PACAP-Related Peptide (PRP), human

PACAP-related peptide (PRP)--molecular evolution and potential functions

PACAP-related peptide (PRP) and PACAP are structurally related peptides that are encoded in the same transcripts. In the past, it was believed that the mammalian PRPs are evolved from GHRHs in non-mammals. With the recent discovery of authentic GHRH and receptor genes in frog and fish, this review aims to (1) coin the name of all GHRH-like peptides in previous literature as PRPs and (2) provide the background for new research direction for PRP in vertebrates. As a goldfish receptor highly specific for PRP with distinct tissue distribution has previously been characterized, it is highly possible that PRP plays a physiological role in non-mammalian vertebrates and the function of PRP has somehow been lost in mammals as a consequence of the loss of its receptor in the genome. This information may provide clues to elucidate functions of PRP in the future.

Molecular cloning and mRNA distribution of pituitary adenylate cyclase-activating polypeptide (PACAP)/PACAP-related peptide in the lungfish

In this article, we report the isolation of a full-length cDNA clone encoding pituitary adenylate cyclase-activating polypeptide (PACAP)/PACAP-related peptide (PRP) from lungfish Protopterus dolloi. When comparing the deduced amino acid sequences, the lungfish PACAP was found to be highly conserved with other vertebrates; however, the PRP shares only lower levels of sequence identity with known PRP sequences. Consistently in phylogenetic analysis, the lungfish PRP, similar to sturgeon PRP, fails to cluster with other PRPs. In addition to the full-length clone, another cDNA encoding a short precursor that lacks the first 32 amino acids of the PRP was also isolated. Interestingly, similar isoforms were also identified in several nonmammalian vertebrates, and it was suggested that exon skipping of PRP/PACAP transcripts was a mechanism that regulated the expression ratio of PACAP to PRP in nonmammalian vertebrates. By real-time PCR, both long and short PRP/PACAP transcripts were found almost exclusively in the brain, and the short isoform is the more abundant transcript (3.7 times more), indicating that PACAP is the major product produced in lungfish brain. The expression patterns of lungfish and previously studied frog PRP/PACAP suggest that the PRP/PACAP gene in the tetrapod lineage may first express in the central nervous system; in the process of evolution, the functions of these peptides diversified and were later found in other tissues.

VIP and PACAP

Vasoactive intestinal polypeptide (VIP) is derived from a 170 amino acid precursor which in addition is processed to preproVIP 22-79, PHI, preproVIP 111-122 and preproVIP 156-170. All preproVIP-derived peptides have been shown in normal tissue and VIP-producing cell lines and elevated quantities occur in plasma and tumour tissues from patients with VIP-producing tumours. In some tissues the dibasic cleavage site after PHI is uncleaved resulting in a C-terminally extended form, PHV. PHI and VIP are present in a 1:1 molar ratio in large dense core vesicles and released in roughly equimolar amounts. Carboxyamidation of VIP and PHI is not critical and glycine-extended forms of both peptides have been demonstrated. Pituitary adenylate cyclase activating polypeptide (PACAP) is derived from a 170 amino acid long precursor, which gives rise to PACAP 38, PACAP 27 and PACAP related peptide (PRP). All peptides are present in tissue, the dominating form being PACAP 38. Prohormone convertase (PC) 1 and 2 seem to be involved in the processing of PACAP, except in the testes and ovary, where the PACAP precursor is substrate for PC4.

PreproPACAP-derived peptides occur in VIP-producing tumours and co-exist with VIP

Pituitary adenylate cyclase activating polypeptide (PACAP) is a newly discovered neuropeptide which exists in two biologically active forms: PACAP-38 consisting of 38 amino acids and PACAP-27, a peptide corresponding to the N-terminal 27 amino acids of PACAP-38. Both PACAPs are derived from a 176 amino acid precursor (preproPACAP) which in addition gives rise to a 29 amino acid peptide, designated PACAP-related peptide (PRP). The presence of the three preproPACAP-derived peptides (PACAP-38, PACAP-27 and PRP) in tumour tissue from nine patients with VIP-producing tumours (pancreatic carcinoma, neuroblastoma, ganglioneuroma and pheochromocytoma) and eleven patients with non-VIP-secreting tumours (gastrinoma, glucagonoma, somatostatinoma, neuroblastoma) was examined by specific radioimmunoassays. In seven out of the nine VIP-secreting tumours elevated concentrations of all the three preproPACAP-derived peptides were found compared with normal tissue, while the concentrations in the non-VIP-secreting tumours were within the normal range. PACAP-38 was in all cases the dominating peptide, the concentration ranging from 41 to 3606 pmol/g. When tumour extracts were fractionated on Sephadex G50 column, tricine gel electrophoresis or reverse-phase HPLC immunoreactive components corresponding to synthetic PACAP-38, PACAP-27 and human PRP were identified, suggesting that preproPACAP was fully processed. Immunocytochemical examination showed PACAP-immunoreactive cells in the tumour tissue which also stained for VIP. This co-localization of PACAP and VIP was confirmed by double-staining experiments on the same sections, demonstrating PHM/VIP mRNA and PACAP-immunostaining in the same cells.

Synthesis, solution structure and biological action of PACAP-related peptide

High quality PACAP-related peptide (PRP), a 29 amino-acid region of the PACAP precursor protein, has been synthesized in quantities sufficient for biological and structural studies. PRP has a distinct biological activity on the gallbladder that is similar to PACAP, but opposite to that of VIP and its related peptide, PHM. Its solution structure has been investigated by circular dichroism spectroscopy and 2D 1H nuclear magnetic resonance spectroscopy. In contrast to the poorly defined structure in aqueous solution alone, the limiting structure, under conditions that mimic a membrane-like environment, possesses stable secondary structure with a helical region between residues 3 and 20, that is terminated by the presence of glycine at residue 21 and is followed by a region of nascent helix. The similarities and differences in the structure of PRP, PACAP27 and GHRH(1-29) are made through comparison of their H alpha chemical shift data and differences in their biological activities assessed.