Conantokin Explained

Conantokins are a small family of helical peptides that are derived from the venom of predatory marine snails of the genus Conus. Conantokins act as potent and specific antagonists of the N-methyl-D-aspartate receptor (NMDAR).[1] They are the only naturally-derived peptides to do so.[2] The subtypes of conantokins exhibit a surprising variability of selectivity across the NMDAR subunits, and are therefore uniquely useful in developing subunit-specific pharmacological probes.[3] [4] [5]

Chemically, conantokins are unique in that they possess a number (generally 4 or 5) of gamma-carboxyglutamyl (Gla) residues, generated by the post-translational modification of glutamyl (Glu) residues. These Gla residues induce a conformational change from a 3 10 helix to an alpha helix on binding to Calcium.[6] In the broader scheme of genetic conotoxin classification, Conanotokins are also known as "Conotoxin Superfamily B."[7]

The word "conantokin" is derived from the Filipino word antokin, meaning sleepy.[8]

Subtypes

Conantokin are in general named after the specific epithet of the Conus species it is found in, using single-letter abbreviations if possible. A conantokin from Conus radiatus is called Conantokin-R, but the latter-discovered ones from Conus rolani are called Canontokin-Rl. If a species makes multiple conantokins, numbers or letters are suffixed to the names. The abbreviation for "Conantokin" in these names is always "Con".

Conantokin-G

Also known as the “sleeper peptide”[9] or CGX-1007,[10] Con-G is a small peptide isolated from the fish-hunting snail, Conus geographus. It is the best-characterized conantokin, and acts as a functional inhibitor of NMDAR.[11]

Con-G shows potential as a neuroprotective agent in ischemic and excitotoxic brain injury, neuronal apoptosis, pain, epilepsy, and as a research tool in drug addiction and Alzheimer's disease.[11] [12] Con-G blocks NMDAR-mediated excitatory postsynaptic currents (EPSCs). Con-G reduces the strength of excitotoxic intracellular Ca2+ actions and blocks different neuronal injuries in vitro.[10] In certain injuries Con-G shows an exceptional prolongation of the therapeutic window.[10] Con-G can reverse established allodynia and can also fully reverse thermal hypersensitivity induced by nerve injury.[4]

Conantokin-T

Con-T is purified from the venom of the fish-hunting cone-snail, Conus tulipa. This peptide has 4 residues of Gla. Con-T acts by inhibiting NMDAR-mediated Ca2+ influx in neurons in the central nervous system.[8]

Conantokin-R and -L

Con-R is a highly potent anticonvulsant compound, derived from Conus radiatus.

Con-L is an efficient anticonvulsant compound, derived from Conus lynceus.[5] It differs from Con-R mainly in the C-terminal amino acids and, like Con-R, it induces sleep-like symptoms in young mice, with faster onset and for a longer duration.[5]

Con-L blocks NMDA-evoked currents in a powerful way, which is only slowly reversible upon washout, similar to Con-R and Con-G.[5]

Conantokin-Pr1, -Pr2 and –Pr3

Each peptide in this group is derived from the same species, Conus parius. Con-Pr3 has three different post-translational modifications. Con-Pr1 and –Pr2 adopt α-helical conformations in the presence of Mg2+ and Ca2+, but otherwise are generally unstructured. Conantokin-Pr3 always adopts an α-helical conformation.[12]

These peptides have highest potency for the NR2B subunits of the NMDAR.[12]

Conantokin-P and -E

Con-P and Con-E were isolated from the only two fish-hunting cone snails of the Americas (Conus purpurascens and Conus ermineus, respectively). Con-P differs from the other known conantokins in that it contains a long disulfide loop with two Gla residues. It is less helical (estimated 44% helical content), but unlike con-G, it does not require calcium for stability of this structure. Another notable distinction is the increased discrimination for NR2B. Con-E is very similar in structure to Con-P, and is likely to have a similar function.[1]

Conantokin-Rl-A

Con-Rl-A, derived from the venom of Conus rolani, is unique among the conantokins in having two distinct conformational states between which it equilibrates. Like Con-P and Con-E, its helical structure (estimated at 50%) does not depend on the presence or absence of calcium. This is likely due to the fact that two of the five Gla residues present in con-G are replaced in con-Rl-A by Lys. Con-R1-A discriminates more effectively than any other known ligand between the NR2B and NR2C subunits of NMDAR.[13]

Conantokin-Br or -S1

Con-Br (or Con-S1,) is isolated from Conus brettinghami (now Conus sulcatus), and is the only known conantokin with a high selectivity for the NR2D subunit of NMDAR.[14]

Synthetic derivatives

Con-G-based

Con-G[γ7A] Con-G[γ7K] and Con-G[S16Y] are synthetic Con-G peptides, where the Gla residue at position 7 is replaced with an alanine or a lysine residue, or the serine at position 16 is replaced with a tyrosine residue, respectively. Con-G[γ7A] is fourfold more potent than the native peptide, Con-G, while Con-G[γ7K] is as potent as Con-G.[3] The first two peptides appear to distinguish NMDAR subtypes in mid-frontal gyri from those in superior temporal gyri in human brain tissue. Both of them are being researched in relation to Alzheimer's disease (AD) and all three evoked 100% inhibition of spermine-enhanced [<sup>3</sup>H]MK-801 binding.[3] [15] Con-G[γ7K] and Con-G[S16Y] also show positive results in morphine withdrawal.[3]

Con-T-based

Con-T[K7γ] is a synthetic Con-T peptide, where the serine at position 7 is replaced with Gla residue. Like Con-G, it has higher affinity for Mg2+ than for Ca2+, but does not dimerize in the presence of Mg2+.[16]

Chemistry

Biochemically, conantokins have a distinctive high γ-carboxyglutamate content and low cysteine content. Conantokins typically lack disulfide bonds, in contrast to most families of conotoxins, which have an unusually high density of disulfide cross-links.

The inhibition of NMDAR-mediated spontaneous EPSCs (sEPSCs) and NMDA-gated currents in cortical neurons might be a result of actions on both diheteromeric (NR1/NR2B) and triheteromeric (NR1/NR2A/NR2B) NMDAR.

Mode of action

Con-G does not act directly at the glycine binding site.[11] [17] It can attenuate both the amplitude and the decay time constant of NMDA-mediated EPSCs[18] and significantly and reversibly affect other different properties of NMDAR-mediated sEPSCs in cultured neurons. The effect of Con-G on the frequency of the sEPSCs most likely relates to antagonizing the NMDAR.[11]

Target

Conantokins target NMDAR. Each subtype selectively targets different subunits of the receptor.

Toxicity

Some of these peptide effects are age-dependent, such as the induction of sleep-like state in young mice and hyperactive behavior in older mice.[3]

Intrathecal administration of doses greater than 300 pmol produced motor impairment in mice.[4]
Con-G, Con-R and Con-L cause behavioral toxicity at similar doses. Thus the difference in the C-terminal sequence might affect the anticonvulsant and behavioral toxicity profile.[5]

External links

Notes and References

  1. Gowd KH, Twede V, Watkins M, Krishnan KS, Teichert RW, Bulaj G, Olivera BM . Conantokin-P, an unusual conantokin with a long disulfide loop . Toxicon . 52 . 2 . 203–13 . August 2008 . 18586049 . 2630528 . 10.1016/j.toxicon.2008.04.178 .
  2. Mena EE, Gullak MF, Pagnozzi MJ, Richter KE, Rivier J, Cruz LJ, Olivera BM . Conantokin-G: a novel peptide antagonist to the N-methyl-D-aspartic acid (NMDA) receptor . Neuroscience Letters . 118 . 2 . 241–4 . October 1990 . 2177176 . 10.1016/0304-3940(90)90637-O . 32784480 .
  3. Wei J, Dong M, Xiao C, Jiang F, Castellino FJ, Prorok M, Dai Q . Conantokins and variants derived from cone snail venom inhibit naloxone-induced withdrawal jumping in morphine-dependent mice . Neuroscience Letters . 405 . 1–2 . 137–41 . September 2006 . 16859831 . 10.1016/j.neulet.2006.06.040 . 35973753 .
  4. Malmberg AB, Gilbert H, McCabe RT, Basbaum AI . Powerful antinociceptive effects of the cone snail venom-derived subtype-selective NMDA receptor antagonists conantokins G and T . Pain . 101 . 1–2 . 109–16 . January 2003 . 12507705 . 10.1016/S0304-3959(02)00303-2 . 25950992 .
  5. Jimenez EC, Donevan S, Walker C, Zhou LM, Nielsen J, Cruz LJ, Armstrong H, White HS, Olivera BM . Conantokin-L, a new NMDA receptor antagonist: determinants for anticonvulsant potency . Epilepsy Research . 51 . 1–2 . 73–80 . September 2002 . 12350383 . 10.1016/S0920-1211(02)00101-8 . 7960889 .
  6. Rigby AC, Baleja JD, Li L, Pedersen LG, Furie BC, Furie B . Role of gamma-carboxyglutamic acid in the calcium-induced structural transition of conantokin G, a conotoxin from the marine snail Conus geographus . Biochemistry . 36 . 50 . 15677–84 . December 1997 . 9398296 . 10.1021/bi9718550 .
  7. Robinson SD, Norton RS . Conotoxin gene superfamilies . Marine Drugs . 12 . 12 . 6058–101 . December 2014 . 25522317 . 4278219 . 10.3390/md12126058 . free .
  8. Haack JA, Rivier J, Parks TN, Mena EE, Cruz LJ, Olivera BM . Conantokin-T. A gamma-carboxyglutamate containing peptide with N-methyl-d-aspartate antagonist activity . The Journal of Biological Chemistry . 265 . 11 . 6025–9 . April 1990 . 10.1016/S0021-9258(19)39285-3 . 2180939 . free .
  9. Olivera BM, McIntosh JM, Clark C, Middlemas D, Gray WR, Cruz LJ . A sleep-inducing peptide from Conus geographus venom . Toxicon . 23 . 2 . 277–82 . 4024137 . 1985 . 10.1016/0041-0101(85)90150-3 .
  10. Williams AJ, Ling G, McCabe RT, Tortella FC . Intrathecal CGX-1007 is neuroprotective in a rat model of focal cerebral ischemia . NeuroReport . 13 . 6 . 821–4 . May 2002 . 11997694 . 10.1097/00001756-200205070-00017 . 30052416 .
  11. Alex AB, Baucum AJ, Wilcox KS . Effect of Conantokin G on NMDA receptor-mediated spontaneous EPSCs in cultured cortical neurons . Journal of Neurophysiology . 96 . 3 . 1084–92 . September 2006 . 16760339 . 10.1152/jn.01325.2005 .
  12. Teichert RW, Jimenez EC, Twede V, Watkins M, Hollmann M, Bulaj G, Olivera BM . Novel conantokins from Conus parius venom are specific antagonists of N-methyl-D-aspartate receptors . The Journal of Biological Chemistry . 282 . 51 . 36905–13 . December 2007 . 17962189 . 10.1074/jbc.M706611200 . free .
  13. Gowd KH, Watkins M, Twede VD, Bulaj GW, Olivera BM . Characterization of conantokin Rl-A: molecular phylogeny as structure/function study . Journal of Peptide Science . 16 . 8 . 375–82 . August 2010 . 20572027 . 4136950 . 10.1002/psc.1249 .
  14. Twede VD, Teichert RW, Walker CS, Gruszczyński P, Kaźmierkiewicz R, Bulaj G, Olivera BM . Conantokin-Br from Conus brettinghami and selectivity determinants for the NR2D subunit of the NMDA receptor . Biochemistry . 48 . 19 . 4063–73 . May 2009 . 19309162 . 3955384 . 10.1021/bi802259a .
  15. Ragnarsson L, Mortensen M, Dodd PR, Lewis RJ . Spermine modulation of the glutamate(NMDA) receptor is differentially responsive to conantokins in normal and Alzheimer's disease human cerebral cortex . Journal of Neurochemistry . 81 . 4 . 765–79 . May 2002 . 12065636 . 10.1046/j.1471-4159.2002.00872.x . free .
  16. Cnudde SE, Prorok M, Castellino FJ, Geiger JH. (June 2010) “Metal ion determinants of conantokin dimerization as revealed in the X-ray crystallographic structure of the Cd(2+)/Mg (2+)-con-T[K7gamma] complex.” J Biol Inorg Chem.15(5):667-75. Cnudde SE, Prorok M, Castellino FJ, Geiger JH . Metal ion determinants of conantokin dimerization as revealed in the X-ray crystallographic structure of the Cd(2+)/Mg (2+)-con-T[K7gamma] complex . Journal of Biological Inorganic Chemistry . 15 . 5 . 667–75 . June 2010 . 20195692 . 3693470 . 10.1007/s00775-010-0633-2 .
  17. Donevan SD, McCabe RT . Conantokin G is an NR2B-selective competitive antagonist of N-methyl-D-aspartate receptors . Molecular Pharmacology . 58 . 3 . 614–23 . September 2000 . 10953056 . 10.1124/mol.58.3.614 .
  18. Huang L, Balsara RD, Sheng Z, Castellino FJ . Conantokins inhibit NMDAR-dependent calcium influx in developing rat hippocampal neurons in primary culture with resulting effects on CREB phosphorylation . Molecular and Cellular Neurosciences . 45 . 2 . 163–72 . October 2010 . 20600930 . 2923249 . 10.1016/j.mcn.2010.06.007 .