Opposing mitogenic regulation by PACAP in sympathetic and cerebral cortical precursors correlates with differential expression of PACAP receptor (PAC1-R) isoforms

Abstract

Neurogenesis in the peripheral and central nervous systems proceeds in region-specific fashion, although underlying mechanisms remain undefined. Emerging evidence indicates that the neuropeptide PACAP and its G-protein-coupled receptor are expressed widely in the embryonic brain, suggesting that the ligand/receptor system plays a role in development. We found previously that PAC1-R activation elicited opposing mitogenic effects in neurogenetic cultures, stimulating peripheral sympathetic neuroblasts while inhibiting cerebral cortical precursors. We have now defined the expression of PAC1-R mRNA isoforms and activation of second-messenger pathways in these model populations. Sympathetic neuroblasts express the "hop" receptor isoform, through which PACAP elicits increased levels of cAMP and activation of the PI signaling pathway. In contrast, cerebral cortical precursors express primarily the "short" (non-insert) receptor isoform and exhibit increased cAMP levels alone following PACAP treatment. Thus, opposing mitogenic regulation in sympathetic and cortical precursors correlates with differential receptor isoform expression and distinct second-messenger signaling. In addition to receptor, PACAP ligand mRNA was expressed by both populations, suggesting that the peptide is produced and acts locally to regulate precursor proliferation. These observations indicate that the PACAP ligand/receptor system is expressed in both the peripheral and central nervous system during development. More generally, these studies suggest that widely expressed extracellular factors mediate region-specific neurogenesis by activating lineage-restricted receptor isoforms and intracellular pathways.

Regulation of neuroblast mitosis is determined by PACAP receptor isoform expression

“…The vast majority of normal cortical precursors express the short isoform of PAC1, defined previously (10,17). Indeed, most cells exhibit PAC1 immunoreactivity after extended peroxidase incubation with DAB substrate (Fig.…”

“…In transfected cell lines, the absence (''short'' isoform) or presence of either one or two 28-aa cassettes, designated ''hip'' and ''hop,'' in the third intracellular loop regulates coupling to phosphoinositide-specific phospholipase C (PLC), although data regarding hop function conflict (14,15). In primary neuroblasts, we previously defined region-specific mitogenic effects of PACAP, which may be attributed to receptor isoforms (17). For example, in embryonic hindbrain (11) and cortical neuroblasts (10,17), which express the short variant, PACAP stimulates AC only and elicits mitotic inhibition.…”

“…In primary neuroblasts, we previously defined region-specific mitogenic effects of PACAP, which may be attributed to receptor isoforms (17). For example, in embryonic hindbrain (11) and cortical neuroblasts (10,17), which express the short variant, PACAP stimulates AC only and elicits mitotic inhibition. In contrast, in sympathetic neuroblasts, which express the hop variant, PACAP activates both AC and PLC pathways and stimulates precursor mitosis (12,17).…”

“…For example, in embryonic hindbrain (11) and cortical neuroblasts (10,17), which express the short variant, PACAP stimulates AC only and elicits mitotic inhibition. In contrast, in sympathetic neuroblasts, which express the hop variant, PACAP activates both AC and PLC pathways and stimulates precursor mitosis (12,17). Thus, alternative usage of the hop exon may determine receptor intracellular signaling and mitogenic response.…”

Direct cAMP Signaling through G-Protein-Coupled Receptors Mediates Growth Cone Attraction Induced by Pituitary Adenylate Cyclase-Activating Polypeptide

“…In particular, PAC1 splice variants activate a number of signaling cascades, including the activation of adenylate cyclase, phosphatidyl inositol turnover, and L-type Ca 2ϩ channels (Pisegna and Wank, 1993;Spengler et al, 1993;Chatterjee et al, 1996). For instance, PAC1 hop splice variant activation increases PI turnover, protein kinase C localization, and intracellular Ca 2ϩ mobilization, whereas PAC1 short activation only stimulates adenylate cyclase (Lu et al, 1998;DiCicco-Bloom et al, 2000;Nicot and DiCicco-Bloom, 2001). Our current data demonstrate that PACAP-induced growth cone attraction was independent of Ca 2ϩ , PLC, and PI3 kinase pathways, implying that PAC1 short , not PAC1 hop , likely mediates the turning.…”