Decoding glutamate receptor activation by the Ca2+ sensor protein hippocalcin in rat hippocampal neurons

Calcium-dependent modulation of BKCa channel activity induced by plasmonic gold nanoparticles in pulmonary artery smooth muscle cells and hippocampal neurons

AIM

Gold nanoparticles are widely used for biomedical applications, but the precise molecular mechanism of their interaction with cellular structures is still unclear. Assuming that intracellular calcium fluctuations associated with surface plasmon-induced calcium entry could modulate the activity of potassium channels, we studied the effect of 5 nm gold nanoparticles on calcium-dependent potassium channels and associated calcium signaling in freshly isolated rat pulmonary artery smooth muscle cells and cultured hippocampal neurons.

METHODS

Outward potassium currents were recorded using patch-clamp techniques. Changes in intracellular calcium concentration were measured using the high affinity Ca²⁺ fluorescent indicator fluo-3 and laser confocal microscope.

RESULTS

In pulmonary artery smooth muscle cells, plasmonic gold nanoparticles increased the amplitude of currents via large-conductance Ca²⁺ -activated potassium channels, which was potentiated by green laser irradiation near plasmon resonance wavelength (532 nm). Buffering of intracellular free calcium with ethylene glycol-bis-N,N,N',N'-tetraacetic acid (EGTA) abolished these effects. Furthermore, using confocal laser microscopy it was found that application of gold nanoparticles caused oscillations of intracellular calcium concentration that were decreasing in amplitude with time. In cultured hippocampal neurons gold nanoparticles inhibited the effect of EGTA slowing down the decline of the BK_Ca current while partially restoring the amplitude of the slow after hyperpolarizing currents.

CONCLUSION

We conclude that fluctuations in intracellular calcium can modulate plasmonic gold nanoparticles-induced gating of BK_Ca channels in smooth muscle cells and neurons through an indirect mechanism, probably involving the interaction of plasmon resonance with calcium-permeable ion channels, which leads to a change in intracellular calcium level.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Calcium Sensors in Neuronal Function and Dysfunction

Calcium signaling in neurons as in other cell types can lead to varied changes in cellular function. Neuronal Ca²⁺ signaling processes have also become adapted to modulate the function of specific pathways over a wide variety of time domains and these can have effects on, for example, axon outgrowth, neuronal survival, and changes in synaptic strength. Ca²⁺ also plays a key role in synapses as the trigger for fast neurotransmitter release. Given its physiological importance, abnormalities in neuronal Ca²⁺ signaling potentially underlie many different neurological and neurodegenerative diseases. The mechanisms by which changes in intracellular Ca²⁺ concentration in neurons can bring about diverse responses is underpinned by the roles of ubiquitous or specialized neuronal Ca²⁺ sensors. It has been established that synaptotagmins have key functions in neurotransmitter release, and, in addition to calmodulin, other families of EF-hand-containing neuronal Ca²⁺ sensors, including the neuronal calcium sensor (NCS) and the calcium-binding protein (CaBP) families, play important physiological roles in neuronal Ca²⁺ signaling. It has become increasingly apparent that these various Ca²⁺ sensors may also be crucial for aspects of neuronal dysfunction and disease either indirectly or directly as a direct consequence of genetic variation or mutations. An understanding of the molecular basis for the regulation of the targets of the Ca²⁺ sensors and the physiological roles of each protein in identified neurons may contribute to future approaches to the development of treatments for a variety of human neuronal disorders.

Measurement of intracellular concentration of fluorescently-labeled targets in living cells

Estimations of intracellular concentrations of fluorescently-labeled molecules within living cells are very important for guidance of biological experiments and interpretation of their results. Here we propose a simple and universal approach for such estimations. The approach is based upon common knowledge that the dye fluorescence is directly proportional to its quantum yield and the number of its molecules and that a coefficient of proportionality is determined by spectral properties of the dye and optical equipment used to record fluorescent signals. If two fluorescent dyes are present in the same volume, then a ratio of their concentrations is equal to a ratio of their fluorescence multiplied by some dye- and equipment-dependent coefficient. Thus, if the coefficient and concentration of one dye is known then the concentration of another dye can be determined. Here we have demonstrated how to calculate this coefficient (called a ratio factor) and how to use it for concentration measurements of fluorescently tagged molecules. As an example of how this approach can be used, we estimated a concentration of exogenously expressed neuronal Ca2+ sensor protein, hippocalcin, tagged by a fluorescent protein in a dendritic tree of rat hippocampal neurons loaded via a patch pipette with Alexa Fluor dye of known concentration. The new approach should allow performing a fast, inexpensive and reliable quantitative analysis of fluorescently-labeled targets in different parts of living cells.

Quantitative Proteomic Analysis of the Hippocampus of Rats with GCR-Induced Spatial Memory Impairment

NASA is planning future missions to Mars, which will result in astronauts being exposed to ∼13 cGy/year of galactic cosmic radiation (GCR). Previous ground-based experiments have demonstrated that low (15 cGy) doses of 1 GeV/n ⁵⁶Fe ions impair hippocampus-dependent spatial memory in rats. However, some irradiated rats maintain a spatial memory performance comparable to that seen in the sham-irradiated rats, suggesting that some of these animals are able to ameliorate the deleterious effects of the GCR, while others are not. This rat model provides a unique opportunity to increase our understanding of how GCR affects neurophysiology, what adaptive responses can be invoked to prevent the emergence of GCR-induced spatial memory impairment, as well as the pathways that are altered when spatial memory impairment occurs. A label-free, unbiased proteomic profiling approach involving quantitative protein/peptide profiling followed by Cytoscape analysis has established the composition of the hippocampal proteome in male Wistar rats after exposure to 15 cGy of 1 GeV/n ⁵⁶Fe, and identified proteins whose expression is altered with respect to: 1. radiation exposure and 2. impaired spatial memory performance. We identified 30 proteins that were classified as "GCR exposure marker" (GEM) proteins (expressed solely or at higher levels in the irradiated rats but not related to spatial memory performance), most notably CD98, Cadps and GMFB. Conversely, there were 252 proteins that were detected only in the sham-irradiated samples, i.e., they were not detected in either of the irradiated cohorts; of these 10% have well-documented roles in neurotransmission. The second aspect of our data mining was to identify proteins whose expression was associated with either impaired or functional spatial memory. While there are multiple changes in the hippocampal proteome in the irradiated rats that have impaired spatial memory performance, with 203 proteins being detected (or upregulated) only in these rats, it would appear that spatial memory impairment may also arise from an inability of these rats to express "good spatial memory" (GSM) proteins, many of which play an important role in neuronal homeostasis and function, axonogenesis, presynaptic membrane organization and G-protein coupled receptor (GCPR) signaling. It may be possible to use this knowledge to develop two alternative countermeasure strategies, one that preserves critical pathways prophylactically and one that invokes restorative pathways after GCR exposure.

Biophysical and functional characterization of hippocalcin mutants responsible for human dystonia

Dystonia is a neurological movement disorder that forces the body into twisting, repetitive movements or sometimes painful abnormal postures. With the advent of next-generation sequencing technologies, the homozygous mutations T71N and A190T in the neuronal calcium sensor (NCS) hippocalcin were identified as the genetic cause of primary isolated dystonia (DYT2 dystonia). However, the effect of these mutations on the physiological role of hippocalcin has not yet been elucidated. Using a multidisciplinary approach, we demonstrated that hippocalcin oligomerises in a calcium-dependent manner and binds to voltage-gated calcium channels. Mutations T71N and A190T in hippocalcin did not affect stability, calcium-binding affinity or translocation to cellular membranes (Ca²⁺/myristoyl switch). We obtained the first crystal structure of hippocalcin and alignment with other NCS proteins showed significant variability in the orientation of the C-terminal part of the molecule, the region expected to be important for target binding. We demonstrated that the disease-causing mutations did not affect the structure of the protein, however both mutants showed a defect in oligomerisation. In addition, we observed an increased calcium influx in KCl-depolarised cells expressing mutated hippocalcin, mostly driven by N-type voltage-gated calcium channels. Our data demonstrate that the dystonia-causing mutations strongly affect hippocalcin cellular functions which suggest a central role for perturbed calcium signalling in DYT2 dystonia.

Proteomic Analysis of Mouse Cortex Postsynaptic Density following Neonatal Brain Hypoxia-Ischemia

Proteomics of the synapses and postsynaptic densities (PSDs) have provided a deep understanding of protein composition and signal networks in the adult brain, which underlie neuronal plasticity and neurodegenerative or psychiatric disorders. However, there is a paucity of knowledge about the architecture and organization of PSDs in the immature brain, and how it is modified by brain injury in an early developing stage. Mass spectrometry (MS)-based proteomic analysis was performed on PSDs prepared from cortices of postnatal day 9 naïve mice or pups which had suffered hypoxic-ischemic (HI) brain injury. 512 proteins of different functional groups were identified from PSDs collected 1 h after HI injury, among which 60 have not been reported previously. Seven newly identified proteins involved in neural development were highlighted. HI injury increased the yield of PSDs at early time points upon reperfusion, and multiple proteins were recruited into PSDs following the insult. Quantitative analysis was performed using spectral counting, and proteins whose relative expression was more than 50% up- or downregulated compared to the sham animals 1 h after HI insult were reported. Validation with Western blotting demonstrated changes in expression and phosphorylation of the N-methyl-D-aspartate receptor, activation of a series of postsynaptic protein kinases and dysregulation of scaffold and adaptor proteins in response to neonatal HI insult. This work, along with other recent studies of synaptic protein profiling in the immature brain, builds a foundation for future investigation on the molecular mechanisms underlying developing plasticity. Furthermore, it provides insights into the biochemical changes of PSDs following early brain hypoxia-ischemia, which is helpful for understanding not only the injury mechanisms, but also the process of repair or replenishment of neuronal circuits during recovery from brain damage.

Brain quantitative proteomic responses reveal new insight of benzotriazole neurotoxicity in female Chinese rare minnow (Gobiocypris rarus)

Benzotriazole (BT) is a high-production volume chemical which has been ubiquitously detected in aquatic environments. Although adverse effects from acute and chronic exposure to BT have been reported, the neurotoxic effect of BT and the mechanisms of toxicity are not well documented. In this study, adult female Chinese rare minnow (Gobiocypris rarus) were exposed to 0.05, 0.5, and 5mg/L BT for 28days. The brain proteome showed that BT exposure mainly involved in metabolic process, signal transduction, stress response, cytoskeleton, and transport. Pathway analysis revealed that cellular processes affected by BT included cellular respiration, G-protein signal cascades, Ca²⁺-dependent signaling, cell cycle and apoptosis. Moreover, data on relative mRNA levels demonstrated that genes related to these toxic pathways were also significantly affected by BT. Furthermore, proteins affected by BT such as CKBB, GS, HPCA, VDAC1, and FLOT1A are associated with neurological disorders. Therefore, our finding suggested that BT induced molecular responses in the brain and could provide new insight into BT neurotoxicity in Chinese rare minnow.

Sense and specificity in neuronal calcium signalling

Changes in the intracellular free calcium concentration ([Ca²⁺]i) in neurons regulate many and varied aspects of neuronal function over time scales from microseconds to days. The mystery is how a single signalling ion can lead to such diverse and specific changes in cell function. This is partly due to aspects of the Ca²⁺ signal itself, including its magnitude, duration, localisation and persistent or oscillatory nature. The transduction of the Ca²⁺ signal requires Ca²⁺binding to various Ca²⁺ sensor proteins. The different properties of these sensors are important for differential signal processing and determine the physiological specificity of Ca(2+) signalling pathways. A major factor underlying the specific roles of particular Ca²⁺ sensor proteins is the nature of their interaction with target proteins and how this mediates unique patterns of regulation. We review here recent progress from structural analyses and from functional analyses in model organisms that have begun to reveal the rules that underlie Ca²⁺ sensor protein specificity for target interaction. We discuss three case studies exemplifying different aspects of Ca²⁺ sensor/target interaction. This article is part of a special issue titled the 13th European Symposium on Calcium.

Ca(2+) sensor proteins in dendritic spines: a race for Ca(2+)

Dendritic spines are believed to be micro-compartments of Ca²⁺ regulation. In a recent study, it was suggested that the ubiquitous and evolutionarily conserved Ca²⁺ sensor, calmodulin (CaM), is the first to intercept Ca²⁺ entering the spine and might be responsible for the fast decay of Ca²⁺ transients in spines. Neuronal calcium sensor (NCS) and neuronal calcium-binding protein (nCaBP) families consist of Ca²⁺ sensors with largely unknown synaptic functions despite an increasing number of interaction partners. Particularly how these sensors operate in spines in the presence of CaM has not been discussed in detail before. The limited Ca²⁺ resources and the existence of common targets create a highly competitive environment where Ca²⁺ sensors compete with each other for Ca²⁺ and target binding. In this review, we take a simple numerical approach to put forth possible scenarios and their impact on signaling via Ca²⁺ sensors of the NCS and nCaBP families. We also discuss the ways in which spine geometry and properties of ion channels, their kinetics and distribution, alter the spatio-temporal aspects of Ca²⁺ transients in dendritic spines, whose interplay with Ca²⁺ sensors in turn influences the race for Ca²⁺.

Understanding the physiological roles of the neuronal calcium sensor proteins

Calcium signalling plays a crucial role in the control of neuronal function and plasticity. Changes in neuronal Ca²⁺ concentration are detected by Ca²⁺-binding proteins that can interact with and regulate target proteins to modify their function. Members of the neuronal calcium sensor (NCS) protein family have multiple non-redundant roles in the nervous system. Here we review recent advances in the understanding of the physiological roles of the NCS proteins and the molecular basis for their specificity.

Between promiscuity and specificity: novel roles of EF-hand calcium sensors in neuronal Ca2+ signalling

In recent years, substantial progress has been made towards an understanding of the physiological function of EF-hand calcium sensor proteins of the Calmodulin (CaM) superfamily in neurons. This deeper appreciation is based on the identification of novel target interactions, structural studies and the discovery of novel signalling mechanisms in protein trafficking and synaptic plasticity, in which CaM-like sensor proteins appear to play a role. However, not all interactions are of plausible physiological relevance and in many cases it is not yet clear how the CaM signaling network relates to the proposed function of other EF-hand sensors. In this review, we will summarize these findings and address some of the open questions on the functional role of EF-hand calcium binding proteins in neurons.