Divergent Pharmacological Activity of Novel Marine-Derived Excitatory Amino Acids on Glutamate Receptors

Abstract

Kainate receptors show a particular affinity for a variety of natural source compounds, including dysiherbaine (DH), a potent agonist derived from the marine sponge Dysidea herbacea. In this study, we characterized the pharmacological activity and structural basis for subunit selectivity of neodysiherbaine (neoDH) and MSVIII-19, which are natural and synthetic analogs of DH, respectively. NeoDH and MSVIII-19 differ from DH in the composition of two functional groups that confer specificity and selectivity for ionotropic glutamate receptors. In radioligand binding assays, neoDH displayed a 15-to 25-fold lower affinity relative to that of DH for glutamate receptor (GluR)5 and GluR6 kainate receptor subunits but a 7-fold higher affinity for kainate (KA)2 subunits, whereas MSVIII-19 displaced [ 3 H]kainate only from GluR5 subunits but not GluR6 or KA2 subunits. NeoDH was an agonist for kainate and ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in patch-clamp recordings; in contrast, MSVIII-19 acted as a potent antagonist for homomeric GluR5 receptor currents with weaker activity on other kainate and AMPA receptors. Neither neoDH nor MSVIII-19 activated group I metabotropic GluRs. Homology modeling suggests that two critical amino acids confer the high degree of selectivity between the dysiherbaine analogs and the GluR5 and KA2 subunits. In summary, these data describe the pharmacological activity of two new compounds, one of which is a selective GluR5 receptor antagonist that will be of use for understanding native receptor function and designing more selective ligands for kainate receptors.

Novel Analogs and Stereoisomers of the Marine Toxin Neodysiherbaine with Specificity for Kainate Receptors

“…2A). The binding affinity of 8-deoxy-neoDH for GluR5-2a subunits (K i ϭ 1.5 nM; n ϭ 3-5) was higher than that of the parent compound neoDH (K i ϭ 7.7 nM; Sanders et al, 2005). Because the subunit isoform of GluR5 that is predominantly expressed in the brain is GluR5-2b, we repeated radioligand binding assays with 8-deoxy-neoDH on GluR5-2b subunits, and we found no substantial change in affinity [K i ϭ 2.0 nM (n ϭ 3-5) versus 1.5 nM for GluR5-2a subunits].…”

“…1). Group 1 analogs lack the hydroxyl group additions at the C8 and C9 ring positions that were previously identified as important determinants of pharmacological activity and selectivity ; therefore, they represent intermediates between the natural dihydroxyl high-affinity agonist neoDH and the dideoxy synthetic analog MSVIII-19, which acts as a selective GluR5 antagonist (Sanders et al, 2005). The other groups consist of stereoisomeric analogs designed to test the importance of the spatial orientation of the C8 and C9 hydroxyl moieties (group 2) and the C2 and C4 carbons within the L-glutamate congener of the parent molecule (group 3) Fig.…”

“…Weak binding to other KAR and AMPAR subunits was observed, with Table 1. For comparison, the K i values for neoDH and MSVIII-19 are included in the table (Sanders et al, 2005). Removal of the C9 hydroxyl eliminated binding to all receptor subunits (K i values estimated at Ͼ100 M), with the exception of GluR5-2a KARs.…”

“…Membrane preparations from HEK-293-T/17 cells were prepared and used in radioligand binding assays as jpet.aspetjournals.org described previously (Sanders et al, 2005 …”

“…DH shares a glutamate congener with other KAR agonists such as kainate and domoate, but it is distinct in that it contains a tetra-substituted hydrofuropyran ring system with two functional groups, at the C8 and C9 positions, that largely control selectivity for AMPA and kainate receptors (Sasaki et al, 1999;Sakai et al, 2001a). Further characterization of a natural analog of DH, neodysiherbaine (neoDH), and a C8/C9 dideoxy synthetic analog of DH, MSVIII-19, revealed that slight structural modifications cause significant changes in the pharmacological activity, including generation of a functional antagonist for GluR5-containing receptors in MSVIII-19 (Sasaki et al, 1999;Sakai et al, 2001a;Sanders et al, 2005).…”