sub-second kinetics, has ATP affinity in the range of tens of nanomolar, and can be used to localize ATP release with subcellular resolution. Using this sensor, we monitored ATP release under a variety of in vitro and in vivo conditions, including stimuli-induced and spontaneous ATP release in primary hippocampal cultures, injury-induced ATP release in a zebrafish model, and lipopolysaccharides-induced ATP-release events in individual astrocytes in the mouse cortex. Thus, the GRAB_ATP1.0 sensor is a sensitive, versatile tool for monitoring ATP release and dynamics under both physiological and pathophysiological conditions.

P2X-GCaMPs as Versatile Tools for Imaging Extracellular ATP Signaling

ATP is an extracellular signaling molecule involved in numerous physiological and pathologic processes. However, in situ characterization of the spatiotemporal dynamic of extracellular ATP is still challenging because of the lack of sensor with appropriate specificity, sensitivity, and kinetics. Here, we report the development of biosensors based on the fusion of cation permeable ATP receptors (P2X) to genetically encoded calcium sensors [genetically encoded calcium indicator (GECI)]. By combining the features of P2X receptors with the high signal-to-noise ratio of GECIs, we generated ultrasensitive green and red fluorescent sniffers that detect nanomolar ATP concentrations in situ and also enable the tracking of P2X receptor activity. We provide the proof of concept that these sensors can dynamically track ATP release evoked by depolarization in mouse neurons or by extracellular hypotonicity. Targeting these P2X-based biosensors to diverse cell types should advance our knowledge of extracellular ATP dynamics in vivo.

New frontiers in probing the dynamics of purinergic transmitters in vivo

Purinergic transmitters such as adenosine, ADP, ATP, UTP, and UDP-glucose play important roles in a wide range of physiological processes, including the sleep-wake cycle, learning and memory, cardiovascular function, and the immune response. Moreover, impaired purinergic signaling has been implicated in various pathological conditions such as pain, migraine, epilepsy, and drug addiction. Examining the function of purinergic transmission in both health and disease requires direct, sensitive, non-invasive tools for monitoring structurally similar purinergic transmitters; ideally, these tools should have high spatial and temporal resolution in in vivo applications. Here, we review the recent progress with respect to the development and application of new methods for detecting purinergic transmitters, focusing on optical tools; in addition, we provide discussion regarding future perspectives.

Multimeric Purinoceptor Detection by Bioluminescence Resonance Energy Transfer

Assays based on bioluminescence resonance energy transfer (BRET) provide a sensitive and simple method to study protein-protein interactions in live cells. Here we describe a protocol using BRET technique to investigate potential interactions between P2X subunits. This approach combined with bimolecular fluorescence complementation (BiFC) can also be employed to determine the stoichiometry of heteromeric P2X receptors.

An Optical Neuron-Astrocyte Proximity Assay at Synaptic Distance Scales

Astrocytes are complex bushy cells that serve important functions through close contacts between their processes and synapses. However, the spatial interactions and dynamics of astrocyte processes relative to synapses have proven problematic to study in adult living brain tissue. Here, we report a genetically targeted neuron-astrocyte proximity assay (NAPA) to measure astrocyte-synapse spatial interactions within intact brain preparations and at synaptic distance scales. The method exploits resonance energy transfer between extracellularly displayed fluorescent proteins targeted to synapses and astrocyte processes. We validated the method in the striatal microcircuitry following in vivo expression. We determined the proximity of striatal astrocyte processes to distinct neuronal input pathways, to D1 and D2 medium spiny neuron synapses, and we evaluated how astrocyte-to-excitatory synapse proximity changed following cortical afferent stimulation, during ischemia and in a model of Huntington's disease. NAPA provides a simple approach to measure astrocyte-synapse spatial interactions in a variety of experimental scenarios. VIDEO ABSTRACT.

Reducing Astrocyte Calcium Signaling In Vivo Alters Striatal Microcircuits and Causes Repetitive Behavior

Astrocytes tile the central nervous system, but their functions in neural microcircuits in vivo and their roles in mammalian behavior remain incompletely defined. We used two-photon laser scanning microscopy, electrophysiology, MINIscopes, RNA-seq, and a genetic approach to explore the effects of reduced striatal astrocyte Ca²⁺ signaling in vivo. In wild-type mice, reducing striatal astrocyte Ca²⁺-dependent signaling increased repetitive self-grooming behaviors by altering medium spiny neuron (MSN) activity. The mechanism involved astrocyte-mediated neuromodulation facilitated by ambient GABA and was corrected by blocking astrocyte GABA transporter 3 (GAT-3). Furthermore, in a mouse model of Huntington's disease, dysregulation of GABA and astrocyte Ca²⁺ signaling accompanied excessive self-grooming, which was relieved by blocking GAT-3. Assessments with RNA-seq revealed astrocyte genes and pathways regulated by Ca²⁺ signaling in a cell-autonomous and non-cell-autonomous manner, including Rab11a, a regulator of GAT-3 functional expression. Thus, striatal astrocytes contribute to neuromodulation controlling mouse obsessive-compulsive-like behavior.

ATP-induced IL-1β secretion is selectively impaired in microglia as compared to hematopoietic macrophages

Under stressful conditions nucleotides are released from dying cells into the extracellular space, where they can bind to purinergic P2X and P2Y receptors. High concentrations of extracellular ATP in particular induce P2X7-mediated signaling, which leads to inflammasome activation. This in turn leads to the processing and secretion of pro-inflammatory cytokines, like interleukin (IL)-1β. During neurodegenerative diseases, innate immune responses are shaped by microglia and we have previously identified microglia-specific features of inflammasome-mediated responses. Here, we compared ATP-induced IL-1β secretion in primary rhesus macaque microglia and bone marrow-derived macrophages (BMDM). We assessed the full expression profile of P2 receptors and characterized the induction and modulation of IL-1β secretion by extracellular nucleotides. Microglia secreted significantly lower levels of IL-1β in response to ATP when compared to BMDM. We demonstrate that this is not due to differences in sensitivity, kinetics or expression of ATP-processing enzymes, but rather to differences in purinergic receptor expression levels and usage. Using a combined approach of purinergic receptor agonists and antagonists, we demonstrate that ATP-induced IL-1β secretion in BMDM was fully dependent on P2X7 signaling, whereas in microglia multiple purinergic receptors were involved, including P2X7 and P2X4. These cell type-specific features of conserved innate immune responses may reflect adaptations to the vulnerable CNS microenvironment. GLIA 2016;64:2231-2246.

Genetically Encoded Fluorescent Indicators for Organellar Calcium Imaging

Optical Ca(2+) indicators are powerful tools for investigating intracellular Ca(2+) signals in living cells. Although a variety of Ca(2+) indicators have been developed, deciphering the physiological functions and spatiotemporal dynamics of Ca(2+) in intracellular organelles remains challenging. Genetically encoded Ca(2+) indicators (GECIs) using fluorescent proteins are promising tools for organellar Ca(2+) imaging, and much effort has been devoted to their development. In this review, we first discuss the key points of organellar Ca(2+) imaging and summarize the requirements for optimal organellar Ca(2+) indicators. Then, we highlight some of the recent advances in the engineering of fluorescent GECIs targeted to specific organelles. Finally, we discuss the limitations of currently available GECIs and the requirements for advancing the research on intraorganellar Ca(2+) signaling.

Genetically targeted single-channel optical recording reveals multiple Orai1 gating states and oscillations in calcium influx

Orai1 comprises the pore-forming subunit of the Ca(2+) release-activated Ca(2+) (CRAC) channel. When bound and activated by stromal interacting molecule 1 (STIM1), an endoplasmic reticulum (ER)-resident calcium sensor, Orai1 channels possess high selectivity for calcium but extremely small conductance that has precluded direct recording of single-channel currents. We have developed an approach to visualize Orai1 activity by fusing Orai1 to fluorescent, genetically encoded calcium indicators (GECIs). The GECI-Orai1 probes reveal local Ca(2+) influx at STIM1-Orai1 puncta. By whole cell recording, these fusions are fully functional as CRAC channels. When GECI-Orai1 and the CRAC-activating domain (CAD) of STIM1 were coexpressed at low levels and imaged using a total internal reflectance fluorescence microscope, cells exhibited sporadic fluorescence transients the size of diffraction-limited spots and the brightness of a few activated GECI proteins. Transients typically rose rapidly and fell into two classes according to duration: briefer "flickers" lasting only a few hundred milliseconds, and longer "pulses" lasting one to several seconds. The size, intensity, trace shape, frequency, distribution, physiological characteristics, and association with CAD binding together demonstrate that GECI-Orai1 fluorescence transients correspond to single-channel Orai1 responses. Single Orai1 channels gated by CAD, and small Orai1 puncta gated by STIM1, exhibit repetitive fluctuations in single-channel output. CAD binding supports a role in open state maintenance and reveals a second phase of CAD/STIM1 binding after channel opening. These first recordings of single-channel Orai1 currents reveal unexpected dynamics, and when paired with CAD association, support multiple single-channel states.

Genetically encoded Ca2+ indicators in cardiac myocytes

Genetically encoded Ca(2+) indicators constitute a powerful set of tools to investigate functional aspects of Ca(2+) signaling in isolated cardiomyocytes, cardiac tissue, and whole hearts. Here, we provide an overview of the concepts, experiences, state of the art, and ongoing developments in the use of genetically encoded Ca(2+) indicators for cardiac cells and heart tissue. This review is supplemented with in vivo viral gene transfer experiments and comparisons of available genetically encoded Ca(2+) indicators with each other and with the small molecule dye Fura-2. In the context of cardiac myocytes, we provide guidelines for selecting a genetically encoded Ca(2+) indicator. For future developments, we discuss improvements of a broad range of properties, including photophysical properties such as spectral spread and biocompatibility, as well as cellular and in vivo applications.

Imaging P2X4 receptor subcellular distribution, trafficking, and regulation using P2X4-pHluorin

A P2X4 receptor labeled with the pH-sensitive GFP superecliptic pHluorin represents a useful probe to investigate P2X4 receptor distribution, trafficking, and up-regulation.

P2X4 receptors are adenosine triphosphate (ATP)-gated cation channels present on the plasma membrane (PM) and also within intracellular compartments such as vesicles, vacuoles, lamellar bodies (LBs), and lysosomes. P2X4 receptors in microglia are up-regulated in epilepsy and in neuropathic pain; that is to say, their total and/or PM expression levels increase. However, the mechanisms underlying up-regulation of microglial P2X4 receptors remain unclear, in part because it has not been possible to image P2X4 receptor distribution within, or trafficking between, cellular compartments. Here, we report the generation of pH-sensitive fluorescently tagged P2X4 receptors that permit evaluations of cell surface and total receptor pools. Capitalizing on information gained from zebrafish P2X4.1 crystal structures, we designed a series of mouse P2X4 constructs in which a pH-sensitive green fluorescent protein, superecliptic pHluorin (pHluorin), was inserted into nonconserved regions located within flexible loops of the P2X4 receptor extracellular domain. One of these constructs, in which pHluorin was inserted after lysine 122 (P2X4-pHluorin123), functioned like wild-type P2X4 in terms of its peak ATP-evoked responses, macroscopic kinetics, calcium flux, current–voltage relationship, and sensitivity to ATP. P2X4-pHluorin123 also showed pH-dependent fluorescence changes, and was robustly expressed on the membrane and within intracellular compartments. P2X4-pHluorin123 identified cell surface and intracellular fractions of receptors in HEK-293 cells, hippocampal neurons, C8-B4 microglia, and alveolar type II (ATII) cells. Furthermore, it showed that the subcellular fractions of P2X4-pHluorin123 receptors were cell and compartment specific, for example, being larger in hippocampal neuron somata than in C8-B4 cell somata, and larger in C8-B4 microglial processes than in their somata. In ATII cells, P2X4-pHluorin123 showed that P2X4 receptors were secreted onto the PM when LBs undergo exocytosis. Finally, the use of P2X4-pHluorin123 showed that the modulator ivermectin did not increase the PM fraction of P2X4 receptors and acted allosterically to potentiate P2X4 receptor responses. Collectively, our data suggest that P2X4-pHluorin123 represents a useful optical probe to quantitatively explore P2X4 receptor distribution, trafficking, and up-regulation.

Conditions and constraints for astrocyte calcium signaling in the hippocampal mossy fiber pathway

The spatiotemporal activities of astrocyte Ca²⁺ signaling in mature neuronal circuits remain unclear. We used genetically encoded Ca²⁺ and glutamate indicators as well as pharmacogenetic and electrical control of neurotransmitter release to explore astrocyte activity in the hippocampal mossy fiber pathway. Our data revealed numerous localized, spontaneous Ca²⁺ signals in astrocyte branches and territories, but these were not driven by neuronal activity or glutamate. Moreover, evoked astrocyte Ca²⁺ signaling changed linearly with the number of mossy fiber action potentials. Under these settings, astrocyte responses were global, suppressed by neurotransmitter clearance, and mediated by glutamate and GABA. Thus, astrocyte engagement in the fully developed mossy fiber pathway was slow and territorial, contrary to that frequently proposed for astrocytes within microcircuits. We show that astrocyte Ca²⁺ signaling functionally segregates large volumes of neuropil and that these transients are not suited for responding to, or regulating, single synapses in the mossy fiber pathway.

Pyridoxal-5′-phosphate (MC-1), a vitamin B6 derivative, inhibits expressed P2X receptors

P2X receptors are cation-permeable ligand-gated ion channels that open in response to the binding of ATP. These receptors are present in many excitable cells, including neurons, striated muscle cells, epithelial cells, and leukocytes. They mediate fast excitatory neurotransmission in the central and peripheral nervous systems and are thought to be involved in neuropathic pain, inflammation, and cell damage following ischemia–reperfusion injuries. P2X receptors are thus a target for the development of new therapeutics to treat chronic pain and inflammation. In this study, we characterized the inhibition caused by pyridoxal-5′-phosphate, a natural metabolite of vitamin B6 (MC-1), of P2X2, P2X4, P2X7, and P2X2/3 receptors stably expressed in HEK293 cells using the patch-clamp technique in the whole-cell configuration. We also tested a new approach using VC6.1, a modified cameleon calcium-sensitive fluorescent protein, to characterize the inhibition of P2X2 and P2X2/3. MC-1 blocked these two P2X receptors, with an IC50 of 7 and 13 μmol/L, respectively. P2X2 exhibited the highest affinity for VC6.1, and the chimeric receptor P2X2/3, the lowest. The patch-clamp and imaging approaches gave similar results and indicated that VC6.1 may be useful for high throughput drug screening. Pyridoxal-5′-phosphate is an efficient P2X blocker and can be classified as a P2X antagonist.

Calcium-dependent regulation of Rab activation and vesicle fusion by an intracellular P2X ion channel

Rab GTPases play key roles in the delivery, docking and fusion of intracellular vesicles. However, the mechanism by which spatial and temporal regulation of Rab GTPase activity is controlled is poorly understood. Here we describe a mechanism by which localized calcium release through a vesicular ion channel controls Rab GTPase activity. We show that activation of P2XA, an intracellular ion channel localized to the Dictyostelium discoideum contractile vacuole system, results in calcium efflux required for downregulation of Rab11a activity and efficient vacuole fusion. Vacuole fusion and Rab11a downregulation require the activity of CnrF, an EF hand containing Rab GAP found in a complex with Rab11a and P2XA. CnrF Rab GAP activity to Rab11a is enhanced by the presence of calcium and the EF-hand domain. These findings suggest that P2XA activation results in vacuolar calcium release, which triggers activation of CnrF Rab GAP activity and subsequent downregulation of Rab11a to allow vacuole fusion.

Neuronal and glial purinergic receptors functions in neuron development and brain disease

Brain development requires the interaction of complex signaling pathways, involving different cell types and molecules. For a long time, most attention has focused on neurons in a neuronocentric conceptualization of central nervous system development, these cells fulfilling an intrinsic program that establishes the brain’s morphology and function. By contrast, glia have mainly been studied as support cells, offering guidance or as the cells that react to brain injury. However, new evidence is appearing that demonstrates a more fundamental role of glial cells in the control of different aspects of neuronal development and function, events in which the influence of neurons is at best weak. Moreover, it is becoming clear that the function and organization of the nervous system depends heavily on reciprocal neuron–glia interactions. During development, neurons are often generated far from their final destination and while intrinsic mechanisms are responsible for neuronal migration and growth, they need support and regulatory influences from glial cells in order to migrate correctly. Similarly, the axons emitted by neurons often have to reach faraway targets and in this sense, glia help define the way that axons grow. Moreover, oligodendrocytes and Schwann cells ultimately envelop axons, contributing to the generation of nodes of Ranvier. Finally, recent publications show that astrocytes contribute to the modulation of synaptic transmission. In this sense, purinergic receptors are expressed widely by glial cells and neurons, and recent evidence points to multiple roles of purines and purinergic receptors in neuronal development and function, from neurogenesis to axon growth and functional axonal maturation, as well as in pathological conditions in the brain. This review will focus on the role of glial and neuronal secreted purines, and on the purinergic receptors, fundamentally in the control of neuronal development and function, as well as in diseases of the nervous system.

Purinergic P2X7 receptors mediate cell death in mouse cerebellar astrocytes in culture

The brain distribution and functional role of glial P2X7 receptors are broader and more complex than initially anticipated. We characterized P2X7 receptors from cerebellar astrocytes at the molecular, immunocytochemical, biophysical, and cell physiologic levels. Mouse cerebellar astrocytes in culture express mRNA coding for P2X7 receptors, which is translated into P2X7 receptor protein as proven by Western blot analysis and immunocytochemistry. Fura-2 imaging showed cytosolic calcium responses to ATP and the synthetic analog 3'-O-(4-benzoyl)benzoyl-ATP (BzATP) exhibited two components, namely an initial transient and metabotropic component followed by a sustained one that depended on extracellular calcium. This latter component, which was absent in astrocytes from P2X7 receptor knockout mice (P2X7 KO), was modulated by extracellular Mg(2+), and was sensitive to Brilliant Blue G (BBG) and 3-(5-(2,3-dichlorophenyl)-1H-tetrazol-1-yl)methyl pyridine (A438079) antagonism. BzATP also elicited inwardly directed nondesensitizing whole-cell ionic currents that were reduced by extracellular Mg(2+) and P2X7 antagonists (BBG and calmidazolium). In contrast to that previously reported in rat cerebellar astrocytes, sustained BzATP application induced a gradual increase in membrane permeability to large cations, such as N-methyl-d-glucamine and 4-[3-methyl-2(3H)-benzoxazolylidene)-methyl]-1-[3-(triethylammonio)propyl]diiodide, which ultimately led to the death of mouse astrocytes. Cerebellar astrocyte cell death was prevented by BBG but not by calmidazolium, removal of extracellular calcium, or treatment with the caspase-3 inhibitor, benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethylketone, thus suggesting a necrotic-type mechanism of cell death. Since this cellular response was not observed in astrocytes from P2X7 KO mice, this study suggests that stimulation of P2X7 receptor may convey a cell death signal to cerebellar astrocytes in a species-specific manner.

Detection of endogenous substances with enzymatic microelectrode biosensors in the kidney

Direct real-time measurements of purinergic agents and reactive oxygen species concentrations have been of great value in understanding the functional roles of these substances in a number of diseases including chronic kidney disease and hypertension. The interstitial concentrations of these intermediate signaling molecules and dynamics of their release are important autocrine and paracrine factors in the kidney, which play a key role in the regulation of oxidative stress, inflammation, and kidney damage. Analysis of signaling mechanisms, especially in vivo and ex vivo, has been slowed by deficiencies of existing methods for direct measurements of the signaling molecules concentrations in whole organs and acute changes in response to endocrine factors. The multienzymatic microelectrode biosensors technique was originally developed and used for the detection of purines release in the brain and in present could be modified to identify the interplay between different substances that could be measured simultaneously in whole organs, such as the kidney. Adaptation of this method for renal and cardiovascular studies represents a unique powerful approach for real-time monitoring of substance level fluctuations in organs or tissues under normal or pathological conditions.

Genetically Encoded Calcium Indicators and Astrocyte Calcium Microdomains

The discovery of intracellular Ca2+ signals within astrocytes has changed our view of how these ubiquitous cells contribute to brain function. Classically thought merely to serve supportive functions, astrocytes are increasingly thought to respond to, and regulate, neurons. The use of organic Ca2+ indicator dyes such as Fluo-4 and Fura-2 has proved instrumental in the study of astrocyte physiology. However, progress has recently been accelerated by the use of cytosolic and membrane targeted genetically encoded calcium indicators (GECIs). Herein, we review these recent findings, discuss why studying astrocyte Ca2+ signals is important to understand brain function, and summarize work that led to the discovery of TRPA1 channel-mediated near-membrane Ca2+ signals in astrocytes and their indirect neuromodulatory roles at inhibitory synapses in the CA1 stratum radiatum region of the hippocampus. We suggest that the use of membrane-targeted and cytosolic GECIs holds great promise to explore the diversity of Ca2+ signals within single astrocytes and also to study diversity of function for astrocytes in different parts of the brain.

Neuromodulation by extracellular ATP and P2X receptors in the CNS

Extracellular adenosine 5' triphosphate (ATP) is a widespread cell-to-cell signaling molecule in the brain, where it activates cell surface P2X and P2Y receptors. P2X receptors define a protein family unlike other neurotransmitter-gated ion channels in terms of sequence, subunit topology, assembly, and architecture. Within milliseconds of binding ATP, they catalyze the opening of a cation-selective pore. However, recent data show that P2X receptors often underlie neuromodulatory responses on slower time scales of seconds or longer. Herein, we review these findings at molecular, cellular and systems levels. We propose that, while P2X receptors are fast ligand-gated cation channels, they are most adept at mediating slow neuromodulatory functions that are more widespread and more physiologically utilized than fast ATP synaptic transmission in the CNS.

P2X2 and P2X5 subunits define a new heteromeric receptor with P2X7-like properties

Ligand-gated ion channels are prototypic oligomeric membrane proteins whose stoichiometry determines their functional properties and subcellular localization. Deciphering the quaternary structure of such protein complexes is an arduous task and usually requires the combination of multiple approaches. ATP-gated P2X receptors are formed by the association of three subunits, but the quaternary arrangement of the seven P2X subunits at the plasma membrane remains poorly characterized. By combining bioluminescence resonance energy transfer, bifunctional fluorescence complementation and protein biochemistry, we developed an experimental approach that allows precise determination of rat P2X receptor quaternary assembly. We found that P2X5 subunits associate with P2X1, P2X2, and P2X4 subunits. We demonstrate that P2X5 and P2X2 subunits interact to form as yet uncharacterized heteromeric receptors with alternate stoichiometries, both present at the plasma membrane. P2X2/5 receptors display functional properties such as pore dilatation, membrane blebbing, and phosphatidylserine exposure that were previously thought to be characteristic hallmarks of the P2X7 receptor. In mouse, P2X2 and P2X5 subunits colocalize and physically interact in specific neuronal populations suggesting that other P2X receptors might contribute to cellular responses typically attributed to P2X7 receptor.

Imaging P2X4 receptor lateral mobility in microglia: regulation by calcium and p38 MAPK

ATP-gated ionotropic P2X4 receptors are up-regulated in activated microglia and are critical for the development of neuropathic pain, a microglia-associated disorder. However, the nature of how plasma membrane P2X4 receptors are regulated in microglia is not fully understood. We used single-molecule imaging to track quantum dot-labeled P2X4 receptors to explore P2X4 receptor mobility in the processes of resting and activated microglia. We find that plasma membrane P2X4 receptor lateral mobility in resting microglial processes is largely random, consisting of mobile and slowly mobile receptors. Moreover, lateral mobility is P2X subunit- and cell-specific, increased in an ATP activation and calcium-dependent manner, and enhanced in activated microglia by the p38 MAPK pathway that selectively regulates slowly mobile receptors. Thus, our data indicate that P2X4 receptors are dynamically regulated mobile ATP sensors, sampling more of the plasma membrane in response to ATP and during the activated state of microglia that is associated with nervous system dysfunction.

Neuronal P2X2 receptors are mobile ATP sensors that explore the plasma membrane when activated

ATP-gated ionotropic P2X2 receptors are widely expressed in neurons. Although the electrophysiological properties of P2X2 receptors have been extensively studied, little is known about the plasma membrane lateral mobility of P2X2 receptors or whether receptor mobility is regulated by ATP. Here we used single-molecule imaging with simultaneous whole-cell voltage-clamp recordings to track quantum dot-labeled P2X2 receptors in the dendrites of rat hippocampal neurons to explore P2X2 receptor mobility and its regulation. We find that plasma membrane P2X2 receptor lateral mobility in dendrites is heterogeneous but mostly Brownian in nature, consisting of mobile and slowly mobile receptor pools. Moreover, lateral mobility is P2X2 subunit and cell specific, is increased in an activation-dependent manner, and is regulated by cytosolic VILIP1, a calcium binding protein. Our data provide the first direct measures of P2X receptor mobility and show that P2X2 receptors are mobile ATP sensors, sampling more of the dendritic plasma membrane in response to ATP.

Measurement of purine release with microelectrode biosensors

Purinergic signalling departs from traditional paradigms of neurotransmission in the variety of release mechanisms and routes of production of extracellular ATP and adenosine. Direct real-time measurements of these purinergic agents have been of great value in understanding the functional roles of this signalling system in a number of diverse contexts. Here, we review the methods for measuring purine release, introduce the concept of microelectrode biosensors for ATP and adenosine and explain how these have been used to provide new mechanistic insight in respiratory chemoreception, synaptic physiology, eye development and purine salvage. We finish by considering the association of purine release with pathological conditions and examine the possibilities that biosensors for purines may one day be a standard part of the clinical diagnostic tool chest.

Time-lapse imaging of neural development: zebrafish lead the way into the fourth dimension

Time-lapse imaging is often the only way to appreciate fully the many dynamic cell movements critical to neural development. Zebrafish possess many advantages that make them the best vertebrate model organism for live imaging of dynamic development events. This review will discuss technical considerations of time-lapse imaging experiments in zebrafish, describe selected examples of imaging studies in zebrafish that revealed new features or principles of neural development, and consider the promise and challenges of future time-lapse studies of neural development in zebrafish embryos and adults.

Monitoring astrocyte calcium microdomains with improved membrane targeted GCaMP reporters

Astrocytes are involved in synaptic and cerebrovascular regulation in the brain. These functions are regulated by intracellular calcium signalling that is thought to reflect a form of astrocyte excitability. In a recent study, we reported modification of the genetically encoded calcium indicator (GECI) GCaMP2 with a membrane-tethering domain, Lck, to generate Lck-GCaMP2. This GECI allowed us to detect novel microdomain calcium signals. The microdomains were random and 'spotty' in nature. In order to detect such signals more reliably, in the present study we further modified Lck-GCaMP2 to carry three mutations in the GCaMP2 moiety (M153K, T203V within EGFP and N60D in the CaM domain) to generate Lck-GCaMP3. We directly compared Lck-GCaMP2 and Lck-GCaMP3 by assessing their ability to monitor several types of astrocyte calcium signals with a focus on spotty microdomains. Our data show that Lck-GCaMP3 is between two- and four-times better than Lck-GCaMP2 in terms of its basal fluorescence intensity, signal-to-noise and its ability to detect microdomains. The use of Lck-GCaMP3 thus represents a significantly improved way to monitor astrocyte calcium signals, including microdomains, and will facilitate detailed exploration of their molecular mechanisms and physiological roles.

Purinergic signaling: a novel mechanism in immune surveillance

Purinergic receptors and the associated signaling cascades are known to play critical roles in cardiovascular, nervous, respiratory, gastrointestinal and urinogenital systems. Recent studies have also shed light on the importance of nucleotides and purinergic receptors in the regulation of the immune response. With a better understanding of the distribution and the receptor subtypes, the purinoceptors have the potential to become important therapeutic targets in inflammation, chemotaxis and immune-related diseases.

A genetically targeted optical sensor to monitor calcium signals in astrocyte processes

Calcium signaling is studied as a potential form of astrocyte excitability that may control astrocyte involvement in synaptic and cerebrovascular regulation. Fundamental questions remain unanswered about astrocyte calcium signaling, as current methods can not resolve calcium in small volume compartments, such as near the cell membrane and in distal cell processes. We modified the genetically encoded calcium sensor GCaMP2 with a membrane-tethering domain, Lck, increasing the level of Lck-GCaMP2 near the plasma membrane tenfold as compared with conventional GCaMP2. Using Lck-GCaMP2 in rat hippocampal astrocyte-neuron cocultures, we measured near-membrane calcium signals that were evoked pharmacologically or by single action potential-mediated neurotransmitter release. Moreover, we identified highly localized and frequent spontaneous calcium signals in astrocyte somata and processes that conventional GCaMP2 failed to detect. Lck-GCaMP2 acts as a genetically targeted calcium sensor for monitoring calcium signals in previously inaccessible parts of astrocytes, including fine processes.

P2X4 receptors in activated C8-B4 cells of cerebellar microglial origin

We investigated the properties and regulation of P2X receptors in immortalized C8-B4 cells of cerebellar microglial origin. Resting C8-B4 cells expressed virtually no functional P2X receptors, but largely increased functional expression of P2X4 receptors within 2–6 h of entering the activated state. Using real-time polymerase chain reaction, we found that P2X4 transcripts were increased during the activated state by 2.4-fold, but this increase was not reflected by a parallel increase in total P2X4 proteins. In resting C8-B4 cells, P2X4 subunits were mainly localized within intracellular compartments, including lysosomes. We found that cell surface P2X4 receptor levels increased by ∼3.5-fold during the activated state. This change was accompanied by a decrease in the lysosomal pool of P2X4 proteins. We next exploited our findings with C8-B4 cells to investigate the mechanism by which antidepressants reduce P2X4 responses. We found little evidence to suggest that several antidepressants were antagonists of P2X4 receptors in C8-B4 cells. However, we found that moderate concentrations of the same antidepressants reduced P2X4 responses in activated microglia by affecting lysosomal function, which indirectly reduced cell surface P2X4 levels. In summary, our data suggest that activated C8-B4 cells express P2X4 receptors when the membrane insertion of these proteins by lysosomal secretion exceeds their removal, and that antidepressants indirectly reduce P2X4 responses by interfering with lysosomal trafficking.

Gated access to the pore of a P2X receptor: structural implications for closed-open transitions

P2X receptors are ligand-gated cation channels that transition from closed to open states upon binding ATP. The crystal structure of the closed zebrafish P2X4.1 receptor directly reveals that the ion-conducting pathway is formed by three transmembrane domain 2 (TM2) alpha-helices, each being provided by the three subunits of the trimer. However, the transitions in TM2 that accompany channel opening are incompletely understood and remain unresolved. In this study, we quantified gated access to Cd(2+) at substituted cysteines in TM2 of P2X2 receptors in the open and closed states. Our data for the closed state are consistent with the zebrafish P2X4.1 structure, with isoleucines and threonines (Ile-332 and Thr-336) positioned one helical turn apart lining the channel wall on approach to the gate. Our data for the open state reveal gated access to deeper parts of the pore (Thr-339, Val-343, Asp-349, and Leu-353), suggesting the closed channel gate is between Thr-336 and Thr-339. We also found unexpected interactions between native Cys-348 and D349C that result in tight Cd(2+) binding deep within the intracellular vestibule in the open state. Interpreted with a P2X2 receptor structural model of the closed state, our data suggest that the channel gate opens near Thr-336/Thr-339 and is accompanied by movement of the pore-lining regions, which narrow toward the cytosolic end of TM2 in the open state. Such transitions would relieve the barrier to ion flow and render the intracellular vestibule less splayed during channel opening in the presence of ATP.

Measuring near plasma membrane and global intracellular calcium dynamics in astrocytes

The brain contains glial cells. Astrocytes, a type of glial cell, have long been known to provide a passive supportive role to neurons. However, increasing evidence suggests that astrocytes may also actively participate in brain function through functional interactions with neurons. However, many fundamental aspects of astrocyte biology remain controversial, unclear and/or experimentally unexplored. One important issue is the dynamics of intracellular calcium transients in astrocytes. This is relevant because calcium is well established as an important second messenger and because it has been proposed that astrocyte calcium elevations can trigger the release of transmitters from astrocytes. However, there has not been any detailed or satisfying description of near plasma membrane calcium signaling in astrocytes. Total internal reflection fluorescence (TIRF) microscopy is a powerful tool to analyze physiologically relevant signaling events within about 100 nm of the plasma membrane of live cells. Here, we use TIRF microscopy and describe how to monitor near plasma membrane and global intracellular calcium dynamics almost simultaneously. The further refinement and systematic application of this approach has the potential to inform about the precise details of astrocyte calcium signaling. A detailed understanding of astrocyte calcium dynamics may provide a basis to understand if, how, when and why astrocytes and neurons undergo calcium-dependent functional interactions.

Optimizing the detergent concentration conditions for immunoprecipitation (IP) coupled with LC-MS/MS identification of interacting proteins

Immunoprecipitation (IP) coupled with LC-MS/MS is a widely used method in proteomics research to identify proteins and to study protein-protein interactions. IP techniques allow purification of proteins of interest and reduce sample complexity before introduction to the mass spectrometer. The effectiveness of IP experiments is an important factor for identification of proteins and protein-protein interactions. In this paper, a variety of IP conditions were studied systematically to improve IP-based protein interaction identification capabilities. Low concentration detergent (around 0.05% NP40/PBS) was found to improve IP effectiveness by decreasing non-specific binding. However, higher concentration detergent (e.g. 1% NP40/PBS) was detrimental. Furthermore, with lower protein concentrations, the IP system showed lower tolerance to detergent. For example, with a detergent concentration higher than 0.05% NP40/PBS, IP experiments were unsuccessful with low protein concentration (e.g. 0.28 microM ADH). In some cases the observed results were even worse than the results obtained without detergent. However, when the protein concentration was high (e.g. 1.12 microM ADH), this effect was not obvious and the high detergent (higher than 0.1%) experimental results were similar to those from low detergent concentration experiments (around 0.05%). Another application of this strategy to a more general system based on FLAG-Bacterial Alkaline Phosphatase (BAP) and anti-FLAG antibody was also performed. These results also suggested that low detergent concentration (0.05% NP40) is helpful for IP experiments, especially for the experiments with low protein concentrations. Considering that in most real applications, the proteins of interest are usually present in low abundance, a low amount of detergent is recommended to be used. The optimized detergent concentration was determined to be 0.05% in these studies. However, the key result presented here illustrates that both detergent and protein concentrations should be carefully considered when one is trying to optimize IP prior to mass spectrometry experiments.

Regulation of P2X2 receptors by the neuronal calcium sensor VILIP1

Extracellular adenosine triphosphate (ATP) activates P2X receptors, which are involved in diverse physiological functions. Using a proteomic approach, we identified the neuronal calcium sensor VILIP1 as interacting with P2X2 receptors. We found that VILIP1 forms a signaling complex in vitro and in vivo with P2X2 receptors and regulates P2X2 receptor sensitivity to ATP, peak response, surface expression, and diffusion. VILIP1 constitutively binds to P2X2 receptors and displays enhanced interactions in an activation- and calcium-dependent manner owing to exposure of its binding segment in P2X2 receptors. VILIP1-P2X2 interactions are also enhanced in hippocampal neurons during conditions of action potential firing known to trigger P2X2 receptor activation. Our data thus reveal a previously unrecognized function for the neuronal calcium sensor protein VILIP1 and a mechanism for regulation of ATP-dependent P2X receptor signaling by neuronal calcium sensors.

Patch-clamp coordinated spectroscopy shows P2X2 receptor permeability dynamics require cytosolic domain rearrangements but not Panx-1 channels

ATP-gated P2X receptors display ion permeability increases within seconds of receptor activation as the channels enter the I(2) state, which is permeable to organic cations and dye molecules. The mechanisms underlying this important behavior are not completely understood. In one model, the I(2) state is thought to be due to opening of Pannexin-1 (Panx-1) channels, and, in the second, it is thought to be an intrinsic P2X property. We tested both models by measuring ion and dye permeability and used a patch-clamp coordinated spectroscopy approach to measure conformational changes. Our data show that Panx-1 channels make no detectable contribution to the P2X(2) receptor I(2) state. However, P2X(2) receptors display permeability dynamics, which are correlated with conformational changes in the cytosolic domain remote from the selectivity filter itself. Finally, the data illustrate the utility of a new approach, using tetracysteine tags and biarsenical fluorophores to measure site-specific conformational changes in membrane proteins.

Molecular shape, architecture, and size of P2X4 receptors determined using fluorescence resonance energy transfer and electron microscopy

P2X receptors are ATP-gated nonselective cation channels with important physiological roles. However, their structures are poorly understood. Here, we analyzed the architecture of P2X receptors using fluorescence resonance energy transfer (FRET) microscopy and direct structure determination using electron microscopy. FRET efficiency measurements indicated that the distance between the C-terminal tails of P2X₄ receptors was 5.6 nm. Single particle analysis of purified P2X₄ receptors was used to determine the three-dimensional structure at a resolution of 21Å_; the orientation of the particle with respect to the membrane was assigned by labeling the intracellular C termini with 1.8-nm gold particles and the carbohydrate-rich ectodomain with lectin. We found that human P2X₄ is a globular torpedo-like molecule with an approximate volume of 270 nm³ and a compact propeller-shaped ectodomain. In this structure, the distance between the centers of the gold particles was 6.1 nm, which closely matches FRET data. Thus, our data provide the first views of the architecture, shape, and size of single P2X receptors, furthering our understanding of this important family of ligand-gated ion channels.

ATP-gated P2X receptors on excitatory nerve terminals onto interneurons initiate a form of asynchronous glutamate release

Previous work has shown that ATP-gated P2X2 receptors are expressed in excitatory nerve terminals onto stratum radiatum interneurons in the mouse hippocampal CA1 region. At these synapses receptor activation results in calcium-dependent facilitation of miniature and spontaneous EPSC frequency. In this study I determined if activation of presynaptic P2X receptors produces these effects by utilizing the vesicles underlying action potential dependent release. Brief trains of electrical stimuli caused short-term synaptic depression of excitatory synapses onto interneurons, in a manner consistent with depletion of the readily releasable pool of vesicles. P2X receptor activation increased the frequency of spontaneous EPSCs, but unexpectedly evoked little effect on synaptic depression. This suggests that P2X receptor activation does not markedly draw on the vesicles underlying action potential dependent glutamate release. However asynchronous EPSCs were increased following synaptic depression and a component of these appeared to be initiated by endogenously released ATP acting on presynaptic P2X receptors. Unexpectedly, the data suggest P2X receptor activation initiates a form of asynchronous glutamate release, rather than detectably affecting the vesicles underlying action potential evoked release.