Identification of key residues that cause differential gallbladder response to PACAP and VIP in the guinea pig

Abstract

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) have opposite actions on the gallbladder; PACAP induces contraction, whereas VIP induces relaxation. Here, we have attempted to identify key residues responsible for their interactions with PACAP (PAC 1) and VIP (VPAC) receptors in the guinea pig gallbladder. We synthesized PACAP-27/VIP hybrid peptides and compared their actions on isolated guinea pig gallbladder smooth muscle strips using isotonic transducers. ]VIP, induced relaxation. In the presence of a PAC1 receptor antagonist, PACAP(6 -38), the contractile response to PACAP-27 was inhibited and relaxation became evident. RT-PCR analysis revealed abundant expressions of PAC1 receptor, "hop" splice variant, and VPAC1 and VPAC2 receptor mRNAs in the guinea pig gallbladder. In conclusion, PACAP-27 induces contraction of the gallbladder via PAC1/hop receptors. Gly 4 and Ile 5 are the key NH2-terminal residues of PACAP-27 that distinguish PAC1/hop receptors from VPAC1/VPAC2 receptors. However, both the NH2-terminal and ␣-helical regions of PACAP-27 are required for initiating gallbladder contraction. PAC1 receptor; VPAC1 receptor; VPAC2 receptor; splice variant PITUITARY ADENYLATE cyclase-activating polypeptide (PACAP) is a member of the secretin/glucagon/vasoactive intestinal polypeptide (VIP) family of peptides (2, 8). It occurs in two bioactive molecules (PACAP-38 and PACAP-27) with identical NH 2 -terminal sequences. PACAP-27 has a 68% sequence homology to VIP, and all are expressed in the central as well as peripheral and enteric nervous systems (2, 22). They are released from nerve terminals as neurotransmitters or neuromodulators and regulate the function of the brain and peripheral organs. PACAP exhibits protean biological effects on the gastrointestinal tract, including motility, secretion, and blood flow (6). PACAP and VIP are coexpressed in nerve fibers and neurons in the ganglia of the guinea pig gallbladder (7). Although both peptides act primarily as inhibitory transmitters on most gastrointestinal and vascular smooth muscle cells (5,6,12), their actions on the gallbladder are opposite; VIP relaxes the gallbladder, whereas PACAP induces the contraction both in vivo (9) and in vitro (15,23,24).Three receptor subtypes that recognize PACAP and VIP have been identified (4,22), and all belong to the group of seven transmembrane G protein-coupled receptors. The PACAP-specific (PAC 1 ) receptor has a much higher affinity for PACAP than VIP, whereas the classical VIP (VPAC 1 ) receptor and VPAC 2 receptor exhibit similar affinities for PACAP and VIP. VPAC 1 and VPAC 2 receptors lead to activation of the adenylate cyclase/cAMP pathway in which elevation of intracellular cAMP, together with nitric oxide, me-

Targeting VIP and PACAP Receptor Signaling: New Insights into Designing Drugs for the PACAP Subfamily of Receptors

“…Due to steric hindrance, the additional methyl group in alanine and the different steric profile of the branched alkyl side of valine compared with glycine and isoleucine, respectively, may restrict the binding of VIP to PAC1R, which may contribute to the selectivity of VIP for the VPAC receptors [ 318 , 319 ]. The substitution of PACAP residues Gly 4P and Ile 5P with Ala 4P and Val 5P of VIP reduced PACAP affinity for PAC1R when tested in guinea pigs and the rat brain [ 320 , 321 ], suggesting these residues are important for receptor selectivity.…”