The synaptic vesicle protein SV2A is the binding site for the antiepileptic drug levetiracetam
Lynch BA, Lambeng N, Nocka K, Kensel-Hammes P, et al. ;
Commented by , 23 Aug 2004
Background
Levetiracetam differs from other antiepileptic drugs (AEDs) because it is inactive in classical animal screening models (ref. 1). Its action has been associated with stereoselective interaction with a unique binding site in the brain (ref. 2)
Aim
To identify the nature of the levetiracetam binding site in the brain.
Methods
Methods included photoaffinity labelling in purified synaptic vesicles, binding studies with expressed protein isoforms, and anti-seizure activity testing in the audiogenic seizure mouse model.
Results
The synaptic vesicle protein SVA2 was singled out as a candidate binding site for levetiracetam due to its distribution characteristics and molecular mass.
Levetiracetam and related compounds bound to synaptic vesicle protein SVA2 expressed in fibroblasts with affinities similar to those for levetiracetam sites in the brain.
Brain membranes and synaptic vesicles from SVA2 knock-out mice did not bind levetiracetam site ligands.
Affinities of levetiracetam analogues for SVA2 correlated with their anticonvulsant potency in audiogenic seizure mice.
Comment
To date, identified mechanisms of AED action included effects on ion channels and interference with GABAergic or glutamatergic transmission. Levetiracetam, however, does not fully fit any of these paradigms and its mode of action has been a mystery (ref. 3; ref. 4).
This study goes a long way towards solving the mistery and represents a potential landmark in CNS research by identifying a novel and hitherto little studied possible target for centrally active drugs.
The investigative work and state-of-the-art technology which led to identification of the levetiracetam binding site is fascinating. A cornerstone in this effort was a little mouse genetically engineered not to express SV2A (ref. 5). The synaptic vesicles of this mouse did not bind levetiracetam site ligands, thus confirming the identity of this site with SVA2.
The molecular action of SVA2 is unknown, though it has been linked to modulation of synaptic vesicle fusion through transport of a vesicle constituent, such as calcium or ATP (ref. 6). Among other evidences linking SV2A to synaptic function, hippocampal neurons from mice lacking SV2A and the related protein SV2B demonstrate altered activity-dependent synaptic depression (ref. 7).
The suggestion that anticonvulsant potency in audiogenic mice correlates with binding affinity to SV2A suggests that SV2A has a critical role in the activity of levetiracetam in epilepsy. However, this hypothesis needs to be confirmed. It would have been nice to show that SV2A knock-out mice are insensitive to the anticonvulsant effects of levetiracetam, but these experiments could not be conducted because these animals, which appear normal at birth, do not survive beyond 3 weeks. Interestingly, by 10 days of age these mice develop a strong seizure phenotype. It would be interesting to know whether these spontaneous seizures respond differentially to levetiracetam and other AEDs.
In conclusion, SV2A seems to be a major target in the anticonvulsant action of levetiracetam, though further experiments are required to confirm this and other actions cannot be excluded (ref. 8; ref. 9; ref. 10). These findings raise many interesting questions:
(i) What is the role of SV2A in regulating synaptic function under physiological and pathological conditions? (ii) How is SV2A function modified after binding with levetiracetam?
(ii) Do other drugs affect SV2A function, and how?
(iii) Could SV2A and other vesicle proteins in the presynaptic terminal be an important pharmacological target for agents active in therapeutic areas other than epilepsy?
References
1. Klitgaard H, Matagne A, Gobert J, Wulfert E. Evidence for a unique profile of levetiracetam in rodent models of seizures and epilepsy. Eur J Pharmacol. 1998;24;353:191-206
2. NoyerM, Gillard M, Matagne A, Henichart JP, Wulfert E . The novel antiepileptic drug levetiracetam (ucb L059) appears to act via a specific binding site in CNS membranes. Eur J Pharmacol. 1995; 286:137-46
3. Klitgaard, H. Keppra: the preclinical profile of a new class of antiepileptic drugs? Epilepsia 2001; 42 (Suppl. 4):13-18
4. Margineanu, D.G. and Klitgaard H. Levetiracetam: mechanisms of action. In: Levy RH, Matttson RH, Meldrum BS, Perucca E, Eds. Antiepileptic Drugs, Fifth Edition, Lippincott Williams & Wilkins, 2002:419-27
5. Crowder KM, Gunther JM, Jones TA, Hale BD, Zhang HZ, Peterson MR, Scheller RH, Chavkin C, Bajjalieh SM. Abnormal neurotransmission in mice lacking synaptic vesicle protein 2A (SV2A). Proc Natl Acad Sci USA 1999;96:15268-73 (Note: Free full text article)
6. Bajjalieh SM, Frantz GD, Weimann JM, McConnell SK, Scheller RH. Differential expression of synaptic vesicle protein 2 (SV2) isoforms. J Neurosci.1994;14:5223-5
7. Janz R, Goda Y, Geppert M, Missler M, Sudhof TC. SV2A and SV2B function as redundant Ca2+ regulators in neurotransmitter release. Neuron 1999; 24:1003-16
8. Madeja M, Margineanu DG, Gorji A, Siep E, Boerrigter P, Klitgaard H, Speckmann EJ. Reduction of voltage-operated potassium currents by levetiracetam: a novel antiepileptic mechanism of action? Neuropharmacology. 2003 ;45:661-71
9. Rigo JM, Hans G, Nguyen L, Rocher V, Belachew S, Malgrange B, Leprince P, Moonen G, Selak I, Matagne A, Klitgaard H. The anti-epileptic drug levetiracetam reverses the inhibition by negative allosteric modulators of neuronal GABA- and glycine-gated currents. Br J Pharmacol. 2002;136:659-72
10. Pisani A, Bonsi P, Martella G, De Persis C, Costa C, Pisani F, Bernardi G, Calabresi P. Intracellular calcium increase in epileptiform activity: modulation by levetiracetam and lamotrigine. Epilepsia. 2004;45:719-28