Drug Repositioning For Allosteric Modulation of VIP and PACAP Receptors.

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two neuropeptides that contribute to the regulation of intestinal motility and secretion, exocrine and endocrine secretions, and homeostasis of the immune system. Their biological effects are mediated by three receptors named VPAC1, VPAC2 and PAC1 that belong to class B GPCRs. VIP and PACAP receptors have been identified as potential therapeutic targets for the treatment of chronic inflammation, neurodegenerative diseases and cancer. However, pharmacological use of endogenous ligands for these receptors is limited by their lack of specificity (PACAP binds with high affinity to VPAC1, VPAC2 and PAC1 receptors while VIP recognizes both VPAC1 and VPAC2 receptors), their poor oral bioavailability (VIP and PACAP are 27- to 38-amino acid peptides) and their short half-life. Therefore, the development of non-peptidic small molecules or specific stabilized peptidic ligands is of high interest. Structural similarities between VIP and PACAP receptors are major causes of difficulties in the design of efficient and selective compounds that could be used as therapeutics. In this study we performed structure-based virtual screening against the subset of the ZINC15 drug library. This drug repositioning screen provided new applications for a known drug: ticagrelor, a P2Y12 purinergic receptor antagonist. Ticagrelor inhibits both VPAC1 and VPAC2 receptors which was confirmed in VIP-binding and calcium mobilization assays. A following analysis of detailed ticagrelor binding modes to all three VIP and PACAP receptors with molecular dynamics revealed its allosteric mechanism of action. Using a validated homology model of inactive VPAC1 and a recently released cryo-EM structure of active VPAC1 we described how ticagrelor could block conformational changes in the region of 'tyrosine toggle switch' required for the receptor activation. We also discuss possible modifications of ticagrelor comparing other P2Y12 antagonist - cangrelor, closely related to ticagrelor but not active for VPAC1/VPAC2. This comparison with inactive cangrelor could lead to further improvement of the ticagrelor activity and selectivity for VIP and PACAP receptor sub-types.

Excitatory actions of vasoactive intestinal peptide on mouse thalamocortical neurons are mediated by VPAC2 receptors.

Thalamic nuclei can generate intrathalamic rhythms similar to those observed at various arousal levels and pathophysiological conditions such as absence epilepsy. These rhythmic activities can be altered by a variety of neuromodulators that arise from brain stem regions as well as those that are intrinsic to the thalamic circuitry. Vasoactive intestinal peptide (VIP) is a neuropeptide localized within the thalamus and strongly attenuates intrathalamic rhythms via an unidentified receptor subtype. We have used transgenic mice lacking a specific VIP receptor, VPAC(2), to identify its role in VIP-mediated actions in the thalamus. VIP strongly attenuated both the slow, 2-4 Hz and spindle-like 5-8 Hz rhythmic activities in slices from wild-type mice (VPAC(2)(+/+)) but not in slices from VPAC(2) receptor knock-out mice (VPAC(2)(-/-)), which suggests a major role of VPAC(2) receptors in the antioscillatory actions of VIP. Intracellular recordings revealed that VIP depolarized all relay neurons tested from VPAC(2)(+/+) mice. In VPAC(2)(-/-) mice, however, VIP produced no membrane depolarization in 80% of neurons tested. In relay neurons from VPAC(2)+/+ mice, VIP enhanced the hyperpolarization-activated mixed cation current, I(h), via cyclic AMP activity, but VIP did not alter I(h) in VPAC(2)-/- mice. In VPAC(2)-/- mice, pituitary adenylate cyclase activating-polypeptide (PACAP) depolarized the majority of relay neurons via I(h) enhancement presumably via PAC(1) receptor activation. Our findings suggest that VIP-mediated actions are predominantly mediated by VPAC(2) receptors, but PAC(1) receptors may play a minor role. The excitatory actions of VIP and PACAP suggest these peptides may not only regulate intrathalamic rhythmic activities, but also may influence information transfer through thalamocortical circuits.