PACAP27: natural brain signaling peptide linked to stress and pain

What this is

PACAP27 is a 27-amino-acid neuropeptide that occurs naturally in the brain, spinal cord, and peripheral nervous system of mammals, including humans. It is one of two forms of pituitary adenylate cyclase-activating polypeptide (PACAP) that arise from the same precursor protein: the shorter PACAP27 corresponds to the amino-terminal segment of the longer PACAP38, sharing its first 27 residues (May and colleagues 2021). PACAP was originally isolated from ovine hypothalamic extracts on the basis of its potent ability to stimulate cyclic AMP production in rat anterior pituitary cells (Liao and colleagues 2019). PACAP27 shares 68% amino acid identity with vasoactive intestinal peptide (VIP), a close structural relative (Frontiers in Cellular Neuroscience 2020). The 13-residue sequence stored on this card represents the N-terminal portion of PACAP27; the full 27-residue peptide additionally carries a C-terminal α-amide cap that is not encoded in the stored sequence but is required for full biological activity.

History

PACAP was first purified from ovine hypothalamic tissue extracts in the early 1990s, identified through bioassay-guided fractionation for compounds that stimulate adenylate cyclase in anterior pituitary cell cultures (Liao and colleagues 2019). Two molecular forms — PACAP38 and PACAP27 — were recognized from the outset; both arise from the same precursor (proPACAP), with PACAP27 mapping to residues 131–157 of rat proPACAP (May and colleagues 2021). PACAP38 is the predominant form in most tissues, with the PACAP38:PACAP27 ratio being tissue-specific. Structurally, PACAP was assigned to the VIP/secretin/glucagon superfamily of GPCRs and their ligands (Liao and colleagues 2019). Cryo-EM structural determination of the PACAP27–VIP1R–Gs heterotrimer complex was achieved in 2020, revealing the molecular details of ligand recognition across class B GPCRs (Liang and colleagues, Nature Communications 2020).

What it does

PACAP27 activates receptors in the central and peripheral nervous systems to regulate a broad range of physiological processes: neuronal survival and differentiation, stress responses, pain signaling, hormone secretion, and protection of tissues against injury. At the pituitary level, PACAP27 stimulates growth hormone release from pituitary cells in a dose-dependent manner (Wong and colleagues 2005). In the pancreas, PACAP and VIP stimulate insulin secretion from pancreatic islets and insulinoma cells (Chu and colleagues 2002). PACAP has also been described as a neuroprotective peptide, with protective effects observed in animal models of cerebral ischemia and retinal ischemia; PACAP mRNA expression rises after myocardial infarction in mice, and cytoprotective effects have been documented in peripheral organs as well as the nervous system (Reglodi and colleagues 2018). PACAP27 shares these biological activities with PACAP38 but is generally considered somewhat less potent in antimicrobial and cytoprotective assays (Bhatt and colleagues 2018).

Evidence

• Human: No published clinical trials for PACAP27 as a standalone therapeutic. PAC1R is under active investigation as a drug target for migraine, PTSD, and metabolic disorders (Liao and colleagues 2019).
• Animal: Protective effects against cerebral and retinal ischemic injury in rodent and other animal models; stimulation of GH secretion from fish pituitary cells; extension of neuronal survival after NGF withdrawal in cell culture (Reglodi and colleagues 2018; Wong and colleagues 2005; JBC 2010).
• In vitro: PACAP27 used as a reference standard for cAMP accumulation assays at human PAC1, VPAC1, and VPAC2 receptors (Chu and colleagues 2002). Cryo-EM structure of PACAP27 in complex with VIP1R (VPAC1) and Gs heterotrimer resolved at molecular resolution (Liang and colleagues 2020).

Known effects

• Neuroprotection — Preclinical (rodent models of cerebral and retinal ischemia)
• Neuronal survival support — Preclinical (cell culture, NGF-withdrawal models)
• Growth hormone stimulation — Preclinical (fish pituitary cell perifusion)
• Insulin secretion stimulation — Preclinical (mouse islets, rat perfused pancreas, insulinoma cells)
• Antimicrobial activity — Preclinical, weaker than PACAP38 (in vitro diffusion and broth assays)
• PAC1R agonism — Mechanistic (in vitro receptor binding and cAMP assays)

Mechanism

PACAP27 acts as an agonist at three G protein-coupled receptors: PAC1R (ADCYAP1R1), VPAC1R (VIPR1), and VPAC2R (VIPR2), all members of the VIP/secretin/glucagon class B GPCR superfamily (Liao and colleagues 2019). PAC1R activation initiates multiple downstream signaling cascades including adenylyl cyclase (cAMP elevation), phospholipase C, MEK/ERK, and Akt pathways that together regulate neuronal homeostasis and tissue protection (Liao and colleagues 2019). In structural terms, PACAP27 adopts an α-helical conformation upon receptor engagement, occupying a V-shaped binding pocket; it contacts all transmembrane helices of VIP1R except TM4, with the N-terminus engaging the helix bundle core and the C-terminal portion docking to the receptor's extracellular N-terminal domain (Liang and colleagues 2020; Blechman and colleagues 2013). Receptor-selectivity determinants reside partly in the N-terminal domain: residues at positions 4 and 5 of PACAP form an encrypted receptor-selective dipeptide, and modification at position 2 (e.g., the [Hyp²]PACAP27 analog) reduces VPAC1R binding while retaining PAC1R and VPAC2R affinity (Liao and colleagues 2019). Alternative splicing of PAC1R produces multiple receptor isoforms that differentially tune the downstream signaling profile (Blechman and colleagues 2013).

Related peptides

• PACAP38 — the full 38-residue form of the same precursor; PACAP38 predominates in most tissues and shares all activities of PACAP27 with generally higher potency. (Card not linked; pep-id unverified.)
• VIP (vasoactive intestinal peptide) — shares 68% sequence identity with PACAP27 and acts at the same VPAC1R and VPAC2R receptors but has lower affinity for PAC1R. (Card not linked; pep-id unverified.)