Fractional contribution of calcium to the cation current through glutamate receptor channels

Estimating the Ca2+ Block of NMDA Receptors with Single-Channel Electrophysiology

In vertebrate central neurons, NMDA receptors are glutamate- and glycine-gated ion channels that allow the passage of Na+ and Ca2+ ions into the cell when these neurotransmitters are simultaneously present. The passage of Ca2+ is critical for initiating the cellular processes underlying various forms of synaptic plasticity. These Ca2+ ions can autoregulate the NMDA receptor signal through multiple distinct mechanisms to reduce the total flux of cations. One such mechanism is the ability of Ca2+ ions to exclude the passage of Na+ ions resulting in a reduced unitary current conductance. In contrast to the well-characterized Mg2+ block, this "channel block" mechanism is voltage-independent. In this chapter, we discuss theoretical and experimental considerations for the study of channel block by Ca2+ using single-channel patch-clamp electrophysiology. We focus on two classic methodologies to quantify the dependence of unitary channel conductance on external concentrations of Ca2+ as the basis for quantifying Ca2+ block.

Estimating the Ca2+ Permeability of NMDA Receptors with Whole-Cell Patch-Clamp Electrophysiology

In the mammalian central nervous system (CNS), fast excitatory transmission relies primarily on the ionic fluxes generated by ionotropic glutamate receptors (iGluRs). Among iGluRs, NMDA receptors (NMDARs) are unique in their ability to pass large, Ca2+-rich currents. Importantly, their high Ca2+ permeability is essential for normal CNS function and is under physiological control. For this reason, the accurate measurement of NMDA receptor Ca2+ permeability represents a valuable experimental step in evaluating the mechanism by which these receptors contribute to a variety of physiological and pathological conditions. In this chapter, we provide a theoretical and practical overview of the common methods used to estimate the Ca2+ permeability of ion channels as they apply to NMDA receptors. Specifically, we describe the principles and methodology used to calculate relative permeability (PCa/PNa) and fractional permeability (Pf), along with the relationship between these two metrics. With increasing knowledge about the structural dynamics of ion channels and of the ongoing environmental fluctuations in which channels operate in vivo, the ability to quantify the Ca2+ entering cells through specific ion channels remains a tool essential to delineating the molecular mechanisms that support health and cause disease.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

A calcium-based plasticity model for predicting long-term potentiation and depression in the neocortex

Pyramidal cells (PCs) form the backbone of the layered structure of the neocortex, and plasticity of their synapses is thought to underlie learning in the brain. However, such long-term synaptic changes have been experimentally characterized between only a few types of PCs, posing a significant barrier for studying neocortical learning mechanisms. Here we introduce a model of synaptic plasticity based on data-constrained postsynaptic calcium dynamics, and show in a neocortical microcircuit model that a single parameter set is sufficient to unify the available experimental findings on long-term potentiation (LTP) and long-term depression (LTD) of PC connections. In particular, we find that the diverse plasticity outcomes across the different PC types can be explained by cell-type-specific synaptic physiology, cell morphology and innervation patterns, without requiring type-specific plasticity. Generalizing the model to in vivo extracellular calcium concentrations, we predict qualitatively different plasticity dynamics from those observed in vitro. This work provides a first comprehensive null model for LTP/LTD between neocortical PC types in vivo, and an open framework for further developing models of cortical synaptic plasticity.

Correlated conformational dynamics of the human GluN1-GluN2A type N-methyl-D-aspartate (NMDA) receptor

N-Methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels found in the nerve cell membranes. As a result of overexcitation of NMDARs, neuronal death occurs and may lead to diseases such as epilepsy, stroke, Alzheimer's disease, and Parkinson's disease. In this study, human GluN1- GluN2A type NMDAR structure is modeled based on the X-ray structure of the Xenopus laevis template and missing loops are added by ab-initio loop modeling. The final structure is chosen according to two different model assessment scores. To be able to observe the structural changes upon ligand binding, glycine and glutamate molecules are docked into the corresponding binding sites of the receptor. Subsequently, molecular dynamics simulations of 1.3 μs are performed for both apo and ligand-bound structures. Structural parameters, which have been considered to show functionally important changes in previous NMDAR studies, are monitored as conformational rulers to understand the dynamics of the conformational changes. Moreover, principal component analysis (PCA) is performed for the equilibrated part of the simulations. From these analyses, the differences in between apo and ligand-bound simulations can be summarized as the following: The girdle right at the beginning of the pore loop, which connects M2 and M3 helices of the ion channel, partially opens. Ligands act like an adhesive for the ligand-binding domain (LBD) by keeping the bi-lobed structure together and consequently this is reflected to the overall dynamics of the protein as an increased correlation of the LBD with especially the amino-terminal domain (ATD) of the protein.

Reply to "TRPA1-dependent calcium transients and CGRP release in DRG neurons require extracellular calcium"

In this issue, Gebhardt et al. (2020. J. Cell Biol.https://doi.org/10.1083/jcb.201702151) express interest in our recently published work (Shang et al. 2016. J. Cell Biol.https://doi.org/10.1083/jcb.201603081). Here, we would like to address their concerns regarding the lysosomal TRPA1-mediated intracellular calcium transients in dorsal root ganglion neurons.

Integrating Bioelectrical Currents and Ca2+ Signaling with Biochemical Signaling in Development and Pathogenesis

Roles of bioelectrical signals are increasingly recognized in excitable and nonexcitable non-neural tissues. Diverse ion-selective channels, pumps, and gap junctions participate in bioelectrical signaling, including those transporting calcium ions (Ca2+). Ca2+ is the most versatile transported ion, because it serves as an electrical charge carrier and a biochemical regulator for multiple molecular binding, enzyme, and transcription activities. We aspire to learn how bioelectrical signals crosstalk to biochemical/biomechanical signals. In this study, we review four recent studies showing how bioelectrical currents and Ca2+ signaling affect collective dermal cell migration during feather bud elongation, affect chondrogenic differentiation in limb development, couple with mechanical tension in aligning gut smooth muscle, and affect mitochondrial function and skeletal muscle atrophy. We observe bioelectrical signals involved in several developmental and pathological conditions in chickens and mice at multiple spatial scales: cellular, cellular collective, and subcellular. These examples inspire novel concept and approaches for future basic and translational studies.

The GluN3 subunit regulates ion selectivity within native N-methyl-d-aspartate receptors

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The GluN3 subunit is the least understood of all subunits that make up functional NMDARs in the brain.

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We show through ion substitution experiments that NMDARs containing GluN3 are more permeable to Ca²⁺ than those containing just GluN1 and GluN2.

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We attribute these differences to their ability to screen for Ca²⁺ over Na⁺.

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Subunit-dependent cation selectivity represents a hitherto unrealized mechanism for finer control of Ca²⁺ influx enhancing the repertoire of synaptic NMDARs.

Glutamatergic N-methyl-d-aspartate receptors (NMDARs) are heterotetrameric proteins whose subunits are derived from three gene families, GRIN1 (codes for GluN1), GRIN2 (GluN2) and GRIN3 (GluN3). In addition to providing binding sites for glutamate and the co-agonist glycine, these subunits in their di (d-) and tri (t-) heteromeric configurations regulate various aspects of receptor function in the brain. For example, the decay kinetics of NMDAR-mediated synaptic currents depend on the type of GluN2 subunit (GluN2A-GluN2D) in the receptor subunit composition. While much is known about the contributions of GluN1 and GluN2 to d-NMDAR function, we know comparatively little about how GluN3 influences the function of t-NMDARs composed of one or more subunits from each of the three gene families. We report here that in addition to altering kinetics and voltage-dependent properties, the GluN3 subunit endows these receptors with ion selectivity wherein influx of Ca²⁺ is preferred over Na⁺. This became apparent in the process of assessing Ca²⁺ permeability through these receptors and is of significance given that NMDARs are generally believed to be nonselective to cations and increased selectivity can lead to enhanced permeability. This was true of two independent brain regions where t-NMDARs are expressed, the somatosensory cortex, where both receptor subtypes are expressed at separate inputs onto single neurons, and the entorhinal cortex, where they are co-expressed at individual synaptic inputs. Based on this data and the sequence of amino acids lining selectivity filters within these subunits, we propose GluN3 to be a regulatory subunit for ion selectivity in t-NMDARs.

Modelling dendritic spines with the finite element method, investigating the impact of geometry on electric and calcic responses

Understanding the relationship between shape and function of dendritic spines is an elusive topic. Several modelling approaches have been used to investigate the interplay between spine geometry, calcium diffusion and electric signalling. We here use a second order finite element method to solve the Poisson-Nernst-Planck equations and describe electrodiffusion in dendritic spines. With this, we obtain relationships between dendritic geometry and calcic as well as electric responses to synaptic events. Our findings support the hypothesis that spine geometry plays a role shaping the electrical responses to synaptic events. Our method was also able to reveal the fine scale distribution of calcium in spines with irregular shapes.

Structural and Mechanistic Insights of CRAC Channel as a Drug Target in Autoimmune Disorder

BACKGROUND

Calcium (Ca2+) ion is a major intracellular signaling messenger, controlling a diverse array of cellular functions like gene expression, secretion, cell growth, proliferation, and apoptosis. The major mechanism controlling this Ca2+ homeostasis is store-operated Ca2+ release-activated Ca2+ (CRAC) channels. CRAC channels are integral membrane protein majorly constituted via two proteins, the stromal interaction molecule (STIM) and ORAI. Following Ca2+ depletion in the Endoplasmic reticulum (ER) store, STIM1 interacts with ORAI1 and leads to the opening of the CRAC channel gate and consequently allows the influx of Ca2+ ions. A plethora of studies report that aberrant CRAC channel activity due to Loss- or gain-of-function mutations in ORAI1 and STIM1 disturbs this Ca2+ homeostasis and causes several autoimmune disorders. Hence, it clearly indicates that the therapeutic target of CRAC channels provides the space for a new approach to treat autoimmune disorders.

OBJECTIVE

This review aims to provide the key structural and mechanical insights of STIM1, ORAI1 and other molecular modulators involved in CRAC channel regulation.

RESULTS AND CONCLUSION

Understanding the structure and function of the protein is the foremost step towards improving the effective target specificity by limiting their potential side effects. Herein, the review mainly focusses on the structural underpinnings of the CRAC channel gating mechanism along with its biophysical properties that would provide the solid foundation to aid the development of novel targeted drugs for an autoimmune disorder. Finally, the immune deficiencies caused due to mutations in CRAC channel and currently used pharmacological blockers with their limitation are briefly summarized.

Holographic two-photon activation for synthetic optogenetics

Optogenetic tools provide users the ability to photocontrol the activity of cells. Commonly, activation is achieved by expression of proteins from photosynthetic organisms, for example, microbial opsins (e.g., ChR2). Alternatively, a sister approach, synthetic optogenetics, enables photocontrol over proteins of mammalian origin by use of photoswitches, visible light (typically), and genetic modification. Thus, synthetic optogenetics facilitates interrogation of native neuronal signaling mechanisms. However, the poor tissue penetration of visible wavelengths impedes the use of the technique in tissue, as two-photon excitation (2PE) is typically required to access the near-infrared window. Here, we describe an alternative technique that uses 2PE-compatible photoswitches (section 1) for photoactivation of genetically modified glutamate receptors (section 2). Furthermore, for fast, multi-region photoactivation, we describe the use of 2P-digital holography (2P-DH) (section 3). We detail how to combine 2P-DH and synthetic optogenetics with electrophysiology, or with red fluorescence Ca²⁺ recordings, for all-optical neural interrogation. The time required to complete the methods, aside from obtaining the necessary reagents and illumination equipment, is ~3 weeks.

Colocalization of distinct NMDA receptor subtypes at excitatory synapses in the entorhinal cortex

The subunit composition of N-methyl-d-aspartate receptors (NMDARs) at synaptic inputs onto a neuron can either vary or be uniform depending on the type of neuron and/or brain region. Excitatory pyramidal neurons in the frontal and somatosensory cortices (L5), for example, show pathway-specific differences in NMDAR subunit composition in contrast with the entorhinal cortex (L3), where we now show colocalization of NMDARs with distinct subunit compositions at individual synaptic inputs onto these neurons. Subunit composition was deduced electrophysiologically based on alterations of current-voltage relationship ( I-V) profiles, amplitudes, and decay kinetics of minimally evoked, pharmacologically isolated, NMDAR-mediated excitatory postsynaptic currents by known subunit-preferring antagonists. The I-Vs were outwardly rectifying in a majority of neurons assayed (~80%), indicating expression of GluN1/GluN2/GluN3-containing triheteromeric NMDARs ( t-NMDARs) and of the conventional type, reversing close to 0 mV with prominent regions of negative slope, in the rest of the neurons sampled (~20%), indicating expression of GluN1/GluN2-containing diheteromeric NMDARs ( d-NMDARs). Blocking t-NMDARs in neurons with outwardly rectifying I-Vs pharmacologically unmasked d-NMDARs, with all responses antagonized using D-AP5. Coimmunoprecipitation assays of membrane-bound protein complexes isolated from the medial entorhinal area using subunit-selective antibodies corroborated stoichiometry and together suggested the coexpression of t- and d-NMDARs at these synapses. Colocalization of functionally distinct NMDAR subtypes at individual synaptic inputs likely enhances the repertoire of pyramidal neurons for information processing and plasticity within the entorhinal cortex. NEW & NOTEWORTHY The subunit composition of a N-methyl-d-aspartate (NMDA) receptor, which dictates most aspects of its function, can vary between neurons in different brain regions and/or between synaptic inputs onto single neurons. Here we demonstrate colocalization of tri- and diheteromeric-NMDA receptors at the same/single synaptic input onto excitatory neurons in the entorhinal cortex. Synaptic colocalization of distinct NMDAR subtypes might endow entorhinal cortical neurons with the ability to encode distinct patterns of neuronal activity through single synapses.

An Inconvenient Truth: Calcium Sensors Are Calcium Buffers

Recent advances in Ca²⁺ imaging have given neuroscientists a tool to follow the activity of large numbers of individual neurons simultaneously in vivo in the brains of animals as they are presented with sensory stimulation, respond to environmental challenges, and engage in behaviors. The Ca²⁺ sensors used to transduce changes in cellular Ca²⁺ into changes in fluorescence must bind Ca²⁺ to produce a signal. By binding Ca²⁺, these sensors can act as buffers, often reducing the magnitude of a Ca²⁺ change severalfold, and producing a proportional slowing of the rates of change. Ca²⁺ probes can thus distort the patterns of activity they are intended to study and modify ongoing Ca²⁺ signaling functions. Recognizing these factors will enhance the use of in vivo Ca²⁺ imaging in the investigation of neural circuit function.

HCN Channels: New Therapeutic Targets for Pain Treatment

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are highly regulated proteins which respond to different cellular stimuli. The HCN currents (I_h) mediated by HCN1 and HCN2 drive the repetitive firing in nociceptive neurons. The role of HCN channels in pain has been widely investigated as targets for the development of new therapeutic drugs, but the comprehensive design of HCN channel modulators has been restricted due to the lack of crystallographic data. The three-dimensional structure of the human HCN1 channel was recently reported, opening new possibilities for the rational design of highly-selective HCN modulators. In this review, we discuss the structural and functional properties of HCN channels, their pharmacological inhibitors, and the potential strategies for designing new drugs to block the HCN channel function associated with pain perception.

Resident Calmodulin Primes NMDA Receptors for Ca2+-Dependent Inactivation

N-methyl-d-aspartate (NMDA) receptors are glutamate- and glycine-gated channels that flux Na+ and Ca2+ into postsynaptic neurons during synaptic transmission. The resulting intracellular Ca2+ transient is essential to physiological and pathological processes related to synaptic development, plasticity, and apoptosis. It also engages calmodulin (CaM) to reduce subsequent NMDA receptor activity in a process known as Ca2+-dependent inactivation (CDI). Here, we used whole-cell electrophysiology to measure CDI and computational modeling to dissect the sequence of events that underlies it. With these approaches, we estimate that CaM senses NMDA receptor Ca2+ influx at ∼9 nm from the channel pore. Further, when we controlled the frequency of Ca2+ influx through individual channels, we found that a kinetic model where apoCaM associates with channels before their activation best predicts the measured CDI. These results provide, to our knowledge, novel functional evidence for CaM preassociation to NMDA receptors in living cells. This particular mechanism for autoinhibitory feedback reveals strategies and challenges for Ca2+ regulation in neurons during physiological synaptic activity and disease.

The enduring legacy of the "constant-field equation" in membrane ion transport

In 1943, David Goldman published a seminal paper in The Journal of General Physiology that reported a concise expression for the membrane current as a function of ion concentrations and voltage. This body of work was, and still is, the theoretical pillar used to interpret the relationship between a cell's membrane potential and its external and/or internal ionic composition. Here, we describe from an historical perspective the theory underlying the constant-field equation and its application to membrane ion transport.

Sodium Dynamics in Pyramidal Neuron Dendritic Spines: Synaptically Evoked Entry Predominantly through AMPA Receptors and Removal by Diffusion

Dendritic spines are key elements underlying synaptic integration and cellular plasticity, but many features of these important structures are not known or are controversial. We examined these properties using newly developed simultaneous sodium and calcium imaging with single-spine resolution in pyramidal neurons in rat hippocampal slices from either sex. Indicators for both ions were loaded through the somatic patch pipette, which also recorded electrical responses. Fluorescence changes were detected with a high-speed, low-noise CCD camera. Following subthreshold electrical stimulation, postsynaptic sodium entry is almost entirely through AMPA receptors with little contribution from entry through NMDA receptors or voltage-gated sodium channels. Sodium removal from the spine head is through rapid diffusion out to the dendrite through the spine neck with a half-removal time of ∼16 ms, which suggests the neck has low resistance. Peak [Na⁺]_i changes during single EPSPs are ∼5 mm Stronger electrical stimulation evoked small plateau potentials that had significant longer-lasting localized [Na⁺]_i increases mediated through NMDA receptors.SIGNIFICANCE STATEMENT Dendritic spines, small structures that are difficult to investigate, are important elements in the fundamental processes of synaptic integration and plasticity. The main tool for examining these structures has been calcium imaging. However, the kinds of information that calcium imaging reveals is limited. We used newly developed, high-speed, simultaneous sodium and calcium imaging to examine ion dynamics in spines in hippocampal pyramidal neurons. We found that following single subthreshold synaptic activation most sodium entry was through AMPA receptors and not through NMDA receptors or through voltage-gated sodium channels and that the spine neck is not a significant resistance barrier. Most spine mechanisms are linear. However, regenerative NMDA conductances can be activated with stronger stimulation.

Local and Use-Dependent Effects of β-Amyloid Oligomers on NMDA Receptor Function Revealed by Optical Quantal Analysis

Beta amyloid (Aβ) triggers the elimination of excitatory synaptic connections in the CNS, an early manifestation of Alzheimer's disease. Oligomeric assemblies of Aβ peptide associate with excitatory synapses resulting in synapse elimination through a process that requires NMDA-type glutamate receptor activation. Whether Aβ affects synaptic NMDA receptor (NMDAR) function directly and acts locally at synapses to which it has bound and whether synaptic activity influences Aβ synaptic binding and synaptotoxicity have remained fundamental questions. Here, we used subcellular Ca²⁺ imaging in rat hippocampal neurons to visualize NMDAR function at individual synapses before and after Aβ application. Aβ triggered a robust impairment of NMDAR Ca²⁺ entry at most, but not all, synapses. NMDAR function was more severely impaired at highly active synapses and synapses with bound Aβ, but activity was not required for Aβ synapse binding. Blocking NMDARs during Aβ exposure prevented Aβ-mediated impairment. Finally, Aβ impaired NMDAR Ca²⁺ entry at doses much lower than those required for NMDAR internalization, revealing a novel, potent mode of NMDAR regulation by Aβ.

SIGNIFICANCE STATEMENT

Amyloid β (Aβ) is strongly implicated in Alzheimer's disease. Aβ triggers the elimination of excitatory synapses through a mechanism that requires NMDA receptors (NMDARs). However, little is known about how or whether Aβ influences synaptic NMDAR function. We used an imaging-based assay to investigate the relationship among Aβ binding, activity, and NMDAR function at individual synapses. Aβ triggered a robust impairment of NMDAR Ca²⁺ entry at most, but not all, synapses. NMDAR function was more severely impaired at highly active synapses and synapses with bound Aβ. Blocking NMDARs during Aβ exposure prevented Aβ-mediated impairment. Together, our experiments reveal a novel use-dependent, potent, and local mode of Aβ-mediated NMDAR impairment.

Intracellular TRPA1 mediates Ca2+ release from lysosomes in dorsal root ganglion neurons

Transient receptor potential A1 (TRPA1) is a nonselective cation channel implicated in thermosensation and inflammatory pain. In this study, we show that TRPA1 (activated by allyl isothiocyanate, acrolein, and 4-hydroxynonenal) elevates the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in dorsal root ganglion (DRG) neurons in the presence and absence of extracellular Ca²⁺ Pharmacological and immunocytochemical analyses revealed the presence of TRPA1 channels both on the plasma membrane and in endolysosomes. Confocal line-scan imaging demonstrated Ca²⁺ signals elicited from individual endolysosomes ("lysosome Ca²⁺ sparks") by TRPA1 activation. In physiological solutions, the TRPA1-mediated endolysosomal Ca²⁺ release contributed to ∼40% of the overall [Ca²⁺]_i rise and directly triggered vesicle exocytosis and calcitonin gene-related peptide release, which greatly enhanced the excitability of DRG neurons. Thus, in addition to working via Ca²⁺ influx, TRPA1 channels trigger vesicle release in sensory neurons by releasing Ca²⁺ from lysosome-like organelles.

Impact of calcium-activated potassium channels on NMDA spikes in cortical layer 5 pyramidal neurons

Active electrical events play an important role in shaping signal processing in dendrites. As these events are usually associated with an increase in intracellular calcium, they are likely to be under the control of calcium-activated potassium channels. Here, we investigate the impact of calcium-activated potassium channels onN-methyl-d-aspartate (NMDA) receptor-dependent spikes, or NMDA spikes, evoked by glutamate iontophoresis onto basal dendrites of cortical layer 5 pyramidal neurons. We found that small-conductance calcium-activated potassium channels (SK channels) act to reduce NMDA spike amplitude but at the same time, also decrease the iontophoretic current required for their generation. This SK-mediated decrease in NMDA spike threshold was dependent on R-type voltage-gated calcium channels and indicates a counterintuitive, excitatory effect of SK channels on NMDA spike generation, whereas the capacity of SK channels to suppress NMDA spike amplitude is in line with the expected inhibitory action of potassium channels on dendritic excitability. Large-conductance calcium-activated potassium channels had no significant impact on NMDA spikes, indicating that these channels are either absent from basal dendrites or not activated by NMDA spikes. These experiments reveal complex and opposing interactions among NMDA receptors, SK channels, and voltage-gated calcium channels in basal dendrites of cortical layer 5 pyramidal neurons during NMDA spike generation, which are likely to play an important role in regulating the way these neurons integrate the thousands of synaptic inputs they receive.

Dendritic sodium spikes are required for long-term potentiation at distal synapses on hippocampal pyramidal neurons

Dendritic integration of synaptic inputs mediates rapid neural computation as well as longer-lasting plasticity. Several channel types can mediate dendritically initiated spikes (dSpikes), which may impact information processing and storage across multiple timescales; however, the roles of different channels in the rapid vs long-term effects of dSpikes are unknown. We show here that dSpikes mediated by Nav channels (blocked by a low concentration of TTX) are required for long-term potentiation (LTP) in the distal apical dendrites of hippocampal pyramidal neurons. Furthermore, imaging, simulations, and buffering experiments all support a model whereby fast Nav channel-mediated dSpikes (Na-dSpikes) contribute to LTP induction by promoting large, transient, localized increases in intracellular calcium concentration near the calcium-conducting pores of NMDAR and L-type Cav channels. Thus, in addition to contributing to rapid neural processing, Na-dSpikes are likely to contribute to memory formation via their role in long-lasting synaptic plasticity.

Quantifying Ca2+ current and permeability in ATP-gated P2X7 receptors

ATP-gated P2X7 receptors are prominently expressed in inflammatory cells and play a key role in the immune response. A major consequence of receptor activation is the regulated influx of Ca(2+) through the self-contained cation non-selective channel. Although the physiological importance of the resulting rise in intracellular Ca(2+) is universally acknowledged, the biophysics of the Ca(2+) flux responsible for the effects are poorly understood, largely because traditional methods of measuring Ca(2+) permeability are difficult to apply to P2X7 receptors. Here we use an alternative approach, called dye-overload patch-clamp photometry, to quantify the agonist-gated Ca(2+) flux of recombinant P2X7 receptors of dog, guinea pig, human, monkey, mouse, rat, and zebrafish. We find that the magnitude of the Ca(2+) component of the ATP-gated current depends on the species of origin, the splice variant, and the concentration of the purinergic agonist. We also measured a significant contribution of Ca(2+) to the agonist-gated current of the native P2X7Rs of mouse and human immune cells. Our results provide cross-species quantitative measures of the Ca(2+) current of the P2X7 receptor for the first time, and suggest that the cytoplasmic N terminus plays a meaningful role in regulating the flow of Ca(2+) through the channel.

Extracellular Ca²⁺ per se inhibits quantal size of catecholamine release in adrenal slice chromaffin cells

Classic calcium hypothesis states that depolarization-induced increase in intracellular Ca(2+) concentration ([Ca(2+)]i) triggers vesicle exocytosis by increasing vesicle release probability in neurons and neuroendocrine cells. The extracellular Ca(2+), in this calcium hypothesis, serves as a reservoir of Ca(2+) source. Recently we find that extracellular Ca(2+)per se inhibits the [Ca(2+)]i dependent vesicle exocytosis, but it remains unclear whether quantal size is regulated by extracellular, or intracellular Ca(2+) or both. In this work we showed that, in physiological condition, extracellular Ca(2+) per se specifically inhibited the quantal size of single vesicle release in rat adrenal slice chromaffin cells. The extracellular Ca(2+) in physiological concentration (2.5 mM) directly regulated fusion pore kinetics of spontaneous quantal release of catecholamine. In addition, removal of extracellular Ca(2+) directly triggered vesicle exocytosis without eliciting intracellular Ca(2+). We propose that intracellular Ca(2+) and extracellular Ca(2+)per se cooperately regulate single vesicle exocytosis. The vesicle release probability was jointly modulated by both intracellular and extracellular Ca(2+), while the vesicle quantal size was mainly determined by extracellular Ca(2+) in chromaffin cells physiologically.

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.

Synaptic integration by NG2 cells

NG2 expressing oligodendrocyte precursor cells stand out from other types of glial cells by receiving classical synaptic contacts from many neurons. This unconventional form of signaling between neurons and glial cells enables NG2 cells to receive information about the activity of presynaptic neurons with high temporal and spatial precision and has been postulated to be involved in activity-dependent myelination. While this still unproven concept is generally compelling, how NG2 cells may integrate synaptic input has hardly been addressed to date. Here we review the biophysical characteristics of synaptic currents and membrane properties of NG2 cells and discuss their capabilities to perform complex temporal and spatial signal integration and how this may be important for activity-dependent myelination.

Fractional Ca(2+) currents through TRP and TRPL channels in Drosophila photoreceptors

Light responses in Drosophila photoreceptors are mediated by two Ca(2+) permeable cation channels, transient receptor potential (TRP) and TRP-like (TRPL). Although Ca(2+) influx via these channels is critical for amplification, inactivation, and light adaptation, the fractional contribution of Ca(2+) to the currents (Pf) has not been measured. We describe a slow (τ ∼ 350 ms) tail current in voltage-clamped light responses and show that it is mediated by electrogenic Na(+)/Ca(2+) exchange. Assuming a 3Na:1Ca stoichiometry, we derive empirical estimates of Pf by comparing the charge integrals of the exchanger and light-induced currents. For TRPL channels, Pf was ∼17% as predicted by Goldman-Hodgkin-Katz (GHK) theory. Pf for TRP (29%) and wild-type flies (26%) was higher, but lower than the GHK prediction (45% and 42%). As predicted by GHK theory, Pf for both channels increased with extracellular [Ca(2+)], and was largely independent of voltage between -100 and -30 mV. A model incorporating intra- and extracellular geometry, ion permeation, diffusion, extrusion, and buffering suggested that the deviation from GHK predictions was largely accounted for by extracellular ionic depletion during the light-induced currents, and the time course of the Na(+)/Ca(2+) exchange current could be used to obtain estimates of cellular Ca(2+) buffering capacities.

Intravenous administration of achyranthes bidentata polypeptides supports recovery from experimental ischemic stroke in vivo

Background

Achyranthes bidentata Blume (A. bidentata) is a commonly prescribed Chinese medicinal herb. A. bidentata polypeptides (ABPP) is an active composite constituent, separated from the aqueous extract of A. bidentata. Our previous studies have found that ABPP have the neuroprotective function in vitro and in rat middle cerebral artery occlusion (MCAO) model in attenuating the brain infract area induced by focal ischemia-reperfusion. However, the ultimate goal of the stroke treatment is the restoration of behavioral function. Identifying behavioral deficits and therapeutic treatments in animal models of ischemic stroke is essential for potential translational applications.

Methodology and Principal Findings

The effect of ABPP on motor, sensory, and cognitive function in an ischemic stroke model with MCAO was investigated up to day 30. The function recovery monitored by the neurological deficit score, grip test, body asymmetry, beam-balancing task, and the Morris Water Maze. In this study, systemic administration of ABPP by i.v after MCAO decreased the neurological deficit score, ameliorated the forepaw muscle strength, and diminished the motor and sensory asymmetry on 7^th and 30^th day after MCAO. MCAO has been observed to cause prolonged disturbance of spatial learning and memory in rats using the MWM, and ABPP treatment could improve the spatial learning and memory function, which is impaired by MCAO in rats, on 30^th day after MCAO. Then, the viable cells in CA1 region of hippocampus were counted by Nissl staining, and the neuronal cell death were significantly suppressed in the ABPP treated group.

Conclusion

ABPP could improve the recovery of sensory, motor and coordination, and cognitive function in MCAO-induced ischemic rats. And this recovery had a good correlation to the less of neuronal injury in brain.

LOTOS-based two-photon calcium imaging of dendritic spines in vivo

Neurons in the mammalian brain receive thousands of synaptic inputs on their dendrites. In many types of neurons, such as cortical pyramidal neurons, excitatory synapses are formed on fine dendritic protrusions called spines. Usually, an individual spine forms a single synaptic contact with an afferent axon. In this protocol, we describe a recently established experimental procedure for measuring intracellular calcium signals from dendritic spines in cortical neurons in vivo by using a combination of two-photon microscopy and whole-cell patch-clamp recordings. We have used mice as an experimental model system, but the protocol may be readily adapted to other species. This method involves data acquisition at high frame rates and low-excitation laser power, and is termed low-power temporal oversampling (LOTOS). Because of its high sensitivity of fluorescence detection and reduced phototoxicity, LOTOS allows for prolonged and stable calcium imaging in vivo. Key aspects of the protocol, which can be completed in 5-6 h, include the use of a variant of high-speed two-photon imaging, refined surgery procedures and optimized tissue stabilization.

Calcium signaling in dendritic spines

Calcium (Ca(2+)) is a ubiquitous signaling molecule that accumulates in the cytoplasm in response to diverse classes of stimuli and, in turn, regulates many aspects of cell function. In neurons, Ca(2+) influx in response to action potentials or synaptic stimulation triggers neurotransmitter release, modulates ion channels, induces synaptic plasticity, and activates transcription. In this article, we discuss the factors that regulate Ca(2+) signaling in mammalian neurons with a particular focus on Ca(2+) signaling within dendritic spines. This includes consideration of the routes of entry and exit of Ca(2+), the cellular mechanisms that establish the temporal and spatial profile of Ca(2+) signaling, and the biophysical criteria that determine which downstream signals are activated when Ca(2+) accumulates in a spine. Furthermore, we also briefly discuss the technical advances that made possible the quantitative study of Ca(2+) signaling in dendritic spines.

Allosteric modulation of Ca2+ flux in ligand-gated cation channel (P2X4) by actions on lateral portals

Human P2X receptors are a family of seven ATP-gated ion channels that transport Na(+), K(+), and Ca(2+) across cell surface membranes. The P2X4 receptor is unique among family members in its sensitivity to the macrocyclic lactone, ivermectin, which allosterically modulates both ion conduction and channel gating. In this paper we show that removing the fixed negative charge of a single acidic amino acid (Glu(51)) in the lateral entrance to the transmembrane pore markedly attenuates the effect of ivermectin on Ca(2+) current and channel gating. Ca(2+) entry through P2X4 receptors is known to trigger downstream signaling pathways in microglia. Our experiments show that the lateral portals could present a novel target for drugs in the treatment of microglia-associated disease including neuropathic pain.

Sustained firing of cartwheel cells in the dorsal cochlear nucleus evokes endocannabinoid release and retrograde suppression of parallel fiber synapses

Neurons in many brain regions release endocannabinoids from their dendrites that act as retrograde signals to transiently suppress neurotransmitter release from presynaptic terminals. Little is known, however, about the physiological mechanisms of short-term endocannabinoid-mediated plasticity under physiological conditions. Here we investigate calcium-dependent endocannabinoid release from cartwheel cells (CWCs) of the mouse dorsal cochlear nucleus (DCN) in the auditory brainstem that provide feedforward inhibition onto DCN principal neurons. We report that sustained action potential firing by CWCs evokes endocannabinoid release in response to submicromolar elevation of dendritic calcium that transiently suppresses their parallel fiber (PF) inputs by >70%. Basal spontaneous CWC firing rates are insufficient to evoke tonic suppression of PF synapses. However, elevating CWC firing rates by stimulating PFs triggers the release of endocannabinoids and heterosynaptic suppression of PF inputs. Spike-evoked suppression by endocannabinoids selectively suppresses excitatory synapses, but glycinergic/GABAergic inputs onto CWCs are not affected. Our findings demonstrate a mechanism of transient plasticity mediated by endocannabinoids that heterosynaptically suppresses subsets of excitatory presynaptic inputs to CWCs that regulates feedforward inhibition of DCN principal neurons and may influence the output of the DCN.

Depolarization gates spine calcium transients and spike-timing-dependent potentiation

Timing-dependent long-term potentiation (t-LTP) is induced when synaptic activity is immediately followed by one or more back-propagating action potentials (bAPs) in the postsynaptic cell. As a mechanistic explanation, it has been proposed that the bAP removes the Mg2+ block of synaptic NMDA receptors, allowing for rapid Ca2+ entry at the active synapse. Recent experimental studies suggest that this model is incomplete: NMDA receptor-based coincidence detection requires strong postsynaptic depolarization, usually provided by AMPA receptor currents. Apparently, the brief AMPA-EPSP does not only enable t-LTP, it is also responsible for the very narrow time window for t-LTP induction. The emerging consensus puts the spine in the center of coincidence detection, as active conductances on the spine together with the electrical resistance of the spine neck regulate the depolarization of the spine head and thus Ca2+ influx during pairing. A focus on postsynaptic voltage during synaptic activation not only encompasses spike-timing-dependent plasticity (STDP), but explains also the cooperativity and frequency-dependence of plasticity.

Calibration of fluorescent calcium indicators

During the past decades, many different fluorescent indicators have been developed for measuring intracellular ion concentrations. Of particular interest are fluorescent calcium indicators because of the fundamental role of Ca2+in various cellular processes such as contraction, secretion, and gene activation. For a quantitative understanding of the physiological roles of Ca2+, fluorescence signals measured with calcium indicators have to be converted to intracellular free calcium concentration ([Ca2+]i). Similarly, changes in [Ca2+]iand the underlying calcium fluxes need to be inferred from the corresponding fluorescence changes. This article describes the theoretical background and the various principal methods for the calibration of calcium imaging data.

Calibration protocols for fluorescent calcium indicators

Fluorescent calcium indicators are useful for measuring intracellular calcium ion concentrations. For a quantitative understanding of the physiological roles of Ca2+, fluorescence signals measured with calcium indicators have to be converted to intracellular free calcium concentration ([Ca2+]i). Most methods for converting a fluorescence signal to [Ca2+]i require the determination of a set of three calibration parameters: (Keff, Rmin, Rmax), (Kd, ΔF/Fmax, [Ca2+]rest), or (Kd, Rf, Fmax) or (Kapp, τmin, τmax). Here we describe the classical procedure for calibration of ratiometric measurements for both in vivo and in vitro calibrations, which is also useful for determining Kd and Rf. The [Ca2+]i dependence of the fluorescence ratio is measured using a set of at least three calibration solutions with known [Ca2+]i levels.

Preferential use of unobstructed lateral portals as the access route to the pore of human ATP-gated ion channels (P2X receptors)

P2X receptors are trimeric cation channels with widespread roles in health and disease. The recent crystal structure of a P2X4 receptor provides a 3D view of their topology and architecture. A key unresolved issue is how ions gain access to the pore, because the structure reveals two different pathways within the extracellular domain. One of these is the central pathway spanning the entire length of the extracellular domain and covering a distance of ≈70 Å. The second consists of three lateral portals, adjacent to the membrane and connected to the transmembrane pore by short tunnels. Here, we demonstrate the preferential use of the lateral portals. Owing to their favorable diameters and equivalent spacing, the lateral portals split the task of ion supply threefold and minimize an ion's diffusive path before it succumbs to transmembrane electrochemical gradients.

Agonist-induced changes in Ca(2+) permeation through the nociceptor cation channel TRPA1

The Ca(2+)-permeable cation channel TRPA1 acts as an ionotropic receptor for various pungent compounds and as a noxious cold sensor in sensory neurons. It is unclear what proportion of the TRPA1-mediated current is carried by Ca(2+) ions and how the permeation pathway changes during stimulation. Here, based on the relative permeability of the nonstimulated channel to cations of different size, we estimated a pore diameter of approximately 11 A. Combined patch-clamp and Fura-2 fluorescence recordings revealed that with 2 mM extracellular Ca(2+), and at a membrane potential of -80 mV, approximately 17% of the inward TRPA1 current is carried by Ca(2+). Stimulation with mustard oil evoked an apparent dilatation of the pore of 3 A and an increase in divalent cation selectivity and fractional Ca(2+) current. Mutations in the putative pore that reduced the divalent permeability and fractional Ca(2+) current also prevented mustard-oil-induced increases in Ca(2+) permeation. It is interesting that fractional Ca(2+) currents for wild-type and mutant TRPA1 were consistently higher than values predicted based on biionic reversal potentials using the Goldman-Hodgkin-Katz equation, suggesting that binding of Ca(2+) in the pore hinders monovalent cation permeation. We conclude that the pore of TRPA1 is dynamic and supports a surprisingly large Ca(2+) influx.

GABAB receptors modulate NMDA receptor calcium signals in dendritic spines

Metabotropic GABA(B) receptors play a fundamental role in modulating the excitability of neurons and circuits throughout the brain. These receptors influence synaptic transmission by inhibiting presynaptic release or activating postsynaptic potassium channels. However, their ability to directly influence different types of postsynaptic glutamate receptors remains unresolved. Here we examine GABA(B) receptor modulation in layer 2/3 pyramidal neurons from the mouse prefrontal cortex. We use two-photon laser-scanning microscopy to study synaptic modulation at individual dendritic spines. Using two-photon optical quantal analysis, we first demonstrate robust presynaptic modulation of multivesicular release at single synapses. Using two-photon glutamate uncaging, we then reveal that GABA(B) receptors strongly inhibit NMDA receptor calcium signals. This postsynaptic modulation occurs via the PKA pathway and does not affect synaptic currents mediated by AMPA or NMDA receptors. This form of GABA(B) receptor modulation has widespread implications for the control of calcium-dependent neuronal function.

Regulation of NMDA receptor Ca2+ signalling and synaptic plasticity

NMDARs (N-methyl-D-aspartate receptors) are critical for synaptic function throughout the CNS (central nervous system). NMDAR-mediated Ca(2+) influx is implicated in neuronal differentiation, neuronal migration, synaptogenesis, structural remodelling, long-lasting forms of synaptic plasticity and higher cognitive functions. NMDAR-mediated Ca(2+) signalling in dendritic spines is not static, but can be remodelled in a cell- and synapse-specific manner by NMDAR subunit composition, protein kinases and neuronal activity during development and in response to sensory experience. Recent evidence indicates that Ca(2+) permeability of neuronal NMDARs, NMDAR-mediated Ca(2+) signalling in spines and induction of NMDAR-dependent LTP (long-term potentiation) at hippocampal Schaffer collateral-CA1 synapses are under control of the cAMP/PKA (protein kinase A) signalling cascade. Thus, by enhancing Ca(2+) influx through NMDARs in spines, PKA can regulate the induction of LTP. An emerging concept is that activity-dependent regulation of NMDAR-mediated Ca(2+) signalling by PKA and by extracellular signals that modulate cAMP or protein phosphatases at synaptic sites provides a dynamic and potentially powerful mechanism for bi-directional regulation of synaptic efficacy and remodelling.

Evaluation of Ca2+ permeability of nicotinic acetylcholine receptors in hypothalamic histaminergic neurons

Hypothalamic histaminergic tuberomammillary (TM) neurons express nicotinic acetylcholine receptors (nAChRs) with kinetic and pharmacological properties resembling those of highly Ca(2+) permeable alpha7 nAChRs. However, the Ca(2+) permeability of TM nAChR channels has not been determined. To directly evaluate the Ca(2+) permeability of TM nAChRs, patch-clamp recordings were conducted using non-cultured acutely dissociated TM neurons and external solutions containing low (2 mM) and high (20 mM) concentrations of Ca(2+). A shift in the reversal potentials was determined from the current-voltage relationships and the permeability ratio, P(Ca)/P(Na), was estimated within the Goldman-Hodgkin-Katz constant field approximation. TM nAChRs were found to be highly Ca(2+) permeable with the permeability ratio, P(Ca)/P(Na)(nAChR) being approximately 5.9 and the fractional Ca(2+) current, P(f)(nAChR) being approximately 10.1% at -60 mV. As a positive control for the applied methods and analysis, the permeability ratio, P(Ca)/P(Na)(NMDAR) being approximately 8.3 and the fractional Ca(2+) current, P(f)(NMDAR) being approximately 13.6% at -60 mV for NMDA receptors were determined using non-cultured acutely dissociated hippocampal pyramidal neurons and found similar to previously reported values. Therefore, these results demonstrate that native TM nAChRs are highly Ca(2+) permeable, but approximately 1.4 fold less permeable to Ca(2+) than native hippocampal pyramidal NMDA receptors.

Gamma-aminobutyric acid and glutamate differentially regulate intracellular calcium concentrations in mouse gonadotropin-releasing hormone neurons

Multiple factors regulate the activity of the GnRH neurons responsible for controlling fertility. Foremost among neuronal inputs to GnRH neurons are those using the amino acids glutamate and gamma-aminobutyric acid (GABA). The present study used a GnRH-Pericam transgenic mouse line, enabling live cell imaging of intracellular calcium concentrations ([Ca(2+)](i)) to evaluate the effects of glutamate and GABA signaling on [Ca(2+)](i) in peripubertal and adult mouse GnRH neurons. Activation of GABA(A), N-methyl-d-aspartate, or alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate acid (AMPA) receptors was found to evoke an increase in [Ca(2+)](i), in subpopulations of GnRH neurons. Approximately 70% of GnRH neurons responded to GABA, regardless of postnatal age or sex. Many fewer (approximately 20%) GnRH neurons responded to N-methyl-d-aspartate, and this was not influenced by postnatal age or sex. In contrast, about 65% of adult male and female GnRH neurons responded to AMPA compared with about 14% of male and female peripubertal mice (P < 0.05). The mechanisms underlying the ability of GABA and AMPA to increase [Ca(2+)](i) in adult GnRH neurons were evaluated pharmacologically. Both GABA and AMPA were found to evoke [Ca(2+)](i) increases through a calcium-induced calcium release mechanism involving internal calcium stores and inositol-1,4,5-trisphosphate receptors. For GABA, the initial increase in [Ca(2+)](i) originated from GABA(A) receptor-mediated activation of L-type voltage-gated calcium channels, whereas for AMPA this appeared to involve direct calcium entry through the AMPA receptor. These observations show that all of the principal amino acid receptors are able to control [Ca(2+)](i) in GnRH neurons but that they do so in a postnatal age- and intracellular pathway-specific manner.

The molecular physiology of CRAC channels

The Ca2+release-activated Ca2+ (CRAC) channel is a highly Ca2+-selective store-operated channel expressed in T cells, mast cells, and various other tissues. CRAC channels regulate critical cellular processes such as gene expression, motility, and the secretion of inflammatory mediators. The identification of Orai1, a key subunit of the CRAC channel pore, and STIM1, the endoplasmic reticulum (ER) Ca2+ sensor, have provided the tools to illuminate the mechanisms of regulation and the pore properties of CRAC channels. Recent evidence indicates that the activation of CRAC channels by store depletion involves a coordinated series of steps, which include the redistributions of STIM1 and Orai1, direct physical interactions between these proteins, and conformational changes in Orai1, culminating in channel activation. Additional studies have revealed that the high Ca2+ selectivity of CRAC channels arises from the presence of an intrapore Ca2+ binding site, the properties of which are finely honed to occlude the permeation of the much more prevalent Na+. Structure-function studies have led to the identification of the potential pore-binding sites for Ca2+, providing a firm framework for understanding the mechanisms of selectivity and gating of the CRAC channel. This review summarizes recent progress in understanding the mechanisms of CRAC channel activation, pore properties, and modulation.