Functional expression of AMPA receptors on central terminals of rat dorsal root ganglion neurons and presynaptic inhibition of glutamate release

No direct evidence has been found for expression of functional AMPA receptors by dorsal root ganglion neurons despite immunocytochemical evidence suggesting they are present. Here we report evidence for expression of functional AMPA receptors by a subpopulation of dorsal root ganglion neurons. The AMPA receptors are most prominently located near central terminals of primary afferent fibers. AMPA and kainate receptors were detected by recording receptor-mediated depolarization of the central terminals under selective pharmacological conditions. We demonstrate that activation of presynaptic AMPA receptors by exogenous agonists causes inhibition of glutamate release from the terminals, possibly via primary afferent depolarization (PAD). These results challenge the traditional view that GABA and GABA(A) receptors exclusively mediate PAD, and indicate that PAD is also mediated by glutamate acting on presynaptically localized AMPA and kainate receptors.     

Effects of ion channel activity on development of dorsal root ganglion neurons

Studies of mouse dorsal root ganglion neurons in vitro demonstrate that ion channel function and regulation can influence a wide range of developmental processes. The work suggests that much as exposure to different trophic factors, the pattern of impulse activity a neuron experiences can have significant structural and functional effects during development. Studies concerning effects of ion channel activity on growth cone motility, axon fasciculation, synaptic plasticity, myelination, and intracellular signaling pathways regulating gene expression are presented in the context of changes in endogenous firing patterns during development. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 158–170, 1998     

缓激肽对背根节神经元钠通道电流的作用

目的：观察缓激肽(bradykinin,BK)对大鼠背根节神经元电压依赖性钠通道电流的作用。方法：采用全细胞膜片钳技术,记录钠通道电流。结果：缓激肽剂量依赖性(0．01～10μmol／L)增高小细胞背根节神经元诱发放电频率;缓激肽剂量依赖性(O．01～10μmol／L)增加小细胞背根节神经元的河豚毒素不敏感(TTX—resistant,TTX—R)钠电流,对TTX敏感(TTX—sensitive,TTX-S)钠电流无明显影响。结论：缓激肽引起炎性痛的机制可能与TTX-R钠通道电流有关。     

The cross channel activities of spider neurotoxin huwentoxin-I on rat dorsal root ganglion neurons

In this paper, we investigated the action of huwentoxin-I (HWTX-I) purified from the venom of the Chinese bird spider Ornithoctonus huwena on Ca(2+), Na(+) channels of adult rat dorsal root ganglion (DRG) neurons. The results showed that huwentoxin-I could reduce the peak currents of N-type Ca(2+) channels (IC(50) approximately 100 nM) and TTX-S Na(+) channels (IC(50) approximately 55 nM), whereas no effect was detected on TTX-R Na(+) channels. The comparative studies indicated that the selectivity of HWTX-I on Ca(2+) channels was higher that of MVIIA and approximately the same as that of GVIA. HWTX-I is the first discovered toxin with the cross channel activities from the spider O. huwena venom similar to micro O-conotoxins MrVIA and MrVIB.     

The PDZ domain of PICK1 differentially accepts protein kinase C-alpha and GluR2 as interacting ligands

The C terminus (ct) of protein kinase C-alpha (PKCalpha) has a type I PDZ binding motif, whereas GluR2 has a type II PDZ binding motif. Both motifs are recognized by the PDZ domain of protein interacting with protein kinase C (PICK1), and PICK1-PKCalpha-controlled phosphorylation regulates the synaptic expression and function of GluR2. Here, we show that a specific mutation within the carboxylate-binding loop of the PDZ domain of PICK1 (K27E; PICK1-KE) results in a loss of interaction with GluR2 but not with PKCalpha. In GST pull-down studies, PICK1-WT (wild type) but not PICK1-KE was retained by GST-ct-GluR2. Furthermore, PICK1-WT co-immunoprecipitated both PKCalpha and GluR2, whereas PICK1-KE only co-immunoprecipitated PKCalpha. In heterologous cells, PICK1-WT, but not PICK1-KE, clustered GluR2 and also clustered GluR1 in a GluR2-dependent manner. However, neither PICK1-WT nor PICK1-KE altered the distribution of PKCalpha, even after phorbol ester-induced redistribution of PKCalpha to the membrane. Finally, PICK1-KE showed no mislocalization when compared with PICK1-WT in neurons. Taken together, it appears that the PDZ domain of PICK1 is less sensitive to mutations for PKCalpha when compared with GluR2 binding. These results suggest that the PDZ domain of PICK1 has distinct PKCalpha and GluR2 binding subsite(s).     

The binding affinities of proteins interacting with the PDZ domain of PICK1

Protein interacting with C kinase (PICK1) is well conserved throughout evolution and plays a critical role in synaptic plasticity by regulating the trafficking and posttranslational modification of its interacting proteins. PICK1 contains a single PSD95/DlgA/Zo-1 (PDZ) protein-protein interaction domain, which is promiscuous and shown to interact with over 60 proteins, most of which play roles in neuronal function. Several reports have suggested the role of PICK1 in disorders such as epilepsy, pain, brain trauma and stroke, drug abuse and dependence, schizophrenia and psychosis. Importantly, lead compounds that block PICK1 interactions are also now becoming available. Here, a new modeling approach was developed to investigate binding affinities of PDZ interactions. Using these methods, the binding affinities of all major PICK1 interacting proteins are reported and the effects of PICK1 mutations on these interactions are described. These modeling methods have important implications in defining the binding properties of proteins interacting with PICK1 as well as the general structural requirements of PDZ interactions. The study also provides modeling methods to support in the drug design of ligands for PDZ domains, which may further aid in development of the family of PDZ domains as a drug target.     

Effect of pH on fast sodium channels in neurons of the rat dorsal root ganglion

Ionic currents through fast sodium channels in the neuronal somatic membrane were measured under voltage clamp conditions using external solutions of normal and low pH. Voltage-dependent inhibition of ionic currents through open channels was observed in acidic solutions. The voltage-dependent block of sodium channels may be explained by the presence of two acid groups at the channel. The parameters of the inner and outer acid groups calculated according to this model are similar to those reported for the nodal membrane.     

Selective internalization of sodium channels in rat dorsal root ganglion neurons infected with herpes simplex virus-1

The neurotropic virus, herpes simplex type 1 (HSV-1), inhibits the excitability of peripheral mammalian neurons, but the molecular mechanism of this effect has not been identified. Here, we use voltage-clamp measurement of ionic currents and an antibody against sodium channels to show that loss of excitability results from the selective, precipitous, and complete internalization of voltage-activated sodium channel proteins from the plasma membrane of neurons dissociated from rat dorsal root ganglion. The internalization process requires viral protein synthesis but not viral encapsulation, and does not alter the density of voltage-activated calcium or potassium channels. However, internalization is blocked completely when viruses lack the neurovirulence factor, infected cell protein 34.5, or when endocytosis is inhibited with bafilomycin A(1) or chloroquine. Although it has been recognized for many years that viruses cause cell pathology by interfering with signal transduction pathways, this is the first example of viral pathology resulting from selective internalization of an integral membrane protein. In studying the HSV-induced redistribution of sodium channels, we have uncovered a previously unknown pathway for the rapid and dynamic control of excitability in sensory neurons by internalization of sodium channels.     

Agonistic effects of xanthines on tetrodotoxin-sensitive sodium channels in the rat dorsal root ganglion neurons

The effects of four xanthine derivatives, caffeine, caffeine benzoate, theophylline, and bromtheophylline, on sodium channels in internally perfused rat dorsal root ganglion neurons were studied under voltage-clamp and whole-cell patch-clamp conditions. Reversible acceleration, enhancement of the amplitude of sodium currents, and shifts of the current-voltage relation (plotted for their maxima), as well as of the steady-state inactivation curve toward more negative potentials, were observed at external applications of the above substances in the concentrations of 0.2–4.0 mM. Under long exposures, inactivation of sodium currents became slower in a part of the cells. Yet, when the exposure to 4 mM or higher concentrations was longer than 10 min, a rise in the passive conductance was obvious, and functional state of the cells became worse. Blocking effects of the xanthine derivatives on transient or delayed potassium currents were not observed. Thus, agonistic action of xanthines on sodium channels has been demonstrated, and it is supposed that a considerable component of their pharmacological effects is provided by the action on Na⁺/Ca²⁺ exchange.     

Zinc binding site in PICK1 is dominantly located at the CPC motif of its PDZ domain

PICK1 ( p rotein i nteracting with C k inase 1) containing a PDZ domain, a BAR domain, and two short acidic regions is as an adaptor protein that plays an important role in α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor trafficking, cell morphology and migration, as well as in some diseases such as cancer, schizophrenia and pain. To better understand the physiological function of PICK1, we expressed the recombinant PICK1 and its truncated mutants in E.coli, and measured their zinc binding properties by fluorescence and competition assay. It is shown that PICK1 has one Zn²⁺-binding site. The Zn²⁺-binding properties of PICK1 are not appreciably affected after the removal of BARC domain (involving BAR domain and C-terminal acidic region). Deleting the N-terminal acidic region of NPDZ domain (involving PDZ domain and N-terminal acidic region) in PICK1 impairs its Zn²⁺-binding capacity.The mutation of the CPC (Cys-Pro-Cys) motif in the PDZ domain of PICK1 abolishes the ability of Zn²⁺-binding. In addition, Zn²⁺ can enhance the lipid-binding ability of PDZ domain as observed in both protein-lipid overlay assay and fluorescence analysis. The results presented in this report suggested that Zn²⁺ plays a regulatory role in the trafficking of PICK1 from the cytoplasm to cell membrane.     

Spermine biphasically affects N-type calcium channel currents in adult dorsal root ganglion neurons of the rat

Spermine (Spe) is a polyamine co-secreted with neurotransmitters. In this work its effects on N-type Ca²⁺ channel (Ca_V2.2) have been studied on adult sensory neurons of the rat by means of whole-cell patch-clamp. Spe exerted biphasic effects when added to the external solution: at 500 μM decreased N-type Ca²⁺ channel currents, reducing the maximum whole-cell conductance, shifting the activation curve to the right on the voltage axes and decreasing its slope; conversely, at lower concentration (500 nM) Spe induced completely opposite effects. In 62% of the neurons the inhibitory effects were accompanied by a slowing down of the activation kinetics relieved by a conditioning pre-pulse to + 50 mV. The biphasic effects and their rapid onset and offset time course may be explained if multiple sites of action with a different affinity for Spe are present directly on the channel. The effects of Spe on HVA Ca²⁺ currents were strongly dependent on [Ca²⁺]_ext, high [Ca²⁺] powerfully reducing Spe effects. This may be explained if we take into account that as Spe has four positive charges at physiological pH; it may compete with divalent cations for some negatively charged regulatory sites. In these experiments, Spe was effective at concentrations possibly reached in physiological conditions.     

Molecular determinants for the complex binding specificity of the PDZ domain in PICK1

PICK1 (protein interacting with C kinase 1) contains a single PDZ domain known to mediate interaction with the C termini of several receptors, transporters, ion channels, and kinases. In contrast to most PDZ domains, the PICK1 PDZ domain interacts with binding sequences classifiable as type I (terminating in (S/T)XPhi; X, any residue) as well as type II (PhiXPhi; Phi, any hydrophobic residue). To enable direct assessment of the affinity of the PICK1 PDZ domain for its binding partners we developed a purification scheme for PICK1 and a novel quantitative binding assay based on fluorescence polarization. Our results showed that the PICK1 PDZ domain binds the type II sequence presented by the human dopamine transporter (-WLKV) with an almost 15-fold and >100-fold higher affinity than the type I sequences presented by protein kinase Calpha (-QSAV) and the beta(2)-adrenergic receptor (-DSLL), respectively. Mutational analysis of Lys(83) in the alphaB1 position of the PDZ domain suggested that this residue mimics the function of hydrophobic residues present in this position in regular type II PDZ domains. The PICK1 PDZ domain was moreover found to prefer small hydrophobic residues in the C-terminal P(0) position of the ligand. Molecular modeling predicted a rank order of (Val > Ile > Leu) that was verified experimentally with up to a approximately 16-fold difference in binding affinity between a valine and a leucine in P(0). The results define the structural basis for the unusual binding pattern of the PICK1 PDZ domain by substantiating the critical role of the alphaB1 position (Lys(83)) and of discrete side chain differences in position P(0) of the ligands.     

Clustering of AMPA receptors by the synaptic PDZ domain-containing protein PICK1

Synaptic clustering of neurotransmitter receptors is crucial for efficient signal transduction and integration in neurons. PDZ domain-containing proteins such as PSD-95/SAP90 interact with the intracellular C termini of a variety of receptors and are thought to be important in the targeting and anchoring of receptors to specific synapses. Here, we show that PICK1 (protein interacting with C kinase), a PDZ domain-containing protein, interacts with the C termini of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptors in vitro and in vivo. In neurons, PICK1 specifically colocalizes with AMPA receptors at excitatory synapses. Furthermore, PICK1 induces clustering of AMPA receptors in heterologous expression systems. These results suggest that PICK1 may play an important role in the modulation of synaptic transmission by regulating the synaptic targeting of AMPA receptors.     

Interaction of the PDZ domain of human PICK1 with class I ADP-ribosylation factors

We have cloned the cDNA encoding human PICK1 (protein interacting with C kinase 1), a PDZ domain-containing protein of 415 amino acids, and also identified the Drosophila homologue by search of the databank. Northern blot analysis shows a single mRNA of about 2.0 kb ubiquitously expressed in human tissues. Although PICK1 proteins harbor a region homologous to arfaptin1 and arfaptin2, two proteins that bind to the ARF (ADP-ribosylation factor), this region of PICK1 does not interact with ARFs in the yeast two-hybrid system. On the other hand, the PDZ domain of PICK1 is capable of interacting with constitutively active, GTP-bound forms of ARF1 and ARF3, but neither with those of ARF5/6 nor with the GDP-bound ARFs. The PICK1-ARF interaction is abrogated by introduction of mutations in the PDZ domain or by deletion of the extreme C-terminus of ARF1. Thus, PICK1 specifically interacts with ARF1/3 in the GTP-bound state, suggesting that PICK1 participates in ARF1/3-mediated cellular processes.     

Upregulation and antiapoptotic role of endogenous Alzheimer amyloid precursor protein in dorsal root ganglion neurons

The amyloid precursor protein (APP) is a transmembrane protein whose abnormal processing is associated with the pathogenesis of Alzheimer's disease. In this study, we examined the expression and role of cell-associated APP in primary dorsal root ganglion (DRG) neurons. When dissociated DRG cells prepared from mouse embryos were treated with nerve growth factor (NGF), neuronal APP levels were transiently elevated. DRG neurons treated with an antibody against cell surface APP failed to mature and underwent apoptosis. When NGF was withdrawn from the cultures after a 36-h NGF treatment, virtually all neurons underwent apoptosis by 48 h. During the course of apoptosis, some neurons with intact morphology contained increased levels of APP immunoreactivity, whereas the APP levels were greatly reduced in apoptotic neurons. Furthermore, affected neurons contained immunoreactivities for activated caspase-3, a caspase-cleaved APP fragment (APPDeltaC31), and Abeta. Downregulation of endogenous APP expression by treatment with an APP antisense oligodeoxynucleotide significantly increased the number of apoptotic neurons in NGF-deprived DRG cultures. Furthermore, overexpression of APP by adenovirus vector-mediated gene transfer reduced the number of apoptotic neurons deprived of NGF. These results suggest that endogenous APP is upregulated to exert an antiapoptotic effect on neurotrophin-deprived DRG neurons and subsequently undergoes caspase-dependent proteolysis.     

Functional properties and toxin pharmacology of a dorsal root ganglion sodium channel viewed through its voltage sensors

The voltage-activated sodium (Nav) channel Nav1.9 is expressed in dorsal root ganglion (DRG) neurons where it is believed to play an important role in nociception. Progress in revealing the functional properties and pharmacological sensitivities of this non-canonical Nav channel has been slow because attempts to express this channel in a heterologous expression system have been unsuccessful. Here, we use a protein engineering approach to dissect the contributions of the four Nav1.9 voltage sensors to channel function and pharmacology. We define individual S3b-S4 paddle motifs within each voltage sensor, and show that they can sense changes in membrane voltage and drive voltage sensor activation when transplanted into voltage-activated potassium channels. We also find that the paddle motifs in Nav1.9 are targeted by animal toxins, and that these toxins alter Nav1.9-mediated currents in DRG neurons. Our results demonstrate that slowly activating and inactivating Nav1.9 channels have functional and pharmacological properties in common with canonical Nav channels, but also show distinctive pharmacological sensitivities that can potentially be exploited for developing novel treatments for pain.     

PDZ binding to the BAR domain of PICK1 is elucidated by coarse-grained molecular dynamics

A key regulator of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor traffic, PICK1 is known to interact with over 40 other proteins, including receptors, transporters and ionic channels, and to be active mostly as a homodimer. The current lack of a complete PICK1 structure determined at atomic resolution hinders the elucidation of its functional mechanisms. Here, we identify interactions between the component PDZ and BAR domains of PICK1 by calculating possible binding sites for the PDZ domain of PICK1 (PICK1-PDZ) to the homology-modeled, crescent-shaped dimer of the PICK1-BAR domain using multiplexed replica-exchange molecular dynamics (MREMD) and canonical molecular dynamics simulations with the coarse-grained UNRES force field. The MREMD results show that the preferred binding site for the single PDZ domain is the concave cavity of the BAR dimer. A second possible binding site is near the N-terminus of the BAR domain that is linked directly to the PDZ domain. Subsequent short canonical molecular dynamics simulations used to determine how the PICK1-PDZ domain moves to the preferred binding site on the BAR domain of PICK1 revealed that initial hydrophobic interactions drive the progress of the simulated binding. Thus, the concave face of the BAR dimer accommodates the PDZ domain first by weak hydrophobic interactions and then the PDZ domain slides to the center of the concave face, where more favorable hydrophobic interactions take over.     

Clustering and synaptic targeting of PICK1 requires direct interaction between the PDZ domain and lipid membranes

Protein interacting with c kinase 1 (PICK1) regulates the trafficking of receptors and ion-channels such as AMPA receptors. Traditionally, the PICK1 PDZ domain is regarded as an adaptor capable of binding to receptors trafficked by PICK1, and the lipid-binding BAR domain functions to tether PICK1 directly to membranes. Here, we show that the PICK1 PDZ domain can directly interact with lipid membranes. The PDZ domain and lipid membrane interaction is mediated by both a polybasic amino-acid cluster and a conserved 'Cys-Pro-Cys' motif located away from the peptide ligand-binding groove. Disruption of the PDZ and lipid membrane interaction totally abolished synaptic targeting of PICK1. Although mutation of the CPC motif did not affect the interaction between PICK1 and AMPA receptors, the mutant PICK1 was unable to cluster the GluR2 subunit of the receptor. In neurons, PICK1 containing the same mutation displayed dramatically compromised capacity in the trafficking of AMPA receptors. Taken together, our findings not only uncovered the novel lipid membrane-binding property of the PICK1 PDZ domain, but also provided direct evidence supporting the functional relevance of the PDZ-lipid interaction.     

Effect of capsaicin and resiniferatoxin on peptidergic neurons in cultured dorsal root ganglion.

The neurotoxic effect of capsaicin has been shown to be selective on a subpopulation of small dorsal root ganglion neurons in newborn animals. The aim of this study was to provide evidence of the long lasting effect of capsaicin and its ultrapotent analog resiniferatoxin (RTX) on sensory peptidergic neurons maintained in organotypic cultures. The effects of the two irritants were examined on neurons that contained substance P (SP) and calcitonin gene-related peptide (CGRP). Exposure of the cultures to 10 microM capsaicin and 100 nM RTX for periods of 2 days or longer resulted in almost complete elimination of SP-immunoreactive (IR) neurites and reduction, but not elimination, of CGRP-IR neurites. In addition, both 10 microM capsaicin and 100 nM RTX significantly reduced the number of SP- and CGRP-IR cell bodies within DRG explants. Capsaicin in 100 microM concentration produced complete elimination of SP-IR fibers and a greater decrease in the number of CGRP-IR fibers, but failed to completely eliminate IR cell bodies. Exposure of the cultures to the irritants in the same concentrations for 90 min did not produce a measurable effect on SP- or CGRP-IR in neurites or cell bodies. It is important to establish that the effect of capsaicin and RTX on cultured neurons was of long duration (longer than 4 days) and is therefore different from depletion of peptides. These findings demonstrate that processes of cultured sensory neurons are much more sensitive to capsaicin and RTX than cell bodies. Furthermore, our results show that SP-IR neuronal elements are more sensitive to capsaicin than CGRP-IR elements. These data suggest that cultured sensory neurons express the functional properties of differentiated sensory neurons in vivo.