Design and synthesis of cyclopropane-based conformationally restricted GABA analogues as selective inhibitors for betaine/GABA transporter 1

Helix-forming aliphatic homo-δ-peptide foldamers based on the conformational restriction effects of cyclopropane

Considerable effort has been directed toward developing artificial peptide-based foldamers. However, detailed structural analysis of δ-peptide foldamers consisting of only aliphatic δ-amino acids has not been reported. Herein, we rationally designed and stereoselectively synthesized aliphatic homo-δ-peptides forming a stable helical structure by using a chiral cyclopropane δ-amino acid as a monomer unit. Structural analysis of the homo-δ-peptides using circular dichroism, infrared, and NMR spectroscopy indicated that they form a stable 14-helical structure in solution. Furthermore, we successfully conducted X-ray crystallographic analysis of the homo-δ-peptides, demonstrating a right-handed 14-helical structure. This helical structure of the crystal was consistent with those predicted by theoretical calculations and those obtained based on NMR spectroscopy in solution. This stable helical structure is due to the effective restriction of the backbone conformation by the structural characteristics of cyclopropane. This work reports the first example of aliphatic homo-δ-peptide foldamers having a stable helical structure both in the solution and crystal states.

Cinnamaldehyde Regulates the Generation of γ-aminobutyric Acid to Exert Sedation via Irreversible Inhibition of ENO1 in the Cerebellar Granular Layer

SCOPE

Glutamate (Glu) and γ-aminobutyric acid (GABA) are the major excitatory and inhibitory neurotransmitters that control information flow in the brain. GABA dysfunction is a general vulnerability factor for mental illness. Cinnamaldehyde (CA) is found to have sedation in a mental illness model. However, the specific targets and molecular mechanisms related to the sedative effects of CA have not been elucidated.

METHODS AND RESULTS

Metabolomics analysis and target fishing showed CA could increase the expression of GABA in vivo, and α-enolase (ENO1) is the primary target protein of CA associated with sedation. CA mainly binds with ENO1 in the cerebellar granular layer of brain, which influences the first transformations of the input signals arriving in the cerebellar cortex. The α,β-unsaturated aldehyde group of CA blocks the hydroxy group of Ser40, which induces a loss in ENO1 activation. CA also disturbs the glycolysis pathway and influences the tricarboxylic acid cycle and oxidative phosphorylation, which activate gluconeogenesis to provide energy to the brain. This mechanism is verified in zebrafish with ENO1 or glutamic acid decarboxylase (GAD) deficiency.

CONCLUSIONS

CA demonstrates sedation and alleviates GABA dysfunction via covalent binding ENO1, which shows the potential to improve the therapy of mental illness.

Molecular Determinants and Pharmacological Analysis for a Class of Competitive Non-transported Bicyclic Inhibitors of the Betaine/GABA Transporter BGT1

The betaine/GABA transporter 1 (BGT1) is a member of the GABA transporter (GAT) family with still elusive function, largely due to a lack of potent and selective tool compounds. Based on modeling, we here present the design, synthesis and pharmacological evaluation of five novel conformationally restricted cyclic GABA analogs related to the previously reported highly potent and selective BGT1 inhibitor (1S,2S,5R)-5-aminobicyclo[3.1.0]hexane-2-carboxylic acid (bicyclo-GABA). Using [³H]GABA radioligand uptake assays at the four human GATs recombinantly expressed in mammalian cell lines, we identified bicyclo-GABA and its N-methylated analog (2) as the most potent and selective BGT1 inhibitors. Additional pharmacological characterization in a fluorescence-based membrane potential assay showed that bicyclo-GABA and 2 are competitive inhibitors, not substrates, at BGT1, which was validated by a Schild analysis for bicyclo-GABA (pK_B value of 6.4). To further elaborate on the selectivity profile both compounds were tested at recombinant α₁β₂γ₂ GABA_A receptors. Whereas bicyclo-GABA showed low micromolar agonistic activity, the N-methylated 2 was completely devoid of activity at GABA_A receptors. To further reveal the binding mode of bicyclo-GABA and 2 binding hypotheses of the compounds were obtained from in silico-guided mutagenesis studies followed by pharmacological evaluation at selected BGT1 mutants. This identified the non-conserved BGT1 residues Q299 and E52 as the molecular determinants driving BGT1 activity and selectivity. The binding mode of bicyclo-GABA was further validated by the introduction of activity into the corresponding GAT3 mutant L314Q (38 times potency increase cf. wildtype). Altogether, our data reveal the molecular determinants for the activity of bicyclic GABA analogs, that despite their small size act as competitive inhibitors of BGT1. These compounds may serve as valuable tools to selectively and potently target BGT1 in order to decipher its elusive pharmacological role in the brain and periphery such as the liver and kidneys.

Structural and molecular aspects of betaine-GABA transporter 1 (BGT1) and its relation to brain function

ɣ-aminobutyric-acid (GABA) functions as the principal inhibitory neurotransmitter in the central nervous system. Imbalances in GABAergic neurotransmission are involved in the pathophysiology of various neurological diseases such as epilepsy, Alzheimer's disease and stroke. GABA transporters (GATs) facilitate the termination of GABAergic signaling by transporting GABA together with sodium and chloride from the synaptic cleft into presynaptic neurons and surrounding glial cells. Four different GATs have been identified that all belong to the solute carrier 6 (SLC6) transporter family: GAT1-3 (SLC6A1, SLC6A13, SLC6A11) and betaine/GABA transporter 1 (BGT1, SLC6A12). BGT1 has emerged as an interesting target for treating epilepsy due to animal studies that reported anticonvulsant effects for the GAT1/BGT1 selective inhibitor EF1502 and the BGT1 selective inhibitor RPC-425. However, the precise involvement of BGT1 in epilepsy remains elusive because of its controversial expression levels in the brain and the lack of highly selective and potent tool compounds. This review gathers the current structural and functional knowledge on BGT1 with emphasis on brain relevance, discusses all available compounds, and tries to shed light on the molecular determinants driving BGT1 selectivity. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.