Inhibition of NMDA-induced currents by conantokin-G and conantokin-T in cultured embryonic murine hippocampal neurons

Peptide antagonists of NMDA receptors: Structure-activity relationships for potential therapeutics

The N-methyl-D-aspartate (NMDA) receptors are heteromeric cation channels involved in memory, learning, and synaptic plasticity. The dysfunction associated with NMDA receptors results in neurodegenerative conditions. The conantokins comprise a family of Conus venom peptides that induce sleep upon intracranial injection into young mice and are known to be NMDA receptor antagonists. This work comprehensibly documents the conantokins that have been characterized to date, focusing on the biochemistry, solution structures in the presence or absence of divalent cations, functions as selective NMDA receptor antagonists, and structure-activity relationships. Furthermore, the applications of conantokins as potential therapeutics for certain neurological conditions, including neuropathic pain, epilepsy, and ischaemia that are linked to NMDA receptor dysfunction are reviewed.

Conopeptide-Functionalized Nanoparticles Selectively Antagonize Extrasynaptic N-Methyl-d-aspartate Receptors and Protect Hippocampal Neurons from Excitotoxicity In Vitro

N-methyl-d-aspartate receptors (NMDARs) are ionotropic glutamate receptors controlling fundamental physiological processes in the central nervous system, such as learning and memory. Excessive activation of NMDARs causes excitotoxicity and results in neurodegeneration, which is observed in a number of pathological conditions. Because of their dichotomous role, therapeutic targeting of NMDAR is difficult. However, several lines of evidence suggest that excitotoxicity is predominantly linked to extrasynaptically located NMDARs. Here, we report on a nanoparticle-based strategy to inhibit extrasynaptic NMDARs exclusively and subtype selectively, while allowing synaptic NMDARs activity. We designed gold nanoparticles (AuNPs) carrying conopeptide derivatives conjugated on their poly(ethylene glycol) coating as allosteric NMDAR inhibitors and show that these nanoparticles antagonize exclusively extrasynaptic NMDAR-mediated currents in cultured hippocampal neurons. Additionally, we show that conopeptide-functionalized AuNPs are neuroprotective in an in vitro model of excitotoxicity. By using AuNPs carrying different allosteric inhibitors with distinct NMDAR subtype selectivity such as peptide conantokin-G or peptide conantokin-R, we suggest activation of extrasynaptic GluN2B-containing diheteromeric NMDARs as the main cause of excitotoxicity.

NMDA Receptors Contribute to Retrograde Synaptic Transmission from Ganglion Cell Photoreceptors to Dopaminergic Amacrine Cells

Recently, a line of evidence has demonstrated that the vertebrate retina possesses a novel retrograde signaling pathway. In this pathway, phototransduction is initiated by the photopigment melanopsin, which is expressed in a small population of retinal ganglion cells. These ganglion cell photoreceptors then signal to dopaminergic amacrine cells (DACs) through glutamatergic synapses, influencing visual light adaptation. We have previously demonstrated that in Mg²⁺-containing solution, α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptors mediate this glutamatergic transmission. Here, we demonstrate that removing extracellular Mg²⁺ enhances melanopsin-based DAC light responses at membrane potentials more negative than −40 mV. Melanopsin-based responses in Mg²⁺-free solution were profoundly suppressed by the selective N-methyl-D-aspartate (NMDA) receptor antagonist D-AP5. In addition, application of NMDA to the retina produced excitatory inward currents in DACs. These data strongly suggest that DACs express functional NMDA receptors. We further found that in the presence of Mg²⁺, D-AP5 reduced the peak amplitude of melanopsin-based DAC responses by ~70% when the cells were held at their resting membrane potential (−50 mV), indicating that NMDA receptors are likely to contribute to retrograde signal transmission to DACs under physiological conditions. Moreover, our data show that melanopsin-based NMDA-receptor-mediated responses in DACs are suppressed by antagonists specific to either the NR2A or NR2B receptor subtype. Immunohistochemical results show that NR2A and NR2B subunits are expressed on DAC somata and processes. These results suggest that DACs express functional NMDA receptors containing both NR2A and NR2B subunits. Collectively, our data reveal that, along with AMPA receptors, NR2A- and NR2B-containing NMDA receptors mediate retrograde signal transmission from ganglion cell photoreceptors to DACs.

Antagonistic action on NMDA/GluN2B mediated currents of two peptides that were conantokin-G structure-based designed

Background

The GluN2B subunit of the N-methyl-d-aspartate receptor (NMDAr) modulates many physiological processes including learning, memory, and pain. Excessive increase in NMDAr/GluN2B activity has been associated with various disorders such neuropathic pain and neuronal death following hypoxia. Thus there is an interest in identifying NMDAr antagonists that interact specifically with the GluN2B subunit. Recently based on structural analysis between the GluN2B subunit and conantokin-G, a toxin that interacts selectively with the GluN2B subunit, we designed various peptides that are predicted to act as NMDAr antagonists by interacting with the GluN2B subunit. In this study we tested this prediction for two of these peptides EAR16 and EAR18.

Results

The effects of EAR16 and EAR18 in NMDA-evoked currents were measured in cultured rat embryonic hippocampal neurons and in HEK-293 cells expressing recombinant NMDAr comprised of GluN1a–GluN2A or GluN1a–GluN2B subunits. In hippocampal neurons, EAR16 and EAR18 reduced the NMDA-evoked calcium currents in a dose-dependent and reversible manner with comparable IC50 (half maximal inhibitory concentration) values of 241 and 176 µM, respectively. At 500 µM, EAR16 blocked more strongly the NMDA-evoked currents mediated by the GluN1a–GluN2B (84%) than those mediated by the GluN1a–GluN2A (50%) subunits. At 500 µM, EAR18 blocked to a similar extent the NMDA-evoked currents mediated by the GluN1a–GluN2B (62%) and the GluN1a–GluN2A (55%) subunits.

Conclusions

The newly designed EAR16 and EAR18 peptides were shown to block in reversible manner NMDA-evoked currents, and EAR16 showed a stronger selectivity for GluN2B than for GluN2A.

Pharmacology of triheteromeric N-Methyl-D-Aspartate Receptors

The N-Methyl-D-Aspartate Receptors (NMDARs) are heteromeric cation channels involved in learning, memory, and synaptic plasticity, and their dysregulation leads to various neurodegenerative disorders. Recent evidence has shown that apart from the GluN1/GluN2A and GluN1/GluN2B diheteromeric ion channels, the NMDAR also exists as a GluN1/GluN2A/GluN2B triheteromeric channel that occupies the majority of the synaptic space. These GluN1/GluN2A/GluN2B triheteromers exhibit pharmacological and electrophysiological properties that are distinct from the GluN1/GluN2A and GluN1/GluN2B diheteromeric subtypes. However, these receptors have not been characterized with regards to their inhibition by conantokins, as well as their allosteric modulation by polyamines and extracellular protons. Here, we show that the GluN1/GluN2A/GluN2B triheteromeric channels showed less sensitivity to GluN2B-specific conantokin (con)-G and con-RlB, and subunit non-specific con-T, compared to the GluN2A-specific inhibitor TCN-201. Also, spermine modulation of GluN1/GluN2A/GluN2B triheteromers switched its nature from potentiation to inhibition in a pH dependent manner, and was 2.5-fold slower compared to the GluN1/GluN2B diheteromeric channels. Unraveling the distinctive functional attributes of the GluN1/GluN2A/GluN2B triheteromers is physiologically relevant since they form an integral part of the synapse, which will aid in understanding spermine/pH-dependent potentiation of these receptors in pathological settings.

Synthetic conantokin peptides potently inhibit N-methyl-D-aspartate receptor-mediated currents of retinal ganglion cells

Retinal ganglion cells (RGCs), which are the sole output neurons of the retina, express N-methyl-D-aspartate receptors (NMDARs), rendering these cells susceptible to glutamate excitotoxicity, with implications for loss of normal RGC excitatory responses in disorders such as glaucoma and diabetic retinopathy. Therefore, antagonists that inhibit NMDAR-mediated currents specifically by targeting the GluN2B component of the ion channel have the potential to serve as a basis for developing potential therapeutics. The roles of peptidic conantokins, which are potent brain neuronal NMDAR inhibitors, were studied. By using patch-clamp whole-cell analyses in dissociated RGCs and retinal whole-mount RGCs, we evaluated the effects of synthetic conantokin-G (conG) and conantokin-T (conT), which are small γ-carboxyglutamate-containing peptides, on NMDA-mediated excitatory responses in mouse RGCs. Both conG and conT inhibited the NMDA-mediated currents of dark-adapted dissociated and whole-mount RGCs in a dose-dependent, reversible, noncompetitive manner. Inhibition of NMDA-mediated steady-state currents by NMDAR nonsubunit-selective conT was approximately threefold greater than GluN2B-selective conG or ifenprodil, demonstrating its potential ability to inhibit both GluN2A- and GluN2B-containing ion channels in RGCs. Because the extent of inhibition of NMDA-evoked currents by conG and the pharmacologic GluN2B-selective inhibitor ifenprodil were similar (40-45%) to that of the GluN2A-selective antagonist NVP-AAM0077, we conclude that the levels of GluN2A and GluN2B subunits are similar in RGCs. These results provide a novel basis for developing effective neuroprotective agents to aid in the prevention of undesired glutamatergic excitotoxicity in neurodegenerative diseases of the retina and demonstrate functional assembly of NMDARs in RGCs.

[Conotoxins: from the biodiversity of gastropods to new drugs]

A review describes general trends in research of conotoxins that are peptide toxins isolated from sea gastropods of the Conus genus, since the toxins were discovered in 1970th. There are disclosed a conotoxin classification, their structure diversity and different ways of action to their molecular targets, mainly, ion channels. In the applied aspect of conotoxin research, drug discovery and development is discussed, the drugs being based on conotoxin structure. A first exemplary drug is a ziconotide, which is an analgesic of new generation.

Opposing action of conantokin-G on synaptically and extrasynaptically-activated NMDA receptors

Synaptic and extrasynaptic activation of the N-methyl-D-aspartate receptor (NMDAR) has distinct consequences on cell signaling and neuronal survival. Since conantokin (con)-G antagonism is NR2B-selective, which is the key subunit involved in extrasynaptic activation of the receptor, its ability to specifically elicit distinct signaling outcomes in neurons with synaptically or extrasynaptically-activated NMDARs was evaluated. Inhibition of Ca(2+) influx through extrasynaptic NMDAR ion channels was neuroprotective, as it effectively enhanced levels of activated extracellular signal-regulated kinase 1/2 (ERK1/2), activated cAMP response element binding protein (CREB), enhanced mitochondrial viability, and attenuated the actin disorganization observed by extrasynaptic activation of NMDARs. Conversely, the pro-signaling pathways stimulated by synaptically-induced Ca(2+) influx were abolished by con-G. Furthermore, subunit non-selective con-T was unable to successfully redress the impairments in neurons caused by extrasynaptically-activated NMDARs, thus indicating that NR2B-specific antagonists are beneficial for neuron survival. Neurons ablated for the NR2B subunit showed weak synaptic Ca(2+) influx, reduced sensitivity to MK-801 blockage, and diminished extrasynaptic current compared to WT and NR2A(-/-) neurons. This indicates that the NR2B subunit is an integral component of both synaptic and extrasynaptic NMDAR channels. Altogether, these data suggest that con-G specifically targets the NR2B subunit in the synaptic and extrasynaptic locations, resulting in the opposing action of con-G on differentially activated pools of NMDARs.

Conjugation of peptides to the passivation shell of gold nanoparticles for targeting of cell-surface receptors

We report the preparation of gold nanoparticles (AuNPs) functionalized with the peptide-toxin conantokin-G and their selective binding to N-methyl-d-aspartate (NMDA) receptors recombinantly expressed by transfected HEK 293 cells. The AuNPs are passivated with a mixed self-assembled monolayer of ω-carboxy- and ω-amino-polyethylene glycol (PEG) thiols. We compare two different passivation systems: the alkyl-PEG600 system is characterized by a C(11)-alkyl chain between the thiol group and the PEG segment, whereas the PEG3000 system lacks this alkyl-chain. We show that only the alkyl-PEG600 passivation system allows selective conjugation of cysteine-terminated peptides to the periphery of the passivation layer via a heterobifunctional linker strategy. In contrast, using the PEG3000 passivation system, peptides are immobilized both on the passivation layer and directly on the gold surface via concurrent place-exchange reaction. We therefore recommend the use of the alkyl-PEG600 system to precisely control the number of immobilized peptides on AuNPs. In fact, we show that the number of conjugated peptides per particle can be varied with good control simply by varying the composition of the self-assembled monolayer. Finally, we demonstrate that conjugation of the conantokin-G peptide to the solvent-exposed interface of the passivation layer results in maximal binding interaction between the peptide-functionalized AuNPs and the targeted NMDA receptors on the cell surface. Conantokin G-coupled AuNP may be used to spatially restrict NMDA-receptor-blockade on neuronal surfaces.

Conantokins inhibit NMDAR-dependent calcium influx in developing rat hippocampal neurons in primary culture with resulting effects on CREB phosphorylation

The effects of conantokin (con)-G, con-R[1-17], and con-T on ion flow through N-methyl-D-aspartate receptor (NMDAR) ion channels were determined in cultured primary rat hippocampal neurons. The potency of con-G diminished, whereas inhibition by con-R[1-17] and con-T did not change, as the neurons matured. Con-G, con-R[1-17], and con-T effectively diminished NMDA-induced Ca(2+) influx into the cells. A similar age-dependent decrease in con-G-mediated inhibition of the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) was observed, compared to con-R[1-17] and con-T. The effects of the conantokins on NMDA-induced cAMP response element-binding protein (CREB) phosphorylation in immature (DIV 9) and mature (DIV 16) neurons showed that, at DIV 9, con-G, con-R[1-17], and con-T inhibited NMDA-mediated P-CREB levels, whereas in DIV 16 neurons the conantokins did not inhibit overall levels of NMDA-induced P-CREB. In contrast, P-CREB levels were enhanced through inhibition of the protein phosphatases, PP1 and PP2B (calcineurin). This ability of conantokins to sustain CREB phosphorylation can thus enhance neuronal survival and plasticity.

Helix-Helix Interactions between Homo- and Heterodimeric γ-Carboxyglutamate-containing Conantokin Peptides and Their Derivatives

The conantokins are a family of small, naturally occurring gamma-carboxyglutamate (Gla)-rich peptides that specifically antagonize the N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptor. One member of this family, conantokin-G (con-G), undergoes Ca(2+)-mediated self-assembly to form an antiparallel helical dimer. Subunit interactions in this complex are incumbent upon intermolecular Ca(2+) bridging of Gla residues spaced at i, i + 4, i + 7, i + 11 intervals within the monomer. Herein, we further probe the molecular determinants governing such helix-helix interactions. Select variants were synthesized to evaluate the contributions of non-Gla residues to conantokin self-association. Con-G dimerization was shown to be exothermic and accompanied by positive heat capacity changes. Using positional Gla variants of conantokin-R (con-R), a non-dimerizing conantokin, i, i + 4, i + 7, i + 11 Gla spacing alone was shown to be insufficient for self-assembly. The Ca(2+)-dependent antiparallel heterodimerization of con-G and con-T(K7 gamma), two peptides that harbor optimal Gla spacing, was established. Last, the effects of covalently constrained con-G dipeptides on NMDA-evoked current in HEK293 cells expressing combinations of NR1a, NR1b, NR2A, and NR2B subunits of the NMDA receptor were investigated. The antiparallel dipeptide was unique in its ability to potentiate current at NR1a/2A receptors and, like monomeric con-G, was inhibitory at NR1a/2B and NR1b/2B combinations. In contrast, the parallel species was completely inactive at all subunit combinations tested. These results suggest that, under physiological Ca(2+) concentrations, equilibrium levels of con-G dimer most likely exist in an antiparallel orientation and exert effects on NMDA receptor activity that differ from the monomer.

Conotoxins down under

In the four decades since toxinologists in Australia and elsewhere started to investigate the active constituents of venomous cone snails, a wealth of information has emerged on the various classes of peptides and proteins that make their venoms such potent bioactive cocktails. This article provides an overview of the current state of knowledge of these venom constituents, several of which are of interest as potential human therapeutics as a consequence of their high potency and exquisite target specificity. With the promise of as many as 50,000 venom components across the entire Conus genus, many more interesting peptides can be anticipated.

Neuroprotection and peptide toxins

Neurodegeneration induced by excitatory neurotransmitter glutamate is considered to be of particular relevance in several types of acute and chronic neurological impairments ranging from cerebral ischaemia to neuropathological conditions such as motor neuron disease, Alzheimer's, Parkinson's disease and epilepsy. The hyperexcitation of glutamate receptors coupled with calcium overload can be prevented or modulated by using well-established competitive and non-competitive antagonists targeting ion/receptor channels. The exponentially increasing body of pharmacological evidence over the years indicates potential applications of peptide toxins, due to their exquisite subtype selectivity on ion channels and receptors, as lead structures for the development of drugs for the treatment of wide variety of neurological disorders. This review comprehensively highlights the overview of the diversity in the molecular as well as neurobiological mechanisms of different peptide toxins derived from venomous animals with particular reference to neuroprotection. In addition, the potential applications of peptide toxins in the diagnosis and treatment of neurological disorders such as neuromuscular disorders, epilepsy, Alzheimer's and Parkinson's diseases, gliomas and ischaemic stroke and their future prospects in the diagnosis as well as in the therapy are addressed.

Conus venoms: a rich source of novel ion channel-targeted peptides

The cone snails (genus Conus) are venomous marine molluscs that use small, structured peptide toxins (conotoxins) for prey capture, defense, and competitor deterrence. Each of the 500 Conus can express approximately 100 different conotoxins, with little overlap between species. An overwhelming majority of these peptides are probably targeted selectively to a specific ion channel. Because conotoxins discriminate between closely related subtypes of ion channels, they are widely used as pharmacological agents in ion channel research, and several have direct diagnostic and therapeutic potential. Large conotoxin families can comprise hundreds or thousands of different peptides; most families have a corresponding ion channel family target (i.e., omega-conotoxins and Ca channels, alpha-conotoxins and nicotinic receptors). Different conotoxin families may have different ligand binding sites on the same ion channel target (i.e., mu-conotoxins and delta-conotoxins to sites 1 and 6 of Na channels, respectively). The individual peptides in a conotoxin family are typically each selectively targeted to a diverse set of different molecular isoforms within the same ion channel family. This review focuses on the targeting specificity of conotoxins and their differential binding to different states of an ion channel.

Marine pharmacology in 1999: compounds with antibacterial, anticoagulant, antifungal, anthelmintic, anti-inflammatory, antiplatelet, antiprotozoal and antiviral activities affecting the cardiovascular, endocrine, immune and nervous systems, and other miscellaneous mechanisms of action

This review, a sequel to the 1998 review, classifies 63 peer-reviewed articles on the basis of the reported preclinical pharmacological properties of marine chemicals derived from a diverse group of marine animals, algae, fungi and bacteria. In all, 21 marine chemicals demonstrated anthelmintic, antibacterial, anticoagulant, antifungal, antimalarial, antiplatelet, antituberculosis or antiviral activities. An additional 23 compounds had significant effects on the cardiovascular, sympathomimetic or the nervous system, as well as possessed anti-inflammatory, immunosuppressant or fibrinolytic effects. Finally, 22 marine compounds were reported to act on a variety of molecular targets, and thus could potentially contribute to several pharmacological classes. Thus, during 1999 pharmacological research with marine chemicals continued to contribute potentially novel chemical leads in the ongoing global search for therapeutic agents for the treatment of multiple disease categories.

Kinetic and mechanistic characterization of NMDA receptor antagonism by replacement and truncation variants of the conantokin peptides

The characterization of conantokin-T (con-T), conantokin-R (con-R), and variants thereof, using the whole-cell patch clamp technique, was undertaken to evaluate the contribution of various residues towards the onset and recovery of N-methyl-D-aspartate (NMDA) receptor inhibition in cultured embryonic murine hippocampal neurons. The results obtained indicate that the two most C-terminal gamma-carboxyglutamic acid (Gla) residues of the conantokins, while not essential for activity, provided for more tenacious binding to the receptor. Specifically, con-T[gamma10K/gamma14K] and con-R[gamma11A/gamma15A] displayed 5.6- and 8.4-fold decreases in tau(off), respectively, compared to the parent peptides. For the truncated con-T variants, con-T[1-9/Q6G], and a sarcosine (Src)-containing species, con-T[1-9/G1Src/Q6G], the tau(off) was over 80- and 40-fold faster, respectively, compared to con-T. For the latter peptide, the coapplication of 300 microM spermine enhanced the onset rate constant from 3.1x10(3)M(-1) x s(-1) to 12.6x10(3)M(-1) x s(-1). From analysis of equilibrium dose-inhibition curves using the Cheng-Prusoff equation, a K(i) value of 1.1 microM for the peptide was obtained. Con-T[1-9/G1Src/Q6G] demonstrated an apparent competitive mode of inhibition relative to NMDA. Schild analysis of the data yielded an equilibrium dissociation constant of 2.4 microM for the interaction of con-T[1-9/G1Src/Q6G] with the receptor.

Point mutations identify the glutamate binding pocket of the N-methyl-D-aspartate receptor as major site of conantokin-G inhibition

Conantokin-G (Con-G), a gamma-carboxylglutamate (Gla) containing peptide derived from the venom of the marine cone snail Conus geographus, acts as a selective and potent inhibitor of N-methyl-D-aspartate (NMDA) receptors. Here, the effect of Con-G on recombinant NMDA receptors carrying point mutations within the glycine and glutamate binding pockets of the NR1 and NR2B subunits was studied using whole-cell voltage-clamp recording from cRNA injected Xenopus oocytes. At wild-type receptors, glutamate-induced currents were inhibited by Con-G in a dose-dependent manner at concentrations of 0.1-100 microM. Substitution of selected residues within the NR2B subunit reduced the inhibitory potency of Con-G, whereas similar mutations in the NR1 subunit had little effect. These results indicate a selective interaction of Con-G with the glutamate binding pocket of the NMDA receptor. Homology-based molecular modeling of the glutamate binding region based on the known structure of the glutamate binding site of the AMPA receptor protein GluR2 suggests how selected amino acid side chains of NR2B might interact with specific residues of Con-G.

The amino acid residue at sequence position 5 in the conantokin peptides partially governs subunit-selective antagonism of recombinant N-methyl-D-aspartate receptors

Whole cell voltage clamp recordings were performed to assess the ability of conantokin-G (con-G), conantokin-T (con-T), and a 17-residue truncated form of conantokin-R (con-R[1-17]) to inhibit N-methyl-d-aspartate (NMDA)-evoked currents in human embryonic kidney 293 cells transiently expressing various combinations of NR1a, NR1b, NR2A, and NR2B receptor subunits. Con-T and con-R[1-17] attenuated ion currents in cells expressing NR1a/NR2A or NR1a/NR2B. Con-G did not affect NMDA-evoked ionic currents in cells expressing NR1a/NR2A, but it showed inhibitory activity in cells expressing NR1a/NR2B receptors and the triheteromeric combination of NR1a/NR2A/NR2B. An Ala-rich con-G analog, con-G[Q6G/gamma7K/N8A/gamma10A/gamma14A/K15A/S16A/N17A] (Ala/con-G, where gamma is Gla), in which all nonessential amino acids were altered to Ala residues, manifested subunit specificity similar to that of con-G, suggesting that the replaced residues are not responsible for selectivity in the con-G framework. A sarcosine-containing con-T truncation analog, con-T[1-9/G1Src/Q6G], inhibited currents in NR1a/NR2A and NR1a/NR2B receptors, eliminating residues 10-21 as mediators of the broad subunit selectivity of con-T. In contrast to the null effects of con-G and Ala/con-G at a NR1a/NR2A-containing receptor, some inhibition ( approximately 40%) of NMDA-evoked currents was effected by these peptides in cells expressing NR1b/NR2A. This finding suggests that the presence of exon 5 in NR1b plays a role in the activity of the conantokins. Analysis of various conantokin analogs demonstrated that Leu(5) of con-G is an important determinant of conantokin selectivity. Taken as a whole, these results suggest that the important molecular determinants on conantokins responsible for NMDA receptor activity and specificity are discretely housed in specific residues of these peptides, thus allowing molecular manipulation of the NMDA receptor inhibitory properties of the conantokins.

Sequence requirements for the N-methyl-D-aspartate receptor antagonist activity of conantokin-R

Conantokin-R (con-R), a gamma-carboxyglutamate-containing 27-residue peptide, is a natural peptide inhibitor of the N-methyl-d-aspartate (NMDA) subtype glutamate receptor. Synthetic analogs of con-R were generated to evaluate the importance of the individual structural elements of this peptide in its NMDA receptor antagonist activity, measured by inhibition of the spermine-enhanced binding of the NMDA receptor-specific channel blocker, [(3)H]MK-801, to rat brain membranes. Progressive C-terminal truncations of the 27-residue peptide revealed stages of severe activity loss. These occurred at con-R[1-11] and con-R[1-7], corresponding to the deletions of Leu(12)-Pro(27) and Met(8)-Pro(27) respectively. A second set of analogs featured single Ala substitutions in the fully active con-R[1-17] fragment. The replacement of Met(8) and Leu(12) by Ala resulted in approximate 20- and 55-fold decreases of inhibitor potency, respectively. In addition to these two residues, the only other positions where a single Ala substitution led to substantial losses (from 11-fold to >1000-fold) of activity were those of the first five N-terminal amino acids. Based on the above findings, the binding epitope of con-R was localized to the N-terminal turn of the helix and other residues on one face along two subsequent turns. This contribution pattern of the side chains in activity closely resembles the results obtained with another member of this peptide family, conantokin-T. The secondary structure and metal ion binding properties of the con-R variants were also evaluated using circular dichroism spectroscopy. Divalent cation-dependent increases of alpha-helix content were observed in most analogs. However, analogs with replacement of Gla(11) and Gla(15), as well as truncation fragments shorter than 15 residues, lost the ability to be stabilized by metal ions. These results confirmed the location of the primary divalent cation binding locus at Gla(11) and Gla(15). Additional interactions were indicated by the reduced alpha-helix stability in the Ala analogs of Gla(4), Lys(7), and Arg(14).

Inhibition of MK801 binding in adult rat brain sections by conantokin-G and conantokin-T

The functional interactions of conantokins with anatomical sites in rat brain have been assessed through displacement of the non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, dizocilpine (MK801). The binding of (+)-3-[125I]-iodo-MK801 (1 nM) to coronal sections from adult rat brain was inhibited in a dose-dependent manner by conantokin-T (con-T) and conantokin-G (con-G). Quantitative densitometry was used to determine IC50 values for conantokin inhibition of [125I]-MK801 binding in the cortex, thalamus and hippocampus. Con-T completely inhibited [125I]-MK801 specific binding in all brain regions at a saturating concentration of 100 microM. Con-G was able to completely displace [125I]-MK801 in the cortex and thalamus, but only inhibited this same binding up to approximately 90% in the hippocampus. Both peptides maintained their inhibitory properties in the presence of 1 mM EDTA, suggesting that divalent cations are not required for their action in this regard. The added presence of spermine (150 microM) resulted in a two-fold increase in [125I]-MK801 binding and a two-fold decrease in the IC50 values for both peptides. The data obtained in this investigation further demonstrate that [125I]-MK801 is a useful probe for the indirect determination of functional NMDAR ligand binding sites in rat brain sections.