Activation of Trk Neurotrophin Receptor Signaling by Pituitary Adenylate Cyclase-activating Polypeptides

Abstract

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide that acts through G proteincoupled receptors, exerts neuroprotective effects upon many neuronal populations. However, the intracellular signaling mechanisms that account for PACAP's trophic effects are not well characterized. Here we have tested the possibility that PACAP uses neurotrophin signaling pathways. We have found that PACAP treatment resulted in an increase in TrkA tyrosine kinase activity in PC12 cells and TrkB activity in hippocampal neurons. The activation of TrkA receptors by PACAP required at least 1 h of treatment and did not involve binding to nerve growth factor. Moreover, PACAP induced an increase in activated Akt through a Trk-dependent mechanism that resulted in increased cell survival after trophic factor withdrawal. The increases in Trk and Akt were blocked by K252a, an inhibitor of Trk receptor activity. In addition, transactivation of TrkA receptors by PACAP could be inhibited with PP1, an inhibitor of Src family kinases or BAPTA/AM, (1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester), an intracellular calcium chelator. Therefore, PACAP can exert trophic effects through a mechanism involving Trk receptors and utilization of tyrosine kinase signaling. This ability may explain several neuroprotective actions of PACAP upon neuronal populations after injury, nerve lesion, or neurotrophin deprivation.Neurotrophic effects are generally associated with the action of polypeptide growth factors, such as the NGF 1 neurotrophin, ciliary neurotrophic factor (CNTF), and glial-derived neurotrophic factor families. Each represents small families of proteins that are essential for the development of the vertebrate nervous system. The effects of these factors on cell survival, differentiation, and cell death events depend upon binding to transmembrane receptors and stimulation of downstream signaling cascades characterized by increased protein tyrosine phosphorylation. Neurotrophins utilize Trk receptor tyrosine kinases, and glial-derived neurotrophic factor family members signal through the c-Ret receptor tyrosine kinase, whereas CNTF utilizes JAK tyrosine kinase activity linked to the CNTF receptor complex (1).It has become increasingly apparent that many other secreted proteins are also capable of providing neuroprotective or neurotrophic actions. The pituitary adenylate cyclase-activating polypeptide (PACAP) belongs to a family of peptides, including secretin, glucagon, and vasoactive intestinal peptide (VIP) (2, 3). PACAP exists in two active forms of 38 (PACAP38) and 27 amino acids (PACAP27). Although PACAP was originally isolated from the hypothalamus, it is distributed throughout the central nervous system, including the hippocampus, olfactory bulb, and cerebral cortex (4 -9).The biological effects of PACAP are mediated by seven-transmembrane-spanning receptors. Two G protein-coupled receptors exist for PACAP. The type 1 PACAP receptor, PAC1, displays high specificity for PACAP (10), whereas type I...

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