The substituted aspartate analogue l-β-threo-benzyl-aspartate preferentially inhibits the neuronal excitatory amino acid transporter EAAT3

The excitatory amino acid transporters (EAATs) play key roles in the regulation of CNS l-glutamate, especially related to synthesis, signal termination, synaptic spillover, and excitotoxic protection. Inhibitors available to delineate EAAT pharmacology and function are essentially limited to those that non-selectively block all EAATs or those that exhibit a substantial preference for EAAT2. Thus, it is difficult to selectively study the other subtypes, particularly EAAT1 and EAAT3. Structure activity studies on a series of β-substituted aspartate analogues identify l-β-benzyl-aspartate (l-β-BA) as among the first blockers that potently and preferentially inhibits the neuronal EAAT3 subtype. Kinetic analysis of d-[3H]aspartate uptake into C17.2 cells expressing the hEAATs demonstrate that l-β-threo-BA is the more potent diastereomer, acts competitively, and exhibits a 10-fold preference for EAAT3 compared to EAAT1 and EAAT2. Electrophysiological recordings of EAAT-mediated currents in Xenopus oocytes identify l-β-BA as a non-substrate inhibitor. Analyzing l-β-threo-BA within the context of a novel EAAT2 pharmacophore model suggests: (1) a highly conserved positioning of the electrostatic carboxyl and amino groups; (2) nearby regions that accommodate select structural modifications (cyclopropyl rings, methyl groups, oxygen atoms); and (3) a unique region l-β-threo-BA occupied by the benzyl moieties of l-TBOA, l-β-threo-BA and related analogues. It is plausible that the preference of l-β-threo-BA and l-TBOA for EAAT3 and EAAT2, respectively, could reside in the latter two pharmacophore regions.