The Section on Molecular Neuroscience studies slow (metabotropic) transmission in the nervous system, to identify novel molecular components underlying cellular plasticity following synaptic activity and slow transmitter release and post-synaptic action. The neuropeptide PACAP, acting through its G-protein coupled receptor PAC1 acts at the adrenomedullary synapse, and in the brain, to simultaneously elevate calcium and cyclic AMP, and uniquely stimulate simultaneous signaling for secretion, and signaling for transcription, in target cells. The subtype of the PAC1 receptor responsible for this unique mode of combinatorial signaling has been shown by T. Mustafa to be the 'hop' variant of the PAC1 receptor (see Mustafa et al., 2007). The 'hop' cassette imparts signaling for both calcium mobilization and calcium influx,in a cell-type-dependent fashion, while all PAC1 receptor variants are able to couple to Gs and stimulate cAMP production in all cells tested. Despite the ubiquity of PACAP signaling through Gs, elevation of cAMP via the PAC1 receptor is unique in that cAMP-dependent ERK activation, and stimulation of target genes such as Ier3, is independent of protein kinase A. Forskolin stimulation of this and other PACAP target genes can be 'switched' to the non-canonical (non-PKA-requiring) pathway through simultaneous activation of calcium influx (e.g. during cell depolarization with elevated KCl). Non-canonical cAMP-initiated signaling by PACAP directs activation of ERK within target cells via a pathway that requires the GTP-binding protein Rap1, and an as-yet unidentified cAMP sensor protein (Gerdin and Eiden, in preparation; Gerdin and Eiden, 2007). Signaling to the PACAP target gene Egr-1 through this pathway is required for neuritogenesis in PC12 cells (Ravni et al., submitted, 2007) and studies are underway to examine PACAP-dependent effects in CNS neurons in culture, and at the adrenomedullary synapse in vivo, mediated through Egr-1 (zif268), shown previously by others to play a role in synaptic events underlying hippocampally dependent learning. ? As described above, PACAP acting through its G-protein coupled receptor PAC1 initiates combinatorial signaling in neuroendocrine cells that activates a unique manifold of target genes that are not stimulated by either calcium or cyclic AMP alone. A meta-analysis of cDNA microarray studies carried out in PC12 cells in culture, and wild-type and PACAP-deficient mice after metabolic stress (adrenal gland) and ischemic insult (cerebral cortex) has revealed a cohort of PACAP-dependent genes including Ier-3, and the neuropeptides enkephalin, neurotensin and substance P, that are potential actors in the 'emergency response' functions of PACAP(see Samal et al., 2007). We have since confirmed initial findings that the up-regulation of several neuropeptides in the adrenal medulla during stress is likewise abrogated in PACAP-deficient mice, suggesting that PACAP's role as a regulator of neuropeptide expression is widespread throughout the nervous system during synaptic activation and/or stress. ? We intend to focus on identifying the cAMP sensor through which genes responsive to combinatorial calcium and cAMP signaling are uniquely activated by slow transmitters such as PACAP, in both central and peripheral PACAP-responsive neurons, and determining the physiological relevance of this novel signaling system to synaptic transmission and cellular plasticity at the adrenomedullary synapse and in the brain in response to stress.
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